SPI_HOST HWIP Technical Specification


This document specifies SPI_HOST hardware IP (HWIP) functionality. This module conforms to the Comportable guideline for peripheral functionality. See that document for integration overview within the broader top-level system.


  • Hardware control for remote devices using the Serial Peripheral Interface (SPI)
  • Primarily designed for serial NOR flash devices such as the Winbond W25Q01JV
  • Number of chip select lines controlled by NumCS parameter
  • Support for Standard SPI, Dual SPI or Quad SPI commands
    • Signals SD[0] through SD[3] are intended to connect to lines IO0 through IO3 respectively, on the target device.
    • Signal SD[0] may also be identified as “MOSI” by other SPI Hosts, while SD[1] is commonly referred to as “MISO”
  • RX and TX data held in separate FIFOs
    • 288 bytes for TX data, 256 bytes for RX data
    • FIFOs loaded/unloaded via 32-bit TL-UL registers
    • Support for arbitrary byte-count in each transaction
    • Parametrizable support for Big- or Little-Endian systems in ordering I/O bytes within 32-bit words.
  • SPI clock rate controlled by separate input clock to core
    • SPI SCK line typically toggles at 1/2 the core clock frequency
    • An additional clock rate divider exists to reduce the frequency if needed
  • Support for all SPI polarity and phases (CPOL, CPHA)
    • Additional support for “Full-cycle” SPI transactions, wherein data can be read a full SPI Clock cycle after the active edge (as opposed to one half cycle as is typical for SPI interfaces)
  • Single Transfer Rate (STR) only (i.e. data received on multiple lines, but only on one clock edge)
    • No support for Dual Transfer Rate (DTR)
  • Pass-through mode for coordination with SPI_DEVICE IP
  • Automatic control of chip select lines
  • Condensed interrupt footprint: Two lines for two distinct interrupt classes: “error” and “spi_event”
    • Fine-grain interrupt masking supplied by secondary enable registers


The Serial Peripheral Interface (SPI) is a synchronous serial interface quite commonly used for NOR flash devices as well as a number of other off-chip peripherals such as ADC’s, DAC’s, or temperature sensors. The interface is a de facto standard (not a formal one), and so there is no definitive reference describing the interface, or establishing compliance criteria.

It is therefore important to consult the data sheets for the desired peripheral devices in order to ensure compatibility. For instance, this OpenTitan SPI Host IP is primarily designed for controlling Quad SPI NOR flash devices, such as the W25Q01JV Serial NOR flash from Winbond or this 1 Gb M25QL NOR flash from Micron. Though this IP implementation aims to be general enough to support a variety of devices, the Winbond serial flash device is used as the primary reference for understanding our host requirements.

There are also a number of good references describing legacy host implementations for this protocol, which are useful for understanding some of the general needs for a wider range of target devices. For instance, the legacy SPI Block Guide from Motorola contains a definitive overview of some of the the general requirements for a standard SPI host, notably the definitions of SPI clock phase and polarity (CPOL and CPHA). In order to potentially support a broad range of devices, this SPI Host IP also supports all four of the standard SPI clock phases.

SPI Protocol Basics

Broadly speaking, the SPI host accepts commands from the TL-UL interface and, based on these commands, serially transmits and receives data on the external SPI interface pins. The timing and data-line formatting of the command sequence depend on the external peripheral device and the nature of the specific command issued.

In each standard SPI command a number of instruction-, address- or data-bytes are transmitted on SD[0], and response bytes are received on SD[1]. So in standard-mode commands, SD[0] is always configured as an output, and SD[1] is always an input. In standard SPI commands the SD[0] and SD[1] lines can be used as a full-duplex communication channel. Not all devices exploit this capability, opting instead to have clear input and output phases for each command. This half-duplex signaling behavior is especially common in devices with also support Dual- and Quad-mode commands, which are always half-duplex. The SPI_HOST IP optionally supports both full-duplex and half-duplex commands in standard mode.

Along with the data lines, the SPI protocol also includes a chip select line, commonly called CS#. In this IP we refer to it as CSB. The SPI bus can be connected to many target peripherals, but each device on the bus gets its own chip select line, and so this active-low signal designates a particular device for each command.

The chip-select line also marks the beginning and end of each command. No device will accept any command input until CSB has been asserted for that device, and most devices (if not all) do not accept a second command unless CSB has been deasserted to mark the end of the previous command. Some simple devices, particularly those that support SPI daisy-chaining, do not process command input at all until after the CSB line has been deasserted. In the case of NOR flash devices, read and write commands are indeterminate in length, and the data transfer ends only when the CSB line is deasserted. So, though the exact details of operation may vary from device to device, the edges of the CSB signal serve as an important markers for delineating the boundaries of each transaction.

The SD and CSB lines are accompanied by a serial clock, SCK. The host is responsible for generating the serial clock, and typically each side asserts outgoing data on one edge of the clock (e.g. on the rising edge) and samples data in the next edge (e.g. on the falling edge). When it comes to devices there is no universal convention on clock polarity (CPOL) or clock phase (CPHA). Some devices expect the clock line to be low when the host is idle, thus the clock should come as a sequence of positive pulses (CPOL = 0). On the other hand, other devices expect CPOL = 1, meaning that the clock line is inverted: high when idle with sequences of negative pulses.

Devices also differ in their expectations of clock phase (CPHA) relative to the data. Devices with CPHA = 0, expect that both the host and device will be sampling data on the leading edge, and asserting data on the trailing edge. (Because of the option for either polarity, the terms “leading” and “trailing” are preferred to “rising” or “falling”). When CPHA = 0, the first output bit is asserted with the falling edge of CSB. Meanwhile if CPHA = 1, data is always is asserted on the leading edge of SCK, and data is always sampled on the trailing edge of SCK.

When operating at the fastest-rated clock speeds, some flash devices (i.e. both the Winbond and Micron devices noted above), require setup times which exceed half a clock-cycle. In order to support these fastest data rates, the SPI_HOST IP offers a modified “Full-cycle” (FULLCYC = 1) timing mode where data can be sampled a full cycle after the target device asserts data on the SD bus. This full cycle mode has no effect on any of the signals transmitted, only on the timing of the sampling of the incoming signals.

As mentioned earlier, the SD[0] and SD[1] lines are unidirectional in Standard SPI mode. On the other hand in the faster Dual- or Quad-modes, all data lines are bidirectional, and in Quad mode the number of data lines increases to four. For the purposes of this IP, Dual or Quad-mode commands can be thought of as consisting of up to four command segments in which the host:

  1. Transmits instructions or data at single-line rate,
  2. Transmits instructions address or data on 2 or 4 data lines,
  3. Holds the bus in a high-impedance state for some number of “dummy” clock cycles (neither side transmits), or
  4. Receives information from the target device.

Each of these segments have a different directionality or speed (i.e., SD bus width). As indicated in the example figure below, input data need only be sampled during the last segment. Likewise, software-provided data is only transmitted in the first two segments. The SPI_HOST command interface allows the user to specify any number of command segments to build larger, more complex transactions.

For even faster transfer rates, some flash chips support double transfer rate (DTR) variations to the SPI protocol wherein the device receives and transmits fresh data on both the leading and trailing edge. This IP only supports single transfer rate (STR), not DTR. A preliminary investigation of DTR transfer mode suggests that proper support for setup and hold times in this mode may require a level of sub-cycle timing control which is not currently planned for this IP.

Theory of Operations

SPI_HOST IP Command Interface

A SPI command consists of at least one segment. Each segment has a different speed (number of active SD lines), direction and length. For example a Quad SPI read transaction consists of 4 segments:

  1. A single byte instruction transmitted at standard data rate
  2. A three or four byte address transmitted at Quad data rate
  3. A number of dummy cycles (no data transmitted or received)
  4. The desired data, received by SPI_HOST at Quad data rate

During a transaction, software can issue multiple segment descriptions to the SPI_HOST IP to control for changes in speed or direction.

Issuing a command then consists of the following steps:

  1. Configure the IP to be compatible with each attached peripheral. The CONFIGOPTS multi-register holds separate sets of configuration settings, one for each CSB line. In principle, the configuration of these device-specific options only needs to be done/performed once at initialization.

  2. Load the TX FIFO with the instructions and data to be transmitted to the remote device by writing to the TXDATA memory window.

  3. Specify which device should receive the next command using the CSID register.

  4. Wait for STATUS.READY before continuing.

  5. Issue speed, direction, and length details for the next command segment using the COMMAND register. If a command consists of multiple segments, then set COMMAND.CSAAT (Chip-select active after transaction) to one for all segments except the last one. Setting COMMAND.CSAAT to zero indicates the end of a transaction, prompting the IP to raise CSB at the end of the segment.

  6. Repeat steps 4 and 5 until all segments have been described.

  7. Read any peripheral response data from the RX FIFO by reading from the RXDATA memory window.

About Command Segments

The structure of a SPI command depends on the device and the command itself.

To support a variety of different I/O sequences the SPI_HOST FSM treats each command as a sequence of segments, each with a defined length, direction and speed.

In case of a standard SPI device the commands are very consistent in structure: the host transmits data on SD[0], and always receives data on SD[1]. For such devices, all commands can in principle be treated as bidirectional, as both the host and device are always transmitting on their respective lines. For bidirectional commands, the SPI_HOST IP will store one byte in the RX FIFO for each byte transmitted from the TX FIFO.

However, even for these standard SPI commands, software may be uninterested in some or all of the device’s response data. For example, for SPI flash devices, standard-mode write commands contain no useful data in the device response, even though the device may be actively asserting signals to SD[1] throughout the transaction. Therefore, for such commands software may choose to specify the entire command as “TX Only”, in which case data placed in the TX FIFO will be transmitted throughout the write command, but signals received from the device will be ignored and will not fill the RX FIFO.

