Pinmux Technical Specification

Overview

This document specifies the functionality of the pin multiplexer (pinmux) peripheral. This module conforms to the OpenTitan guideline for peripheral device functionality. See that document for integration overview within the broader OpenTitan top level system. The module provides a mechanism to reconfigure the peripheral-to-pin mapping at runtime, which greatly enhances the system flexibility. In addition to that, the pinmux also allows the user to control pad attributes (such as pull-up, pull-down, open-drain, drive-strength, keeper and inversion), and it contains features that facilitate low-power modes of the system. For example, the sleep behavior of each pad can be programmed individually, and the module contains additional pattern detectors that can listen on any IO and wake up the system if a specific pattern has been detected.

Features

• Configurable number of chip bidirectional IOs

• Configurable number of peripheral inputs and outputs

• Programmable mapping from peripheral outputs (and output enables) to top-level outputs (and output enables)

• Programmable mapping from top-level inputs to peripheral inputs

• Programmable control of chip pad attributes like output drive-strength, pull-up, pull-down and virtual open-drain

• Programmable pattern detectors to detect wakeup conditions during sleep mode

• Programmable sleep mode behavior

• Support for life-cycle-based JTAG (TAP) isolation and muxing

Theory of Operations

Block Diagram and Overview

The pinmux peripheral is a programmable module designed to wire arbitrary peripheral inputs and outputs to arbitrary multiplexable chip bidirectional pins. It gives much flexibility at the top level of the device, allowing most data pins to be flexibly wired and controlled by many peripherals. Even though the pinmux is referred to as one IP, it is logically split into two modules that are instantiated on the top-level and the chip-level, respectively, as can be seen in the block diagram below. The top-level module pinmux contains the CSRs accessible via the TL-UL interface, the main muxing matrix, retention registers, a set of programmable wakeup detectors, and the HW strap sampling and TAP / JTAG muxing logic. The chip-level module padring instantiates the bidirectional pads and connects the physical pad attributes.

MIO and DIO Signal Categories

The pinmux supports two different IO signal categories: Muxed IO (MIO) signals that are routed through the pinmux matrix, and dedicated IO (DIO) signals that bypass the pinmux matrix. This distinction is useful for accommodating IO signals that are timing critical or that must have a fixed IO mapping for another reason. Note that although DIO signals are not routed through the pinmux matrix, they are still connected to the retention logic and the wakeup detectors (see next section below).

The number of available peripheral IOs, pads, and their assignment to the MIO / DIO categories is done at design time as part of the top-level configuration. This configurability is achieved by representing inputs / outputs as packed arrays, in combination with the SystemVerilog parameters NPeriphIn, NPeriphOut, NMioPads and NDioPads. Note however that the register file is also affected by this configuration and needs to be regenerated for each design instance.

It is assumed that all available pins that the pinmux connects to are bidirectional, controlled by logic within this module. By default, all muxed peripheral inputs are tied to zero. Further, all output enables are set to zero, which essentially causes all pads to be in high-Z state after reset. In addition to wiring programmability, each muxed peripheral input can be set constantly to 0 or 1, and each muxed chip output can be set constantly to 0, 1 or high-Z.

See the muxing matrix section for more details about the mux implementation.

Retention and Wakeup Features

The retention logic allows SW to specify a certain behavior during sleep for each muxed and dedicated output. Legal behaviors are tie low, tie high, high-Z, keeping the previous state, or driving the current value (useful for peripherals that are always on).

The wakeup detectors can detect patterns such as rising / falling edges and pulses of a certain width up to 255 AON clock cycles. Each wakeup detector can listen on any one of the MIO / DIO signals that are routed through the pinmux, and if a pattern is detected, the power manager is informed of that event via a wakeup request.

The pinmux module itself is in the always-on (AON) power domain, and as such does not loose configuration state when a sleep power cycle is performed. However, only the wakeup detector logic will be actively clocked during sleep in order to save power.

See the retention logic and wakeup detectors sections for more details about the mux implementation.

Test and Debug Access

The hardware strap sampling and TAP isolation logic provides test and debug access to the chip during specific life cycle states. This mechanism is explained in more detail in the strap sampling and TAP isolation section.

