UART HWIP Technical Specification


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


  • 2-pin full duplex external interface
  • 8-bit data word, optional even or odd parity bit per byte
  • 1 stop bit
  • 32 x 8b RX buffer
  • 32 x 8b TX buffer
  • Programmable baud rate
  • Interrupt for transmit empty, receive overflow, frame error, parity error, break error, receive timeout


The UART module is a serial-to-parallel receive (RX) and parallel-to-serial (TX) full duplex design intended to communicate to an outside device, typically for basic terminal-style communication. It is programmed to run at a particular baud rate and contains only a transmit and receive signal to the outside world, i.e. no synchronizing clock. The programmable baud rate guarantees to be met up to 1Mbps.


The OpenTitan UART is feature compatible to a specific implementation in Chromium EC. Additional features such as parity have been added.

Theory of Operations

Block Diagram

UART Block Diagram

Hardware Interfaces

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

Primary Clock: clk_i

Other Clocks: none

Bus Device Interface: tlul

Bus Host Interface: none

Peripheral Pins for Chip IO:

Pin namedirectionDescription
rxinputSerial receive bit
txoutputSerial transmit bit


Interrupt NameDescription
tx_watermarkraised if the transmit FIFO is past the high-water mark.
rx_watermarkraised if the receive FIFO is past the high-water mark.
tx_emptyraised if the transmit FIFO has emptied and no transmit is ongoing.
rx_overflowraised if the receive FIFO has overflowed.
rx_frame_errraised if a framing error has been detected on receive.
rx_break_errraised if break condition has been detected on receive.
rx_timeoutraised if RX FIFO has characters remaining in the FIFO without being retrieved for the programmed time period.
rx_parity_errraised if the receiver has detected a parity error.

Security Alerts:

Alert NameDescription

Design Details

Serial interface (both directions)

The TX/RX serial lines are high when idle. Data starts with a START bit (high idle state deasserts, 1–>0) followed by 8 data bits. The least significant bit is sent first. If the parity feature is turned on then an odd or even parity bit follows after the data bits. Finally a STOP (1) bit completes one byte of data transfer.


A write to WDATA enqueues a data byte into the 32 byte deep write FIFO, which triggers the transmit module to start UART TX serial data transfer. The TX module dequeues the byte from the FIFO and shifts it bit by bit out to the UART TX pin on positive edges of the baud clock.

If TX is not enabled, written DATA into FIFO will be stacked up and sent out when TX is enabled.

When the FIFO becomes empty as part of transmittion, a TX FIFO empty interrupt will be raised. This is separate from the TX FIFO water mark interrupt.


The RX module oversamples the RX input pin at 16x the requested baud clock. When the input is detected low the receiver will check half a bit-time later (i.e. 8 cycles of the oversample clock) that the line is still low before detecting the START bit. If the line has returned high the glitch is ignored. After it detects the START bit, the RX module samples at the center of each bit-time and gathers incoming serial bits into a charcter buffer. If the STOP bit is detected as high and the optional partity bit is correct the data byte is pushed into a 32 byte deep RX FIFO. The data can be read out by reading RDATA register.

This behaviour of the receiver can be used to compute the approximate baud clock frequency error that can be tolerated between the transmitter at the other end of the cable and the receiver. The initial sample point is aligned with the center of the START bit. The receiver will then sample every 16 cycles of the 16 x baud clock, the diagram below shows the number of ticks after the centering that each bit is captured. Because of the frequency difference between the transmiter and receiver the actual sample point will drift compared to the ideal center of the bit. In order to correctly receive the STOP bit it must be sampled between the “early” and “late” points shown on the diagram, which are half a bit-time or 8 ticks of the 16x baud clock before or after the center. If the transmitter is considered “ideal” then the local clock must thus differ by no more than plus or minus 8 ticks in 144 or aproximately +/- 5.5%. If parity is enabled the stop bit will be a bit time later, so this becomes 8/160 or about +/- 5%.

In practice, the transmitter and receiver will both differ from the ideal baud rate. Since the worst case difference for reception is 5%, the uart can be expected to work if both sides are within +/- 2.5% of the ideal baud rate.