Meanwhile for other flash commands, such as standard-mode read, the device only transmits useful information during some portions of the transaction. In the case of a basic read (with a 3-byte address), the instruction starts with a 1-byte instruction code (0x3) followed by the three address bytes, during which time the flash device outputs may be high impedance (depending on the device). The device then immediately responds with the requested data in the next SCK cycle, and continues to output data bytes until the CSB line is deasserted. Though such a command could also be treated as entirely bidirectional, the device response can be safely ignored during the instruction and address phase, especially if the SD[1] line is high impedance during this time. Likewise it is not necessary for software to specify any data to transmit while the device is responding. Therefore such a command can be thought of as consisting of two separate segments, the first segment being TX Only and the second segment being RX only, as shown in the following figure. Breaking the command up this way potentially simplifies the job of writing software for this type of command.

In addition to the TX, RX or Bidirectional modes, many SPI commands require periods where neither the host or device are transmitting data. For instance, many flash devices define a Fast Read command in which the host must insert a number of “dummy clocks” between the last address byte and the first data byte from the device. These extra cycles are required for operation at higher clock frequencies, to give the address time to propagate through the flash core. A standard-mode Fast Read (with 3 byte addressing) command then requires three SPI_HOST command segments:

  • 4 bytes TX Only: one for the instruction code (i.e., 0xb for Fast Read), and three for the address.
  • 8 dummy clocks
  • N bytes RX Only for read data response

For standard mode-commands, segments simplify the IO process by identifying which bus cycles have useful RX or TX data. In such cases it is not strictly necessary to the manage the impedance of the SD[0] and SD[1] lines. For Dual- and Quad-mode commands, however, impedance control necessary. The impedance of all data lines (SD[3:0]) must switch between TX and RX segments.

Bidirectional data transfers are not applicable for Dual- or Quad-mode segments.

In addition, the speed-mode changes how data is distributed across the four data lines, and many commands require that some segments are transmitted in standard mode (only on SD[0]), while the bulk of the data is transmitted in Dual- or Quad-mode. For this reason the speed-mode is also adjustable on a segment-by-segment basis.

Specifying Command Segments

The SPI host supports all four possible modes for command segments, and they are controlled writing one of the following values to the 2-bit COMMAND.DIRECTION register:

  • 2’b00: Dummy cycles only (neither side transmits)
  • 2’b01: RX Only
  • 2’b10: TX Only
  • 2’b11: Bidirectional

CSID Register

The CSID register is used to identify the target device for the next command segment. Whenever a command segment descriptor is written to COMMAND, CSID is passed into the FSM along with the command segment descriptor and the corresponding configurations options (taken from the CSID’th element of the CONFIGOPTS multi-register).

This register still exists when instantiated with only one CSB line (i.e. when NumCS=1). However in this case the CSID value is ignored.

Changes in CSID also affect the CSB lines, because a change in CSID can also implicitly end a command, overriding COMMAND.CSAAT. If a change is detected in CSID, but the previous segment was submitted with the CSAAT bit asserted, the FSM terminates the previous command before moving on to the next segment. The previous CSB line is held low for at least CSNTRAIL cycles (as defined by the previous value of CONFIGOPTS.CSNTRAIL) and then brought high. All CSB lines are held high for CSNIDLE cycles (using the new value of CONFIGOPTS.CSNIDLE). The new CSB line is asserted low, and SCK begins toggling after the usual CSNLEAD cycle delay.

Configuration Options

The CONFIGOPTS multi-register has one entry per CSB line and holds clock configuration and timing settings which are specific to each peripheral. Once the CONFIGOPTS multi-register has been programmed for each SPI peripheral device, the values can be left unchanged.

The following sections give details on how the SPI_HOST can be used to control a specific peripheral. For simplicity, this section describes how to interact one device, attached to CSB[0], and as such references are made to the multi-registers CONFIGOPTS and COMMAND. To configure timing and send commands to devices on other CSB lines, instead use the CONFIGOPTS multi-register corresponding to desired CSB line.

The most common differences between target devices are the requirements for a specific SPI clock phase or polarity, CPOL and CPHA, which were described in the previous section SPI Protocol Basics. These clock parameters can be set via the CONFIGOPTS.CPOL or CONFIGOPTS.CPHA register fields. Likewise, as also described in the previous section, if device setup times require a full clock cycle before sampling the output, Full-Cycle Mode can be enabled by asserting the CONFIGOPTS.FULLCYC bit.

Clock rate selection

The SPI clock rate for each peripheral is set by two factors:

  • The SPI_HOST input clock
  • A 16-bit clock divider

The SPI protocol usually requires activity (either sampling or asserting data) on either edge of the SCK clock. For this reason the maximum SCK frequency is at most one half the SPI_HOST core frequency.

Since some peripheral devices attached to the same SPI_HOST may require different clock frequencies, there is also the option to divide the core clock by an additional factor when dealing with slower peripherals.


Chip-select Timing Control

Typically the CSB line is automatically deasserted after the last edge of SCK. However, by asserting COMMAND.CSAAT when issuing a particular command, one can instruct the core to hold CSB low indefinitely after the last clock edge. This is useful for merging two adjacent command segments together, to create more complex commands, such as flash Quad read commands which require a mix of segments with different speeds and directions. The CSB line can then be deasserted by either issuing another command without the COMMAND.CSAAT field, issuing a command to a different device (after changing the CSID register), or simply resetting the core FSM via the CONTROL.RST register.

To avoid spurious clock signals, changes to the CONFIGOPTS parameters take effect only at the end of a command segment and only when all csb lines are deasserted. There are two cases to consider:

  1. Configuration changes detected and CSAAT=0 for the previous segment: This is when configuration changes are typically expected, and in this case, the SPI_HOST waits for the previous segment to complete before moving changing the configuration. The SPI_HOST ensures that all csb lines are held idle long enough to satisfy the configuration requirements both before and after the change.
  2. CSAAT = 1 for the previous segment: Configuration changes are not typically expected after CSAAT segments, and require special treatment as the IP does not usually return the csb lines to the idle/inactive state at this time. In such cases, the SPI_HOST IP closes out the ongoing transaction, ignoring CSAAT, and the configuration is then applied once the SPI_HOST has returned to the idle state. The next segment can then proceed, even though the remote device will likely see the next segment as the start of a new transaction (as opposed to a continuation of the previous transaction), because of the brief intervening idle pulse.

Most devices require at least one-half SCK clock-cycle between either edge of CSB and the nearest SCK edge. However, some devices may require more timing margin and so the SPI_HOST core offers some configuration registers for controlling the timing of the CSB edges when operating under automatic control. The relevant parameters are as follows:

  • TIDLE: The minimum time between each rising edge of CSB and the following falling edge. This time delay is a half SCK cycle by default but can be extended to as long as eight SCK cycles by setting the CONFIGOPTS.CSNIDLE register.
  • TLEAD: The minimum time between each falling edge of CSB and the first leading edge of SCK. This time delay is a half SCK cycle by default but can be extended to as long as eight SCK cycles by setting the CONFIGOPTS.CSNLEAD register.
  • TTRAIL: The minimum time between the last trailing edge of SCK and the following rising edge of CSB. This time delay is a half SCK cycle by default but can be extended to as long as eight SCK cycles by setting the CONFIGOPTS.CSNTRAIL register.

These settings are all minimum bounds, and delays in the FSM implementation may create more margin in each of these timing constraints.

Idle Time Delays When Changing Configurations

It is important that the configuration changes are applied while csb is high to avoid sending spurious sck events to any devices. For example, if two devices have different requirements for CPOL, the clock polarity should not toggle except when csb is high (inactive) for all devices.

Furthermore, csb should be remain high for the minimum idle time both before and after the configuration update. For example, consider a SPI_HOST attached to two devices each with different requirements for the clock divider, clock polarity, and idle time. Consider a configuration where total idle time (as determined by the CONFIGOPTS.CLKDIV and CONFIGOPTS.CSNIDLE multi-registers) works out to 9 idle clocks for the first device, and 4 clocks for the second device. In this scenario then, when swapping from the first device to the second, the SPI_HOST IP will only swap the clock polarity once the first csb line, csb[0], has been high for at least 9 clocks, and will continue to hold the second csb line, csb[1], high for 4 additional clocks before starting the next transaction.

This additional idle time applies not only when switching between devices but when making any changes to the configuration for most recently used device. For instance, even in a SPI_HOST configured for one device, changes to CONFIGOPTS, will trigger this extended idle time behavior to ensure that the change in configuration only occurs in the middle of a long idle period.

Special Command Fields

The COMMAND register must be written once for each command segment. Whenever a command segment is written to COMMAND, the contents of the CONFIGOPTS, CSID, and COMMAND registers are passed through the Config/Command FIFO to the SPI_HOST core FSM. Once the command is issued, the core will immediately deassert STATUS.READY, and once the command has started STATUS.ACTIVE will go high. The command is complete when STATUS.ACTIVE goes low. A spi_event interrupt can also be triggered to go off on completion by setting EVENT_ENABLE.IDLE.

Chip Select Masks

Each instance of the SPI_HOST IP supports a parametrizable number of chip select lines (CSB[NumCS-1:0]). Each CSB line can be routed either to a single peripheral or to a daisy-chain of peripherals. Whenever a segment description is written to the COMMAND register, the CSID is sent along with COMMAND and the CONFIGOPTS multi-register corresponding to CSID to indicate which device is meant to receive the command. The SPI_HOST core typically then manages the details of asserting and deasserting the proper CSB line, subject to the timing parameters expressed in CONFIGOPTS.CSNLEAD, CONFIGOPTS.CSNTRAIL, and CONFIGOPTS.CSNIDLE.