Pad Attributes

Additional pad-specific features such as inversion, pull-up, pull-down, virtual open-drain, drive-strength and input/output inversion etc. can be exercise via the pad attribute CSRs. The pinmux module supports a comprehensive set of such pad attributes, but it is permissible that some of them may not be supported by the underlying pad implementation. For example, certain ASIC libraries may not provide open-drain outputs, and FPGAs typically do not allow all of these attributes to be programmed dynamically at runtime. See the generic pad wrapper section below for more details. Note that static pad attributes for FPGAs are currently not covered in this specification.

Hardware Interfaces

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

Primary Clock: clk_i

Other Clocks: clk_aon_i

Bus Device Interfaces (TL-UL): tl

Bus Host Interfaces (TL-UL): none

Peripheral Pins for Chip IO: none

Interrupts: none

Security Alerts:

Security Countermeasures:

Parameters

The following table lists the main parameters used throughout the pinmux design. Note that the pinmux is generated based on the system configuration, and hence these parameters are placed into a package. The pinout and pinmux mappings are listed under Pinout and Pinmux Mapping for specific top-level configurations.

Signals

The table below lists the pinmux signals. The number of dedicated and muxed IOs is parametric, and hence the signals are stacked in packed arrays.

Muxing Matrix

The diagram below shows connectivity between four arbitrary chip pins, named MIO0 .. MIO3, and several muxed peripheral inputs and outputs. This shows the connectivity available in all directions, as well as the control registers described later in this document. Two example peripherals (uart and spidev) are attached to the pinmux in this example, one with one input and one output, the other with three inputs and one output. The diagram also shows the padring module which instantiates the bidirectional chip pads with output enable control.

Note that apart from selecting a specific input pad, the periph_insel[*] signals can also be used to tie the peripheral input to 0 or 1. Likewise, the output select signals mio_outsel[*] can also be used to constantly drive an output pin to 0/1 or to put it into high-Z state (default). The output enable and the associated data signal (i.e. periph_to_mio and periph_to_mio_oe) are indexed with the same select signal to allow the peripheral hardware to determine the pad direction instead of demoting that control to SW.

Retention Logic

As illustrated in the picture above, all muxing matrix and DIO outputs are routed through the retention logic, which essentially consists of a set of multiplexors and two retention registers per output (one register is for the output data and one for the output enable). This multiplexor can be configured to be automatically activated upon sleep entry in order to either drive the output low, high, high-Z or to the last seen value (keep). If no sleep behavior is specified, the retention logic will continue to drive out the value coming from the peripheral side, which can be useful for peripherals that reside in the AON domain.

The sleep behavior of all outputs is activated in parallel via a trigger signal asserted by the power manager. Once activated, it is the task of SW to disable the sleep behavior for each individual pin when waking up from sleep. This ensures that the output values remain stable until the system and its peripherals have been re-initialized.

Wakeup Detectors

The pinmux contains eight programmable wakeup detector modules that can listen on any of the MIO or DIO pins. Each detector contains a debounce filter and an 8bit counter running on the AON clock domain. The detectors can be programmed via the WKUP_DETECTOR_0 and WKUP_DETECTOR_CNT_TH_0 registers to detect the following patterns:

• rising edge
• falling edge
• rising or falling edge
• positive pulse up to 255 AON clock cycles in length
• negative pulse up to 255 AON clock cycles in length

Note that for all patterns listed above, the input signal is sampled with the AON clock. This means that the input signal needs to remain stable for at least one AON clock cycle after a level change for the detector to recognize the event (depending on the debounce filter configuration, the signal needs to remain stable for multiple clock cycles).

If a pattern is detected, the wakeup detector will send a wakeup request to the power manager, and the cause bit corresponding to that detector will be set in the WKUP_CAUSE register.

Note that the wkup detector should be disabled by setting WKUP_DETECTOR_EN_0 before changing the detection mode. The reason for that is that the pulse width counter is NOT cleared upon a mode change while the detector is enabled.