Setting the baud rate

The baud rate is set by writing to the CTRL.NCO register field. This should be set using the equation below, where f_pclk is the system clock frequency provided to the UART. and f_baud is the desired baud rate (in bits per second).

$$ NCO = 16 \times {{2^{$bits(NCO)} \times f_{baud}} \over {f_{pclk}}} $$

The formula above depends on the NCO CSR width. The logic creates a x16 tick when the NCO counter overflows. So, the computed baud rate from NCO value is below.

$$ f_{baud} = {{1 \over 16} \times {NCO \over {2^{$bits(NCO)}}} \times {f_{pclk}}} $$

Note that the NCO result from the above formula can be a fraction but the NCO register only accepts an integer value. This will create an error if the baud rate is not divisible by the fixed clock frequency. As discussed in the previous section the error rate between the receiver and remote transmitter should be lower than 8 / 144 to latch a correct character value when parity is not used and lower than 8 / 160 when parity is used. In the expectation that the device the other side of the line behaves similarly, this requires each side have a baud rate that is matched to within +/- 2.5% of the ideal baud rate. The contribution to this error if NCO is rounded down to an integer (which will make the actual baud rate always lower or equal to the requested rate) can be computed from:

$$ Error = {{(NCO - INT(NCO))} \over {NCO}} percent $$

In this case if the resulting value of NCO is greater than $$ {1 \over 0.025} = 40 $$ then this will always be less than the 2.5% error target.

For NCO less than 40 the error in baud rate may or may not be acceptable and should be carefully checked and rounding to the nearest integer may achieve better results. If the computed value is close to an integer so that the error in the target range then the baud rate can be supported, however if it is too far off an integer then the baud rate cannot be supported. This check is needed when

$$ {{baud} < {{40 * f_{pclk}} \over {2^{$bits(NCO)+4}}}} \qquad OR \qquad {{f_{pclk}} > {{{2^{$bits(NCO)+4}} * {baud}} \over {40}}} $$

Using rounded frequencies and common baud rates, this implies that care is needed for 9600 baud and below if the system clock is under 250MHz, with 4800 baud and below if the system clock is under 125MHz, 2400 baud and below if the system clock us under 63MHz, and 1200 baud and below if the system clock is under 32MHz.


UART module has a few interrupts including general data flow interrupts and unexpected event interrupts.

tx_watermark / rx_watermark

If the TX FIFO level becomes smaller than the TX water mark level (configurable via FIFO_CTRL.RXILVL and FIFO_CTRL.TXILVL), the tx_watermark interrupt is raised to inform SW. If the RX FIFO level becomes greater than or equal to RX water mark level (configurable via FIFO_CTRL.RXILVL and FIFO_CTRL.TXILVL), the rx_watermark interrupt is raised to inform SW.

Note that the watermark interrupts are edge triggered events. This means the interrupt only triggers when the condition transitions from untrue->true. This is especially important in the tx_watermark case. When the TX FIFO is empty, it by default satisifies all the watermark conditions. In order for the interrupt to trigger then, it is required that software initiates a write burst that is greater than the programmed watermark value.

For example, assume TX watermark is programmed to be less than 4 bytes, and software programs one byte at a time, waits for it to finish transmitting, before supplying the next byte. Under these conditions, the TX watermark interrupt will never trigger because the size of the FIFO never exceeds the watermark level.


If TX FIFO becomes empty as part of transmit, the interrupt tx_empty is asserted. The transmitted contents may be garbage at this point as old FIFO contents will likely be transmitted.


If RX FIFO receives an additional write request when its FIFO is full, the interrupt rx_overflow is asserted and the character is dropped.


The rx_break_err interrupt is triggered if a break condition has been detected. A break condition is defined as the RX pin being continuously low for more than a programmable number of character-times (via CTRL.RXBLVL, either 2, 4, 8, or 16). A character time is 10 bit-times if parity is disabled (START + 8 data + STOP) or 11 bit-times if parity is enabled (START + 8 data + parity + STOP). If the UART is connected to an external connector this would typically indicate the cable has been disconnected (or there is a break in the wire). If the UART is connected to another part on the same board it would typically indicate the other part has reset or rebooted. (If the open connector or resetting peer part causes the RX input to not be actively driven, then a pulldown resistor is needed to ensure a break and a pullup resistor will ensure the line looks idle and no break is generated.) Note that only one interrupt is generated per break – the line must return high for at least half a bit-time before an additional break interrupt is generated. The current break status can be read from the STATUS.BREAK bit. If STATUS.BREAK is set but INTR_STATE.BREAK is clear then the line break has already caused an interrupt that has been cleared but the line break is still going on. If STATUS.BREAK is clear but INTR_STATE.BREAK is set then there has been a line break for which software has not cleared the interrupt but the line is now back to normal.