If Pass-through mode is enabled then the CSB lines are controlled by neither the SPI_HOST hardware nor the firmware register. In Pass-though mode, control of the CSB lines passes directly to the inter-module port, passthrough_i.csb.

Back-to-back Segments

The command interface can allows for any number of segments in a given command.

Since most SPI Flash transactions typically consist of 3 or 4 segments, there is a small command FIFO for submitting segments to the SPI_HOST IP, so that firmware can issue the entire transaction at one time.

Writing a segment description to COMMAND when STATUS.READY is low will trigger an error condition, which must be acknowledged by software. When submitting multiple segments to the the command queue, firmware can also check the STATUS.CMDQD register to determine how many unprocessed segments are in the FIFO.

Data Formatting

Input and Output Byte Ordering

The SPI transactions must be issued with correct bit ordering to properly communicate with a remote device. Based on the requirements for our chosen flash devices, this IP follows these conventions:

  • The relative significance of lines on the SD bus: SD[0] is always the least significant, followed by SD[1] though SD[3] with increasing significance.
  • The relative significance of a sequence of bits on the same SD bus: more significant bits are always transmitted before (or at the same time as) less significant bits.
    • For instance, when transferring a single byte in Quad mode, all four bits of the upper nibble (bits 7 through 3) are transferred in the first clock cycle and the entire lower nibble (bits 3 through 0) is transferred in the second cycle.

The programming model for the IP should meanwhile make it easy to quickly program the peripheral device, with a minimum amount of byte shuffling. It should be intuitive to program the specific flash devices we are targeting, while following the conventions above:

  • When transferring data in from the RXDATA memory window or out to the TXDATA window, the IP should fully utilize the TL-UL bus, using 32-bit I/O instructions.
  • The SPI_HOST should make it easy to arrange transaction data in processor memory, meaning that bytes should be sequentially transmitted in order of ascending memory address.
    • When using 32-bit I/O instructions, this requires some knowledge of the processor byte-order.

Based on these requirements, data read from RXDATA or placed in TXDATA are handled as follows:

  • 32-bit words placed in TXDATA are transmitted in first-in-first-out order. Likewise, words received from the SPI data lines are made available for reading from RXDATA in first-in-first-out order.
  • Within a 32-bit word, the ByteOrder parameter controls the order in which bytes are transmitted, and also the manner in which received bytes are eventually arranged in the 32-bit RXDATA register. By default (ByteOrder = 1, for Little-Endian processors), the LSB of TXDATA (i.e bits 7 though 0) is transmitted first, and the other bytes follow in order of increasing significance. Similarly, the first byte received is packed into the LSB of RXDATA, and the subsequent bytes of each RXDATA word are packed in order of increasing significance.

On the other hand, if ByteOrder is set to 0 (for Big-Endian processors), the MSB is transmitted first from TXDATA, and received data is loaded first into the MSB of RXDATA.

  • The default choice of Little-Endian reflects native byte-order of the Ibex processor.
  • Finally within a given byte, the most significant bits are transmitted and received first. For Dual and Quad transactions the least significant bit in any instantaneous pair or nibble is transmitted or received on SD[0], and the remaining SD bits (1 though 3) are populated in order of increasing significance.

The following figure shows how data appears on the serial data bus when the hardware reads it from TXDATA or writes it to RXDATA.

As shown in the following figure, a similar time-ordering scheme applies for Dual- and Quad-mode transfers. However many bits of similar significance are packed into multiple parallel SD data lines, with the least significant going to SD[0].

Command Length and Alignment in DATA

Even though the TXDATA memory window typically accepts 32-bit words, command segments do not need to use all the bytes from every word.

For TX (or Bidirectional) segments, unused bytes from the latest TX FIFO word are simply ignored at the end of a segment. For RX (or Bidirectional) segments, if the last few bytes received do not fill an entire DATA word, the partial word will be zero-padded and inserted into the RX FIFO once the segment is completed. If ByteOrder=1 (the default, Little-Endian case), this padding will fill the unused most-significant bytes of the final RX DATA word, otherwise the padding will fill the unused least-significant bytes.

The following waveform illustrates an example SPI transaction, where neither the data transmitted nor the data received in each segment fit into an even number of 32-bit words. In this example, the values I[31:0], A[31:0] and B[31:0], have been previously written into TXDATA via firmware, and afterwards one word, X[31:0], is available for reading from RXDATA. All data in the waveform is transferred using 32-bit instructions.

When packing data into the TX FIFO, there are also no restrictions on the alignment of the data written to the TXDATA memory window, as it supports byte-enable signals. This means that when copying bytes into TXDATA from unaligned firmware memory addresses, it is possible to use byte or half-word instructions. Full-word instructions should however be used whenever possible, because each write consumes a full word of data in the TX FIFO regardless of the instruction size. Smaller writes will thus make inefficient use of the TX FIFO.

Filtering out disabled bytes consumes clock cycles in the data pipeline, and can create bubbles in the transmission of SPI_DATA. In the worst case, such bubbles can also be interpreted as transient underflow conditions in the TX FIFO, and could trigger spurious interrupts. The longest delays occur whenever a word is loaded into the TX FIFO with only one byte enabled.

When writing to the TXDATA window, only three types of data are expected: individual bytes, half-words, and full-words. Other types of write transactions (i.e., non-contiguous, zero-byte and three-byte writes) are not supported by most processors. Therefore it is assumed that if such transactions do appear, it is likely a sign of a system integrity error, and so these other classes of writes are not supported.

If such transactions ever occur, they trigger an “Invalid Access” error event, which suspends the processing of future commands until the error has been cleared by setting the ERROR_STATUS.ACCESSINVAL bit.

The RX FIFO has no special provisions for packing received data in any unaligned fashion. Depending on the ByteOrder parameter, the first byte received is always packed into either the most- or least-significant byte read from the RXDATA memory window.

Pass-through Mode

The SPI_HOST also supports a special “Pass-through” mode, which allows for the direct control of the serial interface by another block (namely SPI_DEVICE). This feature is entirely controlled by intermodule signals passthrough_i and passthrough_o, which control a set of multiplexers. If passthrough_i.passthrough_en is asserted the SPI_HOST peripheral bus signals reflect the corresponding signals in the passthrough_i structure. Otherwise, the peripheral signals are controlled by the SPI_HOST FSM and the internal shift register.

Interrupt Aggregation

In order to reduce the total number of interrupts in the system, the SPI_HOST has only two interrupt lines: error and spi_event. Within these two interrupt classes, there are a number of conditions which can trigger them.

Each interrupt class has a secondary status and mask register, to control which sub-classes of SPI events will cause an interrupt.

SPI Events and Event Interrupts

The SPI_HOST supports interrupts for the following SPI events:

  • IDLE: The SPI_HOST is idle.
  • READY: The SPI_HOST is ready to accept a new command.
  • RXFULL: The SPI_HOST has run out of room in the RXFIFO.
  • RXWM: The number of 32-bit words in the RXFIFO currently exceeds the value set in CONTROL.RX_WATERMARK.
  • TXEMPTY: The SPI_HOST has transmitted all the data in the TX FIFO.
  • TXWM: The number of 32-bit words in the TX FIFO currently is currently less than the value set in CONTROL.TX_WATERMARK

Most SPI events signal a particular condition that persists until it is fixed, and these conditions can be detected by polling the corresponding field in the STATUS register.

In addition to these events, there are also two additional diagnostic fields in the STATUS register:

  • RXSTALL: The RX FIFO is full, and the SPI_HOST is stalled and waiting for firmware to remove some data.
  • TXSTALL: The TX FIFO is not only empty, but the SPI_HOST is stalled and waiting for firmware to add more data.

These bits can provide diagnostic data for tuning the throughput of the device, but do not themselves generate event interrupts.

By default none of these SPI events trigger an interrupt. They need to be enabled by writing to the corresponding field in EVENT_ENABLE.

The SPI event interrupt is signaled only when the IP enters the corresponding state. For example if an interrupt is requested when the TX FIFO is empty, the IP will only generate one interrupt when the last data word is transmitted from the TX FIFO. In this case, no new interrupts will be created until more data has been added to the FIFO, and all of it has been transmitted.

Stall Conditions

The SPI_HOST IP will temporarily suspend operations if it detects a potential overflow of the RX FIFO or an attempted underflow of the TX FIFO. During a stall event, csb remains active, and there are no sck clock ticks until there is more data to transmit or there is some space to receive more data. The RXSTALL and TXSTALL status bits are meant to inform firmware of such halts. Due to implementation details the SPI_HOST IP will also pause, and signal a stall condition, if there are delays related to packing or unpacking the SPI_DATA into 32-bit words. The exact conditions for these transient stall conditions are implementation dependent, and described in detail in the Design Details section.

Error Interrupt Conditions

There are six types of error events which each represent a violation of the SPI_HOST programming model:

All of these programming violations will create an error event when they occur. They will also halt the IP until the corresponding bit is cleared in the ERROR_STATUS register. Whenever an error event occurs, the error must be acknowledged by clearing (write 1 to clear) the corresponding bit in ERROR_STATUS.

By default all error events will trigger an error interrupt. Clearing the bit corresponding bit in the ERROR_ENABLE register in the suppresses interrupts for that class of error event and allows the IP to proceed even if one of these errors has occurred. The ERROR_STATUS register will continue to report all violations even if a particular class of error event has been disabled.