Strap Sampling and TAP Isolation

The pinmux contains a set of dedicated HW “straps”, which are essentially signals that are multiplexed onto fixed MIO pad locations. Depending on the life cycle state, these straps are either continuously sampled, or latched right after POR.

There are two groups of HW straps:

1. Three DFT straps that determine the DFT mode. These bits are output via the dft_strap_test_o signal such that they can be routed to the tool-inserted DFT controller.
2. Two TAP selection straps for determining which TAP should be multiplexed onto the JTAG IOs.

The conditions under which these two strap groups are sampled are listed in the tables below. Note that the HW straps can be used just like regular GPIOs once they have been sampled.

Once at boot: Sampled once after life cycle initialization (sampling event is initiated by pwrmgr).

Continously: Sampled continuously after life cycle initialization.

The TAP muxing logic is further qualified by the life cycle state in order to isolate the TAPs in certain life cycle states. The following table lists the TAP strap encoding and the life cycle states in which the associated TAPs can be selected and accessed.

Note that the tool-inserted DFT controller may assert the dft_hold_tap_sel_i during a test (e.g. boundary scan) in which case the pinmux will temporarily pause sampling of the TAP selection straps.

Also, it should be noted that the pad attributes of all JTAG IOs will be gated to all-zero temporarily, while the JTAG is enabled (this does not affect the values in the CSRs). This is to ensure that any functional attributes like inversion or pull-ups / pull-downs do not interfere with the JTAG while it is in use.

For more information about the life cycle states, see Life Cycle Controller Specification and the Life Cycle Definition Table.

Generic Pad Wrapper

The generic pad wrapper is intended to abstract away implementation differences between the target technologies by providing a generic interface that is compatible with the padring module. It is the task of the RTL build flow to select the appropriate pad wrapper implementation.

A specific implementation of a pad wrapper may choose to instantiate a technology primitive (as it is common in ASIC flows), or it may choose to model the functionality behaviorally such that it can be inferred by the technology mapping tool (e.g., in the case of an FPGA target). It is permissible to omit the implementation of all IO attributes except input/output inversion.

The generic pad wrapper must expose the following IOs and parameters, even if they are not connected internally. In particular, the pad attribute struct attr_i must contain all fields listed below, even if not all attributes are supported (it is permissible to just leave them unconnected in the pad wrapper implementation).

Note that PadType is a technology-specific parameter. The generic pad wrapper only implements variant BidirStd, but for other target technologies, this parameter can be used to select among a variety of different pad flavors.

The ScanRole parameter determines the behavior when scanmode is enabled. Depending on whether a given pad acts as a scan input or output, certain pad attributes and functionalities need to be bypassed. This parameter is typically only relevant for ASIC targets and therefore not modeled in the generic pad model.

Also note that the pad wrapper may implement a “virtual” open-drain termination, where standard bidirectional pads are employed, but instead of driving the output high for a logic 1 the pad is put into tristate mode.

Note that the corresponding pad attribute registers MIO_PAD_ATTR_0 and DIO_PAD_ATTR_0 have “writes-any-reads-legal” (WARL) behavior (see also pad attributes).

Programmers Guide

Pad Attributes

Software should determine and program the pad attributes at startup, or reprogram the attributes when the functionality requirements change at runtime.

This can be achieved by writing to the MIO_PAD_ATTR_0 and DIO_PAD_ATTR_0 registers. Note that the IO attributes should be configured before enabling muxed IOs going through the pinmux matrix in order to avoid undesired electrical behavior and/or contention at the pads.

The pad attributes configuration can be locked down individually for each pad via the MIO_PAD_ATTR_REGWEN_0 and DIO_PAD_ATTR_REGWEN_0 registers. The configuration can then not be altered anymore until the next system reset.

The following pad attributes are supported by this register layout by default:

Since some of the pad attributes may not be implemented, SW can probe this capability by writing the CSRs and read them back to determine whether the value was legal. This behavior is also referred to as “writes-any-reads-legal” or “WARL” in the RISC-V world. For example, certain pads may only support two drive-strength bits, instead of four. The unsupported drive-strength bits in the corresponding CSRs would then always read as zero, even if SW attempts to set them to 1.