The rx_frame_err interrupt is triggered if the RX module receives the START bit (0) and a series of data bits but did not detect the STOP bit (1). This can happen because of noise affecting the line or if the transmitter clock is fast or slow compared to the receiver. In a real frame error the stop bit will be present just at an incorrect time so the line will continue to signal both high and low. The start of a line break (described above) matches a frame error with all data bits zero and one frame error interrupt will be raised. If the line stays zero until the break error occurs, the frame error will be set at every char-time. Frame errors will continue to be reported after a break error.

The effects of the line being low for certain periods are summarized in the table:

Line low (bit-times) Frame Err? Break? Comment
<10 If STOP=0 No Normal operation
10 (with parity) No No Normal zero data with STOP=1
10 (no parity) Yes No Frame error since STOP=0
11 - RXBLVL*char Yes No Break less than detect level
>RXBLVL*char Yes Once Frame error signalled at every char-time, break at RXBLVL char-times


The rx_timeout interrupt is triggered when the RX FIFO has data sitting in it without software reading it for a programmable number of bit times (using the baud rate clock as reference, programmable via TIMEOUT_CTRL). This is used to alert software that it has data still waiting in the FIFO that has not been handled yet. The timeout counter is reset whenever the FIFO depth is changed or an rx_timeout event occurs. If the RX FIFO is full and new character is received, it won’t reset the timeout value. The software is responsible for keeping the RX FIFO in the level below the watermark. The actual timeout time can vary based on the reset of the timeout timer and the start of the transaction. For instance, if the software resets the timeout timer by reading a character from the RX FIFO and right after it there is a baud clock tick and the start of a new RX transaction from the host, the timeout time is reduced by 1 and half baud clock periods.


The rx_parity_err interrupt is triggered if parity is enabled and the RX parity bit does not match the expected polarity as programmed in CTRL.PARITY_ODD.

Programmers Guide


The following code snippet demonstrates initializing the UART to a programmable baud rate, clearing the RX and TX FIFO, setting up the FIFOs for interrupt levels, and enabling some interrupts. The NCO register controls the baud rate, and should be set using the equation below, where f_pclk is the fixed clock frequency and f_baud is the baud rate in bits per second. The UART uses the primary clock clk_i as a clock source.

$$ NCO = {{2^{20} * f_{baud}} \over {f_{pclk}}} $$

Note that the NCO result from the above formula can be a fraction but the NCO register only accepts an integer value. See the the Reception and Setting the baud rate sections for more discussion of the baud rate error target and when care is needed.

Also note that because the baud rate is multiplied by 2^20 care is needed not to overflow 32-bit registers. Baud rates can easily be more than 12 bits. The code below is careful to force 64-bit arithmetic. (Even if the complier is pre-computing constants there can be unexpected overflow).

#define CLK_FIXED_FREQ_HZ (50ULL * 1000 * 1000)

void uart_init(unsigned int baud) {
  // nco = 2^20 * baud / fclk. Assume NCO width is 16bit.
  uint64_t uart_ctrl_nco = ((uint64_t)baud << 20) / CLK_FIXED_FREQ_HZ;
  REG32(UART_CTRL(0)) =
      ((uart_ctrl_nco & UART_CTRL_NCO_MASK) << UART_CTRL_NCO_OFFSET) |
      (1 << UART_CTRL_TX) |
      (1 << UART_CTRL_RX);

  // clear FIFOs and set up to interrupt on any RX, half-full TX
      UART_FIFO_CTRL_RXRST                 | // clear both FIFOs
      UART_FIFO_CTRL_TXRST                 |
      (UART_FIFO_CTRL_RXILVL_RXFULL_1 <<3) | // intr on RX 1 character
      (UART_FIFO_CTRL_TXILVL_TXFULL_16<<5) ; // intr on TX 16 character