Of the six error event classes, ACCESSINVAL error events are the only ones which cannot be disabled. This is because ACCESSINVAL events are caused by anomalous TLUL byte-enable masks that do not correspond to any known software instructions, and can only occur through a fault in the hardware integration.

When handling SPI_HOST error interrupts, the ERROR_STATUS bit should be cleared before clearing the error interrupt in the INTR_STATE register. Failure do to so may result in a repeated interrupt.

Status Indicators

The STATUS register contains a number of fields that should be queried for successful operation or troubleshooting.

The register STATUS.ACTIVE indicates whether a command segment is currently being processed by the FSM. Even if STATUS.ACTIVE is high it is often still possible to insert another command segment into the command FIFO. The register STATUS.READY indicates that there is room in the command FIFO.

The STATUS.BYTEORDER field indicates the fixed value of the ByteOrder parameter, which is presented to software to confirm the byte ordering used in the RXDATA and TXDATA windows.

The 8-bit fields STATUS.RXQD and STATUS.TXQD respectively indicate the number of words currently stored in the RX and TX FIFOs.

The remaining fields in the STATUS register are all flags related to the management of the TX and RX FIFOs, which are described in the section on SPI Events.

Other Registers


The SPI_HOST state machine is disabled on reset. Before any commands are processed, the block must be enabled by writing one to the CONTROL.SPIEN register. Writing a zero to this register temporarily suspends any previously submitted transactions. If the block is re-enabled by writing a one to CONTROL.SPIEN, any previously executing commands will continue from wherever they left off.

An unacknowledged error event suspends the core state machine.

SPI_HOST Output Enable

In addition to enabling the SPI_HOST FSM, the SPI_HOST outputs must also be enabled for successful operation. This can be achieved by also setting the CONTROL.OUTPUT_EN field when enabling the SPI_HOST FSM.

Component reset

In addition to the global hardware reset, there is a software reset option which completely resets the SPI host. To use this reset, assert CONTROL.SW_RST, and then wait for the device to reset ( STATUS.ACTIVE, STATUS.TXQD and STATUS.RXQD to all go to zero), before releasing CONTROL.SW_RST.

Block Diagram

Hardware Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module spi_host has the following hardware interfaces defined.

Primary Clock: clk_i

Other Clocks: none

Bus Device Interfaces (TL-UL): tl

Bus Host Interfaces (TL-UL): none

Peripheral Pins for Chip IO:

Pin namedirectionDescription

SPI Clock


Chip Select# (One hot, active low). The size of this port should match NumCS.


SPI data bus

Inter-Module Signals: Reference

Inter-Module Signals
Port Name Package::Struct Type Act Width Description
passthrough spi_device_pkg::passthrough req_rsp rsp 1
tl tlul_pkg::tl req_rsp rsp 1


Interrupt NameTypeDescription

Error-related interrupts, see ERROR_ENABLE register for more information.


Event-related interrupts, see EVENT_ENABLE register for more information.

Security Alerts:

Alert NameDescription

This fatal alert is triggered when a fatal TL-UL bus integrity fault is detected.

Security Countermeasures:

Countermeasure IDDescription

End-to-end bus integrity scheme.

Design Details

Component Overview

Transaction data words flow through the SPI_HOST IP in a path which starts with the TX FIFOs, shown in the block diagram above. At the output of the TX FIFOs each data word is separated into individual bytes by the Byte Select block, which is also responsible for parsing the byte-enable mask and discarding unwanted bytes. Selected bytes are then passed into the shift register, where they are played out at Standard, Dual, or Quad speed. For receive segments, outputs from the shift register are passed into the Byte Merge block to be packed into 32-bit words. Finally the repacked words are inserted into the RX FIFO to be read by firmware.

All of the blocks in the data path use ready-valid handshakes for flow control. In addition, the Byte Select block expects a flush pulse from the shift register to signify when no further data is needed for the current segment, and so any remaining data in the current word can be discarded. Likewise, the Byte Merge block receives a last signal from the shift register to identify the end of a command segment so that any partial words can be passed into the RX FIFO (regardless of whether the last byte forms a complete 32-bit word). The shift register is then responsible for driving and receiving data on the cio_sd lines. It coordinates all of the data flow to and from the Byte Select and Byte Merge blocks.

The SPI_HOST FSM parses the software command segments and orchestrates the proper transmission of data through its control of the shift register. The FSM directly drives the cio_sck and cio_csb signals at the commanded speed. It also controls the shift register: dictating the correct timing for sending out each beat of data, loading bytes from the Byte Select, and sending bytes on to the Byte Merge block.


The RX and TX FIFOs store the transmitted and received data, which are stored in synchronous FIFOs. The RX FIFO is 32 bits wide, matching the width of the TLUL register bus. The TX FIFO on the other hand is 36 bits wide, with 32 bits of SPI data (again to match the TLUL bus width) plus 4 byte enable-bits, which are passed into the core to allow the processing of unaligned writes.

The depth of these FIFOs is controlled by two independent parameters for the RX and TX queues.

Byte Select

The Byte Select unit is responsible for loading words from the FIFO and feeding individual bytes into the shift register. This unit takes two data inputs: a data word, word_i[31:0], and a byte enable signal, word_be_i[3:0]. There is a single output, byte_o[7:0], which feeds the following shift register. There are ready/valid signals for managing flow control on all inputs and outputs. The shift register asserts ready to request new bytes, based on control inputs from the SPI_HOST FSM.

When the SPI_HOST FSM indicates the final byte for a segment, the shift register asserts the flush_i signal with byte_ready_i as it requests the last byte from the Byte Select. This instructs the Byte Select block to send one more byte from current word, and then discard any remaining unused bytes, before immediately loading the next available word from the TX FIFO.

It is assumed that the input data-words and byte enables have already been byte-swapped at the IP top level, as needed. The bytes are transmitted to the shift register in decreasing significance, starting with word_i[31:24], followed by word_i[23:16], word_i[15:8], and finally word_i[7:0].

Some bytes may be skipped however if the corresponding value of word_be_i[3:0] is zero. For example if word_be_i[3:0] equals 4'b0011, then the first two input bytes will be skipped, and only word_i[15:8] and word_i[7:0] will be forwarded, in that order.

The following waveform illustrates the operation of the Byte Select module, highlighting the effect of the flush_i signal (in the first input word), as well as the effect of the byte enable signal (shown in the second word).

Byte Merge

The Byte Merge block is responsible for accumulating bytes from the shift register and packing them into words. Like the Byte Select block, it is based on the prim_packer_fifo primitive.

The Byte Merge block has a data byte input, and a data word output, which are both controlled by their corresponding ready/valid signals. There are no byte-enable outputs for the byte merge, as it is assumed that software can infer the relevant bytes based on the length of the relevant read command segment.

There is byte_last_i signal, to indicate the final byte in a word. If byte_last_i is asserted whenever a byte is loaded, the new byte will be added to the output word, and any remaining bytes will be set to zero, before the word is be loaded into the RX FIFO.

Input bytes are packed into the output word in decreasing significance. The first byte in each segment is loaded into word_o[31:24]. The following bytes are packed into word_o[23:16], word_o[15:8], and then word_o[7:0]. For partially filled words, the zero padding goes into the least significant byte positions.

Any ByteOrder swapping is performed at the other end of the RX FIFO.

Shift Register

The SPI_HOST shift register serially transmits and receives all bytes to the sd_o[3:0] and sd_i[3:0] signals, based on the following timing-control signals from the FSM:

  • speed_i: Controls the speed of the current data segment, ranging from Standard or Dual to Quad
  • wr_en_i: Writes a new byte from the Byte Select into the 8-bit shift register This is usually the first signal issued to the shift register in command segments with data to transmit (i.e., TX only, or bidirectional segments)
    • There is also a wr_ready_o output to tell the FSM that there is no data currently available. If wr_ready_o is deasserted when the FSM asserts wr_en_i, the FSM will stall.
  • last_write_i: When asserted at the same time as wr_en_i, this indicates that the current byte is the last of its command segment, and thus the tx_flush_o signal should be asserted when requesting this byte from the Byte Select block.
  • shift_en_i: Advances the shift register by 1, 2, or 4 bits, depending on the value of speed_i
  • full_cyc_i: Indicates full-cycle operation (i.e., input data are sampled from sd_i whenever new data is shifted out to sd_o)
  • sample_en_i: Samples sd_i[3:0] into a temporary register, sd_i_q[3:0] so it can be loaded into the shift register with the next assertion of shift_en_i Explicit sampling is particularly required for Standard SPI bidirectional segments, where new input data arrives before the first output shift operation. For consistency in timing, the sd_i_q buffer is used in all other modes as well, unless full_cyc_i is asserted. The sample_en_i signal is ignored during full-cycle operation, in which case data is copied directly into the shift register during shift operations.
  • rd_en_i: Indicates that the current byte from the shift register should be transferred on to the Byte Merge block
    • The rd_ready_o output informs the FSM whenever all data storage (the RX FIFO plus any intervening buffers) is full and no further data can be acquired.
  • last_read_i: When asserted at the same time as rd_en_i, this indicates that the current byte is the last of its command segment, and thus the rx_last_o signal should be asserted when passing this byte to the Byte Merge block.

The connection from the shift register to the sd bus depends on the speed of the current segment.

  • In Standard-mode, only the most significant shift register bit, sr_q[7] is connected to the outputs using sd_o[0]. In this mode, each shift_en_i pulse is induces a shift of only one bit.
  • In Dual-mode, the two most significant bits, sr_q[7:6], are connected to sd_o[1:0] and the shift register shifts by two bits with every shift_en_i pulse.
  • In Quad-mode, the four most significant bits, sr_q[7:4], are connected to sd_o[3:0] and the shift register shifts four bits with every pulse.