Pinmux Configuration

Upon POR, the pinmux state is such that all MIO outputs are high-Z, and all MIO peripheral inputs are tied off to 0. Software should determine and program the pinmux mapping at startup, or reprogram it when the functionality requirements change at runtime. This can be achieved by writing the following values to the PERIPH_INSEL_0 and MIO_OUTSEL_0 registers.

The global default at reset is 0, but the default of individual signals can be overridden at design time, if needed.

The global default at reset is 2, but the default of individual signals can be overridden at design time, if needed.

Note that the pinmux configuration should be sequenced after any IO attribute-specific configuration in the MIO_PAD_ATTR_0 and DIO_PAD_ATTR_0 registers to avoid any unwanted electric behavior and/or contention. If needed, each select signal can be individually locked down via MIO_PERIPH_INSEL_REGWEN_0 or MIO_OUTSEL_REGWEN_0. The configuration can then not be altered anymore until the next system reset.

Sleep Features

The sleep behavior of each individual MIO or DIO can be defined via the ( MIO_PAD_SLEEP_EN_0, DIO_PAD_SLEEP_EN_0, MIO_PAD_SLEEP_MODE_0 and DIO_PAD_SLEEP_MODE_0) registers. Available sleep behaviors are:

Note that if the behavior is set to “Drive”, the sleep mode will not be activated upon sleep entry. Rather, the retention logic continues to drive the value coming from the peripheral side. Also note that the sleep logic is located after the pinmux matrix, hence the sleep configuration is per MIO pad and not per MIO peripheral.

Before sleep entry, SW should configure the appropriate sleep behavior of all MIOs/DIOs via MIO_PAD_SLEEP_MODE_0, DIO_PAD_SLEEP_MODE_0. This configuration can be optionally locked down, in which case it cannot be modified again until POR. The configured behavior is then activated for all pads that have sleep mode set to enabled ( MIO_PAD_SLEEP_EN_0 and DIO_PAD_SLEEP_EN_0) at once by the power manager during the sleep entry sequence.

When exiting sleep, the task of disabling the sleep behavior is however up to SW. I.e., it must clear the per-pad sleep status bits in registers MIO_PAD_SLEEP_STATUS_0 and DIO_PAD_SLEEP_STATUS_0 that have been set upon sleep entry. The rationale for this is that it may not be desirable to disable sleep behavior on all pads at once due to some additional book keeping / re-initialization that needs to be performed while exiting sleep.

Wakeup Features

The pinmux contains eight wakeup detectors. These detectors can be individually enabled and disabled regardless of the sleep state. This ensures that SW can set them up before and disable them after sleep in order to ensure that no events are missed during sleep entry and exit.

For more information on the patterns supported by the wakeup detectors, see wakeup detectors.

A typical programming sequence for the wakeup detectors looks as follows:

1. Before initiating any sleep mode, SW should configure the wakeup detectors appropriately and enable them via the WKUP_DETECTOR_0, WKUP_DETECTOR_CNT_TH_0 and WKUP_DETECTOR_PADSEL_0 registers.

2. Optionally, lock the wakeup detector configuration via the WKUP_DETECTOR_REGWEN_0 registers.

3. During sleep, the wakeup detectors will trigger a wakeup request if a matching pattern has been observed. A bit corresponding to the wakeup detector that has observed the pattern will be set in the WKUP_CAUSE register.

4. When exiting sleep, SW should read the wake info register in the power manager to determine the reason(s) for the wakeup request.

5. If the wakeup request was due to a pin wakeup pattern detector, SW should inspect the WKUP_CAUSE registers in order to determine the exact cause.

6. SW should in any case disable the wakeup detectors and clear the WKUP_CAUSE registers once it is safe to do so (in order to not miss any events). Note that the WKUP_CAUSE registers reside in the slow AON clock domain, and hence clearing them takes a few uS to take effect. If needed, a SW readback can be performed to ensure that the clear operation has completed successfully.

Pinout and Pinmux Mapping

The tables below summarize the pinout and pinmux connectivity for certain top-level designs.

Top Earlgrey

Register Table

The register description below matches the instance in the Earl Grey top level design.

Similar register descriptions can be generated with different parameterizations.