  // enable only RX, overflow, and error interrupts

  // at the processor level, the UART interrupts should also be enabled

Common Examples

The following code shows the steps to transmit a string of characters.

int uart_tx_rdy() {

void uart_send_char(char val) {
  while(!uart_tx_rdy()) {}
  *UART_WDATA_REG = val;

void uart_send_str(char *str) {
  while(*str != \0) {

Do the following to receive a character, with -1 returned if RX is empty.

int uart_rx_empty() {
          (0 << UART_FIFO_STATUS_RXLVL_LSB)) ? 1 : 0;

int uart_rcv_char() {
    return -1;
  return *UART_RDATA_REG;

Interrupt Handling

The code below shows one example of how to handle all UART interrupts in one service routine.

void uart_interrupt_routine() {
  volatile uint32 intr_state = *UART_INTR_STATE_REG;
  uint32 intr_state_mask = 0;
  char uart_ch;
  uint32 intr_enable_reg;

  // Turn off Interrupt Enable
  intr_enable_reg = *UART_INTR_ENABLE_REG;
  *UART_INTR_ENABLE_REG = intr_enable_reg & 0xFFFFFF00; // Clr bits 7:0

  if (intr_state & UART_INTR_STATE_RX_PARITY_ERR_MASK) {
    // Do something ...

    // Store Int mask
    intr_state_mask |= UART_INTR_STATE_RX_PARITY_ERR_MASK;

  if (intr_state & UART_INTR_STATE_RX_BREAK_ERR_MASK) {
    // Do something ...

    // Store Int mask
    intr_state_mask |= UART_INTR_STATE_RX_BREAK_ERR_MASK;

  // .. Frame Error

  // TX/RX Overflow Error

  // RX Int
    while(1) {
      uart_ch = uart_rcv_char();
      if (uart_ch == 0xff) break;
    // Store Int mask
    intr_state_mask |= UART_INTR_STATE_RX_WATERMARK_MASK;

  // Clear Interrupt State
  *UART_INTR_STATE_REG = intr_state_mask;

  // Restore Interrupt Enable
  *UART_INTR_ENABLE_REG = intr_enable_reg;

One use of the rx_timeout interrupt is when the FIFO_CTRL.RXILVL is set greater than one, so an interrupt is only fired when the fifo is full to a certain level. If the remote device sends fewer than the watermark number of characters before stopping sending (for example it is waiting an acknowledgement) then the usual rx_watermark interrupt would not be raised. In this case an rx_timeout would generate an interrupt that allows the host to read these additional characters. The rx_timeout can be selected based on the worst latency experienced by a character. The worst case latency experienced by a character will happen if characters happen to arrive just slower than the timeout: the second character arrives just before the timeout for the first (resetting the timer), the third just before the timeout from the second etc. In this case the host will eventually get a watermark interrupt, this will happen ((RXILVL - 1)*timeout) after the first character was received.

Device Interface Functions (DIFs)

To use this DIF, include the following C header:

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

This header provides the following device interface functions:

Register Table

uart.INTR_STATE @ + 0x0
Interrupt State Register
Reset default = 0x0, mask 0xff
  rx_parity_err rx_timeout rx_break_err rx_frame_err rx_overflow tx_empty rx_watermark tx_watermark
0rw1c0x0tx_watermarkraised if the transmit FIFO is past the high-water mark.
1rw1c0x0rx_watermarkraised if the receive FIFO is past the high-water mark.
2rw1c0x0tx_emptyraised if the transmit FIFO has emptied and no transmit is ongoing.
3rw1c0x0rx_overflowraised if the receive FIFO has overflowed.
4rw1c0x0rx_frame_errraised if a framing error has been detected on receive.
5rw1c0x0rx_break_errraised if break condition has been detected on receive.
6rw1c0x0rx_timeoutraised if RX FIFO has characters remaining in the FIFO without being retrieved for the programmed time period.
7rw1c0x0rx_parity_errraised if the receiver has detected a parity error.