The connections to the shift register inputs are similar. Depending on the speed, the sd_i inputs are routed to the the 1, 2, or 4 least significant inputs of the shift register. In full-cycle mode, the shift register LSB’s are updated directly from the sd_i inputs. Otherwise the data first passes through an input sampling register, sd_i_q[3:0], which allows the input sampling events to be staggered from the output shift events.

Bubbles in the Data Pipeline

Temporary delays in the transmission or receipt data are a performance issue. Stall events, which temporarily halt operation of the SPI_HOST IP, often indicate that software is not keeping up with data in the TX and RX FIFOs. For this reason the SPI_HOST IP can create interrupts to help monitor the frequency of these stall events, in order to identify correctable performance delays.

There is also the possibility of encountering bubble events, which cause transient stalls in the data pipeline. Transient stalls only occur for Quad-mode segments, and only when transmitting or receiving words with only one valid byte.

When transmitting at full clock speed, Quad-mode segments need to process one byte every four clock cycles. If a particular Quad TX segment pulls only one byte from a particular data word (for reasons related either to the segment length or odd data alignment), the prim_packer_fifo used in the Byte Select block can generate delays of up to four clocks before releasing the next byte. This can cause temporary stall conditions either during the Quad segment, or–if there is another TX segment immediately following–just before the following segment.

Similar delays exist when receiving Quad-mode data, which are similarly worst when packing words with just one byte (i.e., when receiving segments of length 4n+1). The RX pipeline is however much more robust to such delays, thanks to buffering in the shift register outputs. There is some sensitivity to repeated 4 clock delays, but it takes at least six of them to cause a temporary stall. So transient RX stalls only occur when receiving more than six consecutive one-byte segments. As this is an unlikely use case, transient stalls are considered an unlikely occurrence in the RX path.

Dual- and Standard-mode segments can tolerate byte-to-byte delays of 7 or 15 clocks, so there are no known mechanism for transient stalls at these speeds.

Please refer to the the Appendix for a detailed analysis of transient stall events.

SPI_HOST Finite State Machine (FSM)

The SPI_HOST FSM is responsible for parsing the input command segments and configuration settings, which it uses to control the timing of the sck and csb signals. It also controls the timing of shift register operations, coordinating I/O on the sd bus with the other SPI signals.

This section describes the SPI_HOST FSM and its control of the sck and csb lines as well as its interactions with the Shift Register and the Command FIFO.

Clock Divider

The SPI_HOST FSM is driven by the rising edge of the input clock, however the FSM state registers are not enabled during every cycle. There is an internal clock counter clk_cntr_q which repeatedly counts down from CONFIGOPTS.CLKDIV to 0, and the FSM is only enabled when clk_cntr_q == 0.

The exception is when the FSM is one of the two possible Idle states (Idle or IdleCSBActive), in which case clk_cntr_q is constantly held at zero, making it possible to immediately transition out of the idle state as soon as a new command appears. Once the FSM transitions out of the idle state, clk_cntr_q resets to CONFIGOPTS.CLKDIV, and FSM transitions are only enabled at the divided clock rate.

As shown in the waveform below, this has the effect of limiting the FSM transitions to only occur at discrete timeslices of duration:

$$T_\textrm{timeslice} = \frac{T_{\textrm{clk},\textrm{clk}}}{\texttt{clkdiv}+1}.$$

Other Internal Counters

In addition to the FSM state register, the SPI_HOST FSM block also has a number of internal registers to track the progress of a given command segment.

  • wait_cntr_q: This counter is used the hold the FSM in a particular state for several timeslices, in order to implement the CSNIDLE, CSNLEAD or CSNTRAIL delays required for a particular device.

  • byte_cntr_q, bit_cntr_q: These counters respectively track the number of bytes left to transmit for the current segment and the number of bits left to transmit in the current byte.

  • Finally, there are registers corresponding to each configuration field (csid_q, cpol_q, cpha_, csnidle_q, csnlead_q, csntrail_q, and full_cyc_q) and to each command segment field (csaat, cmd_rd_en, cmd_wr_en, and cmd_speed). This registers are sampled whenever a new command comes in, allowing the command inputs to change.

Basic Operation

The state machine itself is easiest understood by first considering a simple case, with CSAAT set to zero. For this initial discussion it is assumed that every command consists of one single segment. Multi-segment commands are considered in the following sections. In this case the state machine can be simplified to the following figure.

The operation of the state machine is the same regardless of the polarity (CPOL) or phase (CPHA) of the current command. Commands with CPOL==0 or CPOL==1 are processed nearly identically, since the only difference is in the polarity of the sck output. The state machine drives an internal sck clock signal, which is low except when the FSM is in the InternalClockHigh state. If CPOL==0 this clock is registered as is to the external sck ports. If CPOL==1 the internal clock is inverted before the final sck output register.

In the following description of the individual states, it is assumed that CPOL==0. To understand the IP’s behavior for transactions with CPOL==1, simply invert the value of sck.

  1. Idle state: In this initial reset state, The sck signal is low, and all csb lines are high (i.e., inactive). An input command is registered whenever command_valid_i and command_ready_o are both high (i.e., when the signal new_command = command_valid_i & command_ready_o is high), in which case the state machine transitions to the WaitLead state.

  2. WaitLead state: In this state, sck remains low, and the csb line corresponding to csid is asserted-low. The purpose of this state is to hold sck low for at least csnlead + 1 timeslices, before the first rising edge of sck. For his reason, the FSM uses the wait_cntr to track the number of timeslices spent in this state, and only exits when wait_cntr counts down to zero, at which point the FSM transitions to the InternalClkHigh state.

  3. InternalClkHigh state: Entering this state drives sck high. This state repeats many times per segment, and usually transitions to the InternalClkLow state. The FSM transitions to the WaitTrail state only when the entire segment has been transmitted/received (as indicated by the signals last_bit and last_byte). This state machine usually only lasts stays in this state for one timeslice, except when the FSM is disabled or stalled.

  4. InternalClkLow state: This state serves to drive sck low between visits to the InternalClkHigh state. This state always returns back to the InternalClkHigh state in the next timeslice.

  5. WaitTrail state: Similar to the WaitLead, this state serves to control the timing of the csb line. The FSM uses the wait_cntr register to ensure that it remains in this state for csntrail+1 timeslices, during which time the active csb is still held low. The wait_cntr register resets to CONFIGOPTS.CSNTRAIL upon entering this state, and is decremented once per timeslice. This state transitions to WaitIdle when wait_cntr is zero.

  6. WaitIdle state: In this timing control state, the FSM uses the wait_cntr register to ensure that all csb lines are held high for at least csnidle+1 timeslices. The wait_cntr register resets to CONFIGOPTS.CSNIDLE upon entering this state, and is decremented once per timeslice. This state transitions to Idle when wait_cntr reaches zero.

Milestone Signals, Serial Data Lines & Shift Register Control

The FSM manages I/O on the sd bus by controlling the timing of the shift register control signals: shift_en_o, sample_en_o, rd_en_o, last_read_o, wr_en_o, and last_write_o.

The shift register control signals are managed through the use of three intermediate signals:

  • byte_starting: This signal indicates the start of a new byte on the sd bus in the following clock cycle. For Bidirectional or TX segments this signal would indicate that it is time to load a new byte into the shift register. This signal corresponds to the FSM’s wr_en_o port, though that output is suppressed during RX or dummy segments.
  • byte_ending: This signal indicates the end of the current sd byte in the current clock cycle (i.e., the next clock cycle either marks the beginning new byte or the end of the current segment). As illustrated in the following waveform, the byte_starting and byte_ending signals are often asserted at the same time, though there is an extra byte_starting pulse at the beginning of each command and an extra byte_ending pulse at the end. For RX and bidirectional command segments, a byte_ending pulse generates a rd_en_o pulse to the shift register, which transfers the 8-bit contents of the shift register into the RX FIFO via the Byte Merge block.
  • bit_shifting: This signal drives the shift_en_o control line to the shift register, ejecting the most-significant bits, and updating the sd outputs.

These milestone signals mark the progress of each command segment.

The coordination of the milestone signals and the shift register controls are shown in the following waveform. Since the milestone signal pulses coincide with entering particular FSM states, they are derived from the state register inputs (i.e., state_d), as opposed to the state register outputs (state_q).

When working from a CPHA=0 configuration, the milestone signals are directly controlled by transitions in the FSM state register, as described in the following table.

Milestone SignalFSM Triggers
byte_startingEntering WaitLead
Entering InternalClkLow and bit_cntr == 0
bit_shiftingEntering InternalClkLow and bit_cntr != 0
byte_endingExiting InternalClkHigh and bit_cntr == 0

When working from a CPHA=1 configuration, the milestone signals exploit the fact that there is usually a unique correspondence between csb/sck events and FSM transitions. There are some exceptions to this pattern since, as discussed below, CSAAT- and multi-csb-support requires the creation of multiple flavors of idle states. However, there are no milestone signal pulses in any of the transitions between these various idle states. Thus in CPHA=1 mode, the milestone signals are delayed by one-state transition. For example, in a CPHA=0 configuration the first data burst should be transmitted as the csb line is asserted low, that is, when the FSM enters the WaitLead state. Thus a byte_starting pulse is generated at this transition. On the other hand, in CPHA=1 configuration the first data burst should be transmitted after the first edge of sck, which happens on the next state transition as illustrated in the following waveform.

That said, there are two copies of each milestone signal:

  • the original FSM-driven copy, for use when operating with CPHA=0, and
  • a delayed copy, for use in CPHA=1 operation.

Milestone Signals and Control of the the Bit and Byte Counters

The previous waveform also highlights the relationship between the milestone signals and the bit and byte counters. At the beginning of each byte bit_cntr_q is reset to a speed-specific value, to trigger the correct number of shift operations required for each byte:

  • 7 for Standard-mode
  • 6 for Dual-mode
  • 4 for Quad-mode

The reset of the bit_cntr_q counter is triggered by the byte_starting register. Meanwhile the bit_shifting signal triggers a decrement of the bit-shifting register. The size of the decrement also depends on the speed of the current segment:

  • 1 for Standard-mode
  • 2 for Dual-mode
  • 4 for Quad-mode

The byte_cntr_q register is updated from the COMMAND.LEN register value, at the beginning of each segment, and decremented after each byte_ending pulse until the counter reaches zero.

This relationship between the milestone signals and the bit and byte counters is also illustrated in the previous waveform.

Implementation of Configuration Change Delays

As described in the Theory of Operation, changes in configuration only occur when the SPI_HOST is idle. The configuration change must be preceded by enough idle time to satisfy the previous configuration, and followed by enough idle time to satisfy the new configuration.

In order to support these idle time requirements, the SPI_HOST FSM has two idle waiting states.

  • The WaitIdle state manages the idle time requirements of the preceding command segment, and usually transitions to the Idle state afterwards.
  • From the Idle state the FSM monitors for changes in configuration, and transitions to the ConfigSwitch state if any changes are detected in the next incoming command segment. This state introduces delays long enough the satisfy the idle time requirements of following command segment. From the ConfigSwitch state, the state machine directly enters the WaitLead state to start the next command segment.

A complete state diagram, including the ConfigSwitch state, is shown in the following section.

The following waveform illustrates how a change in a single CONFIGOPTS, here CONFIGOPTS.CPOL, triggers an entry into the ConfigSwitch Idle state, and how the new configuration is applied at the transition from WaitIdle to ConfigSwitch thereby ensuring ample idle time both before and after the configuration update.

CSAAT Support

In addition to omitting the ConfigSwitch state, the simplified state machine illustrated above does not take into account commands with multiple segments, where the CSAAT bit is enabled for all but the last segment.

When the CSAAT bit in enabled there is no idle period between the current segment and the next, nor are the two adjoining segments separated by a Trail or Lead period. Usually the end of each segment is detected in the InternalClkHigh state, at which point, if CSAAT is disabled, the FSM transitions to the WaitTrail state to close out the transaction. However, if CSAAT is enabled the WaitTrail state is skipped, and the next state depends on whether there is another command segment available for processing (i.e., both command_ready_o and command_valid_i are both asserted).

In order to support seamless, back-to-back segments the ConfigSwitch state can be skipped if a new segment is already available when the previous ends, in which case the FSM transitions directly to the InternalClkLow at the end of the previous segment.

If there is no segment available yet, the FSM must pause and idly wait for the next command in the special IdleCSBActive state. This state serves a similar purpose to the Idle state since in this state the IP is doing nothing but waiting for new commands. It is different from the Idle state though in that during this state the active csb is held low. When a command segment is received in the IdleCSBActive state, it transitions immediately to the InternalClkLow state to generate the next sck pulse and process the next segment.

The following figure shows the complete state transition diagram of for the SPI_HOST FSM.

Skipped idle states

The Idle and IdleCSBActive states are unique from the others in that:

  1. In order to respond to an incoming command the FSM can exit these idle states at any time, regardless of the current timeslice definition. (In fact, since different commands may use different values for the CLKDIV configuration parameter, the concept of a timeslice is poorly defined when idle).
  2. These idle states may be bypassed in order to support more efficient transitions from one command segment to the next. If an incoming command is detected as the FSM is about to enter an idle state, that idle state is skipped, and the FSM immediately transitions to the next logical state, based on the contents of the new incoming command.

These bypassable states, which are highlighted in the previous diagram, represent a number of possible transitions from one pre-idle state to a following post-idle state. For clarity such transitions are left implicit in the diagram above. However they could also be explicitly added to the state diagram. For example, the implicit transitions around the Idle are shown in the following figure.


Whenever the shift register needs to transfer data in (or out) of the RX (TX) FIFOs, but they are full (or empty), the FSM immediately stalls to wait for new data.

During this stall period none of the FSM internal registers are updated. Normal operation proceeds only when the stall condition has been resolved or the SPI_HOST has been reset.

In the SPI_HOST FSM this is realized by disabling all flop updates whenever a stall is detected.

Furthermore, all control signals out of the FSM are suppressed during a stall condition.

From an implementation standpoint, the presence of a stall condition has two effects on the SPI_HOST FSM:

  1. No flops or registers may be updated during a stall condition. Thus the FSM may not progress while stalled.

  2. All handshaking or control signals to other blocks must be suppressed during a stall condition, placing backpressure on the rest the blocks within the IP to also stop operations until the stall is resolved.

Programmer’s Guide

The operation of the SPI_HOST IP proceeds in seven general steps.

To initialize the IP:

  1. Program the CONFIGOPTS multi-register with the appropriate timing and polarity settings for each csb line.
  2. Set the desired interrupt parameters
  3. Enable the IP

Then for each command:

  1. Load the data to be transmitted into the FIFO using the TXDATA memory window.
  2. Specify the target device by programming the CSID
  3. Specify the structure of the command by writing each segment into the COMMAND register
    • For multi-segment transactions, be sure to assert COMMAND.CSAAT for all but the last command segment
  4. For transactions which expect to receive a reply, the data can then be read back from the RXDATA window.

These latter four steps are then repeated for each command. Each step is covered in detail in the following sections.

For concreteness, this Programmer’s Guide uses examples from one of our primary target devices, the W25Q01JV flash from Winbond. The SPI_HOST IP is however suitable for interacting with any number of SPI devices, and the same mode of operation can be used for any SPI device.

Initializing the IP

Per-target Configuration

The CONFIGOPTS multi-register must be programmed to reflect the requirements of the attached target devices. As such these registers can be programmed once at initialization, or whenever a new device is connected (e.g., via changes in the external pin connections, or changes in the pinmux configuration). The proper settings for the CONFIGOPTS fields (e.g., CPOL and CPHA, clock divider, ratios, and other timing or sampling requirements) will all depend on the specific device attached as well as the board level delays.

Interrupt configuration

The next step is to configuration the interrupts for the SPI_HOST. This should also be done at initialization using the following register fields:

  • The ERROR_ENABLE register should be configured to indicate what types of error conditions (if any) should be ignored to not trigger an interrupt. At reset, these fields are all set indicating that all error classes trigger an interrupt.

  • For interrupt driven I/O the EVENT_ENABLE register must be configured to select the desired event interrupts to signal the desired conditions (e.g. “FIFO empty”, “FIFO at the watermark level”, or “ready for next command segment”). By default, this register is all zeros, meaning all event interrupts are disabled, and thus all transactions must be managed by polling the status register.

  • The event and error interrupts must finally be enabled using the INTR_ENABLE register.

Enabling the SPI_HOST

The IP must be enabled before sending the first command by asserting the CONTROL.SPIEN bit.

Issuing Transactions

As mentioned above, each command is typically specified in three phases: loading the TX data, specifying the command segments/format, and reading the RX data. In principle, the first two steps can be performed in either order. If the SPI_HOST does not find any data to transmit it will simply stall until data is inserted. Meanwhile, the RX data is only available after the command format has been specified and processed by the state machine.

For longer transactions, with data larger than the capacity of the FIFOs, the command sequence may become more complex. For instance, to send 1024 bytes of data in a single transaction, the TX data may need to be loaded several times if using a 256-byte FIFO. In this instance, the programming sequence will consist of at least four iterations of entering TX data and waiting for the TX FIFO to drain.

Loading TX data

SPI transactions expect each command to start with some command sequence from the host, and so usually data will be transmitted at least in the first command segment. The TXDATA window provides a simple interface to the TX FIFO. Data can be written to the window using 8-, 16- or 32-bit instructions.

Some attention, however, should be paid to byte-ordering and segmenting conventions.

Byte-ordering Conventions

For SPI flash applications, it is generally assumed that most of the payload data will be directly copied from embedded SRAM to the flash device.

If this data is to copied to the TXDATA window using 32-bit instructions, the SPI_HOST should be parameterized such that the ByteOrder parameter matches the byte order of the embedded CPU (i.e., for Ibex, ByteOrder should be left set to 1 to indicate a Little-Endian CPU). This will ensure that data is transmitted to the flash (and thus also stored in flash) in address-ascending order. For example, consider the transfer of four bytes, D[3:0][7:0], to SPI via the TXDATA window.

  • It is assumed for this example that all four bytes are contiguously stored in SRAM at a word-aligned address, with D[0] at the lowest byte-address.
  • When these bytes are loaded into the Ibex CPU they are arranged as the 32-bit word: W[31:0] = {D[3][7:0], D[2][7:0], D[1][7:0], D[0][7:0]}.
  • After this word are loaded into the TXDATA window, the LSB (i.e., W[7:0] = D[0][7:0]) is transmitted first, by virtue of the ByteOrder == 1 configuration.

In this way, configuring ByteOrder to match the CPU ensures that data is transmitted in memory-address order.

The value of the ByteOrder parameter can be confirmed by firmware by reading the STATUS.BYTEORDER register field.

Not all data to the SPI device will come from memory however. In many cases the transaction command codes or headers will be constructed or packed on the fly in CPU registers. The order these register bytes are transmitted on the bus will depend on the value of the ByteOrder parameter, as discussed in the Theory of Operation section, and for multi-bit values, such as flash addresses), some byte-swapping may be required to ensure that data is transmitted in the proper order expected by the target device.