uart.INTR_ENABLE @ + 0x4
Interrupt Enable Register
Reset default = 0x0, mask 0xff
  rx_parity_err rx_timeout rx_break_err rx_frame_err rx_overflow tx_empty rx_watermark tx_watermark
0rw0x0tx_watermarkEnable interrupt when INTR_STATE.tx_watermark is set
1rw0x0rx_watermarkEnable interrupt when INTR_STATE.rx_watermark is set
2rw0x0tx_emptyEnable interrupt when INTR_STATE.tx_empty is set
3rw0x0rx_overflowEnable interrupt when INTR_STATE.rx_overflow is set
4rw0x0rx_frame_errEnable interrupt when INTR_STATE.rx_frame_err is set
5rw0x0rx_break_errEnable interrupt when INTR_STATE.rx_break_err is set
6rw0x0rx_timeoutEnable interrupt when INTR_STATE.rx_timeout is set
7rw0x0rx_parity_errEnable interrupt when INTR_STATE.rx_parity_err is set

uart.INTR_TEST @ + 0x8
Interrupt Test Register
Reset default = 0x0, mask 0xff
  rx_parity_err rx_timeout rx_break_err rx_frame_err rx_overflow tx_empty rx_watermark tx_watermark
0wo0x0tx_watermarkWrite 1 to force INTR_STATE.tx_watermark to 1
1wo0x0rx_watermarkWrite 1 to force INTR_STATE.rx_watermark to 1
2wo0x0tx_emptyWrite 1 to force INTR_STATE.tx_empty to 1
3wo0x0rx_overflowWrite 1 to force INTR_STATE.rx_overflow to 1
4wo0x0rx_frame_errWrite 1 to force INTR_STATE.rx_frame_err to 1
5wo0x0rx_break_errWrite 1 to force INTR_STATE.rx_break_err to 1
6wo0x0rx_timeoutWrite 1 to force INTR_STATE.rx_timeout to 1
7wo0x0rx_parity_errWrite 1 to force INTR_STATE.rx_parity_err to 1

uart.CTRL @ + 0xc
UART control register
Reset default = 0x0, mask 0xffff03f7
0rw0x0TXTX enable
1rw0x0RXRX enable
2rw0x0NFRX noise filter enable. If the noise filter is enabled, RX line goes through the 3-tap repetition code. It ignores single IP clock period noise.
4rw0x0SLPBKSystem loopback enable. If this bit is turned on, any outgoing bits to TX are received through RX. See Block Diagram. Note that the TX line goes 1 if System loopback is enabled.
5rw0x0LLPBKLine loopback enable. If this bit is turned on, incoming bits are forwarded to TX for testing purpose. See Block Diagram. Note that the internal design sees RX value as 1 always if line loopback is enabled.
6rw0x0PARITY_ENIf true, parity is enabled in both RX and TX directions.
7rw0x0PARITY_ODDIf PARITY_EN is true, this determines the type, 1 for odd parity, 0 for even.
9:8rw0x0RXBLVLTrigger level for RX break detection. Sets the number of character times the line must be low to detect a break.
0break22 characters
1break44 characters
2break88 characters
3break1616 characters
31:16rw0x0NCOBAUD clock rate control.

uart.STATUS @ + 0x10
UART live status register
Reset default = 0x3c, mask 0x3f
0ro0x0TXFULLTX buffer is full
1ro0x0RXFULLRX buffer is full
2ro0x1TXEMPTYTX FIFO is empty
3ro0x1TXIDLETX FIFO is empty and all bits have been transmitted
4ro0x1RXIDLERX is idle
5ro0x1RXEMPTYRX FIFO is empty

uart.RDATA @ + 0x14
UART read data
Reset default = 0x0, mask 0xff

uart.WDATA @ + 0x18
UART write data
Reset default = 0x0, mask 0x0

uart.FIFO_CTRL @ + 0x1c
UART FIFO control register
Reset default = 0x0, mask 0x7c
0woxRXRSTRX fifo reset. Write 1 to the register resets RX_FIFO. Read returns 0
1woxTXRSTTX fifo reset. Write 1 to the register resets TX_FIFO. Read returns 0
4:2rw0x0RXILVLTrigger level for RX interrupts. If the FIFO depth is greater than or equal to the setting, it raises rx_watermark interrupt.
0rxlvl11 character
1rxlvl44 characters
2rxlvl88 characters
3rxlvl1616 characters
4rxlvl3030 characters
Other values are reserved.
6:5rw0x0TXILVLTrigger level for TX interrupts. If the FIFO depth is