For example, SPI flash devices generally expect flash addresses (or any other multi-byte values) to be transmitted MSB-first. This is illustrated in the following figure, which depicts a Fast Quad Read I/O command. Assuming that ByteOrder is set to 1 for Little-Endian devices such as Ibex, byte-swapping will be required for these addresses, otherwise the device will receive the addresses LSB first.

Byte ordering on the bus can also be managed by writing TXDATA as a sequence of discrete bytes using 8-bit transactions, since partially-filled data-words are always sent in the order they are received.

A few examples related to using SPI flash devices on a Little-Endian platform:

  • A 4-byte address can be loaded into the TX FIFO as four individual bytes using 8-bit I/O instructions.
  • The above read command (with 4-byte address) can be loaded into the FIFO by first loading the command code into TXDATA as a single byte, and the address can be loaded into TXDATA using 32-bit instructions, provided the byte order is swapped before loading.
  • Flash transactions with 3-byte addressing require some care, as there are no 24-bit I/O instructions, though there are a several options:
    • After the 8-bit command code is sent, the address can either be sent in several I/O operations (e.g., the MSB is sent as an 8-bit command, and the remaining 16-bits can be sent after swapping)
    • If bandwidth efficiency is a priority, the address, A[23:0], and command code, C[7:0], can all be packed together into a single 4-byte quantity W[31:0] = {A[7:0], A[15:8], A[23:16], C[7:0]}, which when loaded into TXDATA will ensure that the command code is sent first, followed by the address in MSB-first order.

Segmenting Considerations

Data words are not shared across segments. If at the end of each TX (or bidirectional) segment there is a partially transmitted data word then any unsent bytes will be discarded as the SPI_HOST IP closes the segment. For the next TX segment, the transmitted data will start with the following word from the TX FIFO.

Refilling the TX FIFO

For extremely long transactions, the TX FIFO may not have enough capacity to hold all the data being transmitted. In this case software can either poll the STATUS.TXQD register to determine the number of elements in the TX FIFO, or enable the SPI_HOST IP to send an interrupt when the FIFO drains to a certain level. If INTR_ENABLE.spi_event and EVENT_ENABLE.TXWM are both asserted, the IP will send an interrupt whenever the number of elements in the TX FIFO falls below CONTROL.TX_WATERMARK.

Specifying the Segments

Each write to the COMMAND register corresponds to a single command segment. The length, CSAAT flag, direction and speed settings for that segment should all be packed into a single 32-bit register and written simultaneously to COMMAND.

The COMMAND should only be written when STATUS.READY is asserted.

While each command segment is being processed, the SPI_HOST has room to queue up exactly one additional segment descriptor in the Command Clock Domain Crossing. Once a second command segment descriptor has been submitted, software must wait for the state machine to finish processing the current segment before submitting more. Software can poll the STATUS.READY field to determine when it is safe to insert another segment descriptor. Otherwise the EVENT_ENABLE.IDLE bit can be enabled (along with INTR_ENABLE.spi_event) to trigger an event interrupt whenever STATUS.READY is asserted.

Reading Back the Device Response

Once an RX segment descriptor has been submitted to the SPI_HOST, the received data will be available in the RX FIFO after the first word has been received.

The number of words in the FIFO can be polled by reading the STATUS.RXQD field. The SPI_HOST IP can also configured to generate watermark event interrupts whenever the number of words received reaches (or exceeds) CONTROL.RX_WATERMARK. To enable interrupts when ever the RX FIFO reaches the watermark, assert EVENT_ENABLE.RXWM along with INTR_ENABLE.spi_event.

Exception Handling

The SPI_HOST will assert one of the ERROR_STATUS bits in the event of a firmware programming error, and will become unresponsive until firmware acknowledges the error by clearing the corresponding error bit.

The SPI_HOST interrupt handler should clear any bits in ERROR_STATUS bit before clearing INTR_STATE.error.

In addition to clearing the ERROR_STATUS register, firmware can also trigger a complete software reset via the CONTROL.SW_RST bit, as described in the next section.

Other system-level errors may arise due to improper programming of the target device (e.g., due to violations in the device programming model, or improper configuration of the SPI_HOST timing registers). Given that the SPI protocol provides no mechanism for the target device to stall the bus, the SPI_HOST will continue to function even if the remote device becomes unresponsive. In case of an unresponsive device, the RX FIFO will still accumulate data from the bus during RX segments, though the data values will be undefined.

Software Reset Procedure

In the event of an error the SPI_HOST IP can be reset under software control using the following procedure:

  2. Poll IP status registers for confirmation of successful state machine reset:
  3. Clear {{ < regref “CONTROL.SW_RST” >}}.

Device Interface Functions (DIFs)

To use this DIF, include the following C header:

#include "sw/device/lib/dif/dif_spi_host.h"

This header provides the following device interface functions:

Register Table

Name Offset Length Description
spi_host.INTR_STATE 0x0 4

Interrupt State Register

spi_host.INTR_ENABLE 0x4 4

Interrupt Enable Register

spi_host.INTR_TEST 0x8 4

Interrupt Test Register

spi_host.ALERT_TEST 0xc 4

Alert Test Register

spi_host.CONTROL 0x10 4

Control register

spi_host.STATUS 0x14 4

Status register

spi_host.CONFIGOPTS 0x18 4

Configuration options register.

spi_host.CSID 0x1c 4

Chip-Select ID

spi_host.COMMAND 0x20 4

Command Register

spi_host.RXDATA 0x24 4

SPI Receive Data.

spi_host.TXDATA 0x28 4

SPI Transmit Data.

spi_host.ERROR_ENABLE 0x2c 4

Controls which classes of errors raise an interrupt.

spi_host.ERROR_STATUS 0x30 4

Indicates that any errors that have occurred. When an error occurs, the corresponding bit must be cleared here before issuing any further commands.

spi_host.EVENT_ENABLE 0x34 4

Controls which classes of SPI events raise an interrupt.

spi_host.INTR_STATE @ 0x0

Interrupt State Register

Reset default = 0x0, mask 0x3
  spi_event error

Error-related interrupts, see ERROR_ENABLE register for more information.


Event-related interrupts, see EVENT_ENABLE register for more information.

spi_host.INTR_ENABLE @ 0x4

Interrupt Enable Register

Reset default = 0x0, mask 0x3
  spi_event error

Enable interrupt when INTR_STATE.error is set.


Enable interrupt when INTR_STATE.spi_event is set.

spi_host.INTR_TEST @ 0x8

Interrupt Test Register

Reset default = 0x0, mask 0x3
  spi_event error

Write 1 to force INTR_STATE.error to 1.


Write 1 to force INTR_STATE.spi_event to 1.

spi_host.ALERT_TEST @ 0xc

Alert Test Register

Reset default = 0x0, mask 0x1

Write 1 to trigger one alert event of this kind.

spi_host.CONTROL @ 0x10

Control register

Reset default = 0x7f, mask 0xe000ffff

If EVENT_ENABLE.RXWM is set, the IP will send an interrupt when the depth of the RX FIFO reaches RX_WATERMARK words (32b each).


If EVENT_ENABLE.TXWM is set, the IP will send an interrupt when the depth of the TX FIFO drops below TX_WATERMARK words (32b each).


Enable the SPI host output buffers for the sck, csb, and sd lines. This allows the SPI_HOST IP to connect to the same bus as other SPI controllers without interference.


Clears the entire IP to the reset state when set to 1, including the FIFOs, the CDC's, the core state machine and the shift register. In the current implementation, the CDC FIFOs are drained not reset. Therefore software must confirm that both FIFO's empty before releasing the IP from reset.


Enables the SPI host. On reset, this field is 0, meaning that no transactions can proceed.

spi_host.STATUS @ 0x14

Status register

Reset default = 0x0, mask 0xffdfffff

Transmit queue depth. Indicates how many unsent 32-bit words are currently in the TX FIFO. When active, this result may be an overestimate due to synchronization delays,


Receive queue depth. Indicates how many unread 32-bit words are currently in the RX FIFO. When active, this result may an underestimate due to synchronization delays.


Command queue depth. Indicates how many unread 32-bit words are currently in the command segment queue.


If high, the number of 32-bits in the RX FIFO now exceeds the CONTROL.RX_WATERMARK entries (32b each).


The value of the ByteOrder parameter, provided so that firmware can confirm proper IP configuration.


If high, signifies that an ongoing transaction has stalled due to lack of available space in the RX FIFO


When high, indicates that the receive fifo is empty. Any reads from RX FIFO will cause an error interrupt.


When high, indicates that the receive fifo is full. Any ongoing transactions will stall until firmware reads some data from RXDATA.


If high, the amount of data in the TX FIFO has fallen below the level of CONTROL.TX_WATERMARK words (32b each).


If high, signifies that an ongoing transaction has stalled due to lack of data in the TX FIFO


When high, indicates that the transmit data fifo is empty.


When high, indicates that the transmit data fifo is full. Any further writes to RXDATA will create an error interrupt.


When high, indicates the SPI host is processing a previously issued command.


When high, indicates the SPI host is ready to receive commands. Writing to COMMAND when READY is low is an error, and will trigger an interrupt.

spi_host.CONFIGOPTS @ 0x18

Configuration options register.

Reset default = 0x0, mask 0xefffffff

Contains options for controlling each peripheral. One register per cs_n line


Core clock divider. Slows down subsequent SPI transactions by a factor of (CLKDIV+1) relative to the core clock frequency. The period of sck, T(sck) then becomes 2(CLK_DIV+1)T(core)


Minimum idle time between commands. Indicates the minimum number of sck half-cycles to hold cs_n high between commands. Setting this register to zero creates a minimally-wide CS_N-high pulse of one-half sck cycle.


CS_N Trailing Time. Indicates the number of half sck cycles, CSNTRAIL+1, to leave between last edge of sck and the rising edge of cs_n. Setting this register to zero corresponds to the minimum delay of one-half sck cycle.


CS_N Leading Time. Indicates the number of half sck cycles, CSNLEAD+1, to leave between the falling edge of cs_n and the first edge of sck. Setting this register to zero corresponds to the minimum delay of one-half sck cycle


Full cycle. Modifies the CPHA sampling behaviour to allow for longer device logic setup times. Rather than sampling the SD bus a half cycle after shifting out data, the data is sampled a full cycle after shifting data out. This means that if CPHA = 0, data is shifted out on the trailing edge, and sampled a full cycle later. If CPHA = 1, data is shifted and sampled with the trailing edge, also separated by a full cycle.


The phase of the sck clock signal relative to the data. When CPHA = 0, the data changes on the trailing edge of sck and is typically sampled on the leading edge. Conversely if CPHA = 1 high, data lines change on the leading edge of sck and are typically sampled on the trailing edge. CPHA should be chosen to match the phase of the selected device. The sampling behavior is modified by the CONFIGOPTS.FULLCYC bit.


The polarity of the sck clock signal. When CPOL is 0, sck is low when idle, and emits high pulses. When CPOL is low, sck is high when idle, and emits a series of low pulses.

spi_host.CSID @ 0x1c

Chip-Select ID

Reset default = 0x0, mask 0xffffffff

Controls which device to target with the next command. This register is passed to the core whenever COMMAND is written. The core then asserts cio_csb_o[CSID] during the execution of the command.


Chip Select ID

spi_host.COMMAND @ 0x20

Command Register

Reset default = 0x0, mask 0x3fff

Parameters specific to each command segment. Unlike the CONFIGOPTS multi-register, there is only one command register for controlling all attached SPI devices


Segment Length.

For read or write segments, this field controls the number of 1-byte bursts to transmit and or receive in this command segment. The number of cyles required to send or received a byte will depend on COMMAND.SPEED. For dummy segments, (COMMAND.DIRECTION == 0), this register controls the number of dummy cycles to issue. The number of bytes (or dummy cycles) in the segment will be equal to COMMAND.LEN + 1.


Chip select active after transaction. If CSAAT = 0, the chip select line is raised immediately at the end of the command segment. If COMMAND.CSAAT = 1, the chip select line is left low at the end of the current transaction segment. This allows the creation longer, more complete SPI transactions, consisting of several separate segments for issuing instructions, pausing for dummy cycles, and transmitting or receiving data from the device.


The speed for this command segment: "0" = Standard SPI. "1" = Dual SPI. "2"=Quad SPI, "3": RESERVED.


The direction for the following command: "0" = Dummy cycles (no TX/RX). "1" = Rx only, "2" = Tx only, "3" = Bidirectional Tx/Rx (Standard SPI mode only).

spi_host.RXDATA @ + 0x24
1 item ro window
Byte writes are not supported

SPI Receive Data.

Reads from this window pull data from the RXFIFO.

The serial order of bit transmission is chosen to match SPI flash devices. Individual bytes are always transmitted with the most significant bit first. Only four-bute reads are supported. If ByteOrder = 0, the first byte received is packed in the MSB of RXDATA. For some processor architectures, this could lead to shuffling of flash data as compared to how it is written in memory. In which case, choosing ByteOrder = 1 can reverse the byte-order of each data read, causing the first byte received to be packed into the LSB of RXDATA. (Though within each byte the most significant bit is always pulled from the bus first.)

spi_host.TXDATA @ + 0x28
1 item wo window
Byte writes are supported

SPI Transmit Data.

Data written to this window is placed into the TXFIFO. Byte-enables are supported for writes.

The serial order of bit transmission is chosen to match SPI flash devices. Individual bytes are always transmitted with the most significant bit first. Multi-byte writes are also supported, and if ByteOrder = 0, the bits of TXDATA are transmitted strictly in order of decreasing signficance (i.e. most signicant bit first). For some processor architectures, this could lead to shuffling of flash data as compared to how it is written in memory. In which case, choosing ByteOrder = 1 can reverse the byte-order of multi-byte data writes. (Though within each byte the most significant bit is always sent first.)

spi_host.ERROR_ENABLE @ 0x2c

Controls which classes of errors raise an interrupt.

Reset default = 0x1f, mask 0x1f

Command Error: If this bit is set, the block sends an error interrupt whenever a command is issued while busy (i.e. a 1 is when STATUS.READY is not asserted.)


Overflow Errors: If this bit is set, the block sends an error interrupt whenever the TX FIFO overflows.


Underflow Errors: If this bit is set, the block sends an error interrupt whenever there is a read from RXDATA but the RX FIFO is empty.


Invalid Command Errors: If this bit is set, the block sends an error interrupt whenever a command is sent with invalid values for COMMAND.SPEED or COMMAND.DIRECTION.


Invalid CSID: If this bit is set, the block sends an error interrupt whenever a command is submitted, but CSID exceeds NumCS.

spi_host.ERROR_STATUS @ 0x30

Indicates that any errors that have occurred. When an error occurs, the corresponding bit must be cleared here before issuing any further commands.

Reset default = 0x0, mask 0x3f

Indicates a write to COMMAND when STATUS.READY = 0.


Indicates that firmware has overflowed the TX FIFO


Indicates that firmware has attempted to read from RXDATA when the RX FIFO is empty.


Indicates an invalid command segment, meaning either an invalid value of COMMAND.SPEED or a request for bidirectional data transfer at dual or quad speed


Indicates a command was attempted with an invalid value for CSID.


Indicates that TLUL attempted to write to TXDATA with no bytes enabled. Such 'zero byte' writes are not supported.

spi_host.EVENT_ENABLE @ 0x34

Controls which classes of SPI events raise an interrupt.

Reset default = 0x0, mask 0x3f

Assert to send a spi_event interrupt whenever STATUS.RXFULL goes high


Assert to send a spi_event interrupt whenever STATUS.TXEMPTY goes high


Assert to send a spi_event interrupt whenever the number of 32-bit words in the RX FIFO is greater than CONTROL.RX_WATERMARK. To prevent the reassertion of this interrupt, read more data from the RX FIFO, or increase CONTROL.RX_WATERMARK.


Assert to send a spi_event interrupt whenever the number of 32-bit words in the TX FIFO is less than CONTROL.TX_WATERMARK. To prevent the reassertion of this interrupt add more data to the TX FIFO, or reduce CONTROL.TX_WATERMARK.


Assert to send a spi_event interrupt whenever STATUS.READY goes high


Assert to send a spi_event interrupt whenever STATUS.ACTIVE goes low


Analysis of Transient Datapath Stalls

Even if the RX (or TX) FIFOs have free-space (or data) available, stall events can still happen due to momentary backlogs or bubbles in the data pipeline. For instance, the Byte Merge and Byte Select blocks occasionally need some extra cycles to clean out the internal prim_packer_fifo. These delays are likely to cause transient stalls particularly, when constructing transactions with many short (less than 4-byte) segments. Transient stalls could lead to false diagnostics when trying to optimize SPI_HOST throughput. Thus it is useful to analyze the shift register’s tolerance to bubble events, particularly for the highest bandwidth Quad SPI mode.

Transient Stalls in TX direction.

The transient analysis stall analysis is simpler for the TX direction. There is no buffering on the shift register TX data inputs because it would complicate the byte_flush operation on the Byte Select block.

In Quad mode,the shift register will demand one new byte as often as once every four clock cycles. (This rate is slowed down if for a non-trivial clock-divide ratio). Meanwhile, the Byte Select Block pauses once for every disabled byte, and once more at the end of each word. Thus if the Byte Select block is loaded with three-consecutive bytes-disables (either in the same word or across two separate words), this will create a pause of 4-clock cycles between two bytes causing a transient stall event.

Assuming that each TX Word has at least one byte enabled, the longest possible transient delay between two Byte Select outputs is 7 clock cycles (with three byte-disables in two adjacent words, respectively aligned for maximal delay and assuming no delays in the TX FIFOs themselves). Dual- and Standard-mode segments can tolerate inter-byte delays of 7 or 15 clocks respectively, and thus transient stalls should not be a problem after Dual- or Standard-mode segments.

Transient Stalls in the RX direction

Similar to the Byte Select, the Byte Merge block must pause for at least one cycle between each word. Also when at the end of a segment the Byte Merge packs less than four bytes into the last word, there is also an additional cycle of delay for each unused byte. Thus if the last word in a given segment has only one valid byte, the total delay will be four clock cycles.

Such stalls however are a much smaller concern in the RX direction due to the buffering of the Shift Register outputs. As shown in the following waveform, even in Quad-mode, this buffer means the shift register can tolerate as many as six clock cycles of temporary back-pressure before creating a stall.

Even though such long delays are tolerable, it takes some time for shift register to catch up completely and clear the backlog. For example, if after a 6-clock delay the shift-register encounters another 4-clock backlog this can also introduce a stall condition, as shown in the waveform below.

Delays of 3-clocks or less do not create any internal backlog in the system. However, the Byte Merge block can create a 4-clock delay each time it processes a single-byte segment. In practice, this is unlikely to cause a problem, as no Quad-SPI Flash transactions require even two back-to-back RX segments. However with enough (at least six) consecutive one-byte segments, the accumulated delay can eventually create a stall event on the RX path as well, as seen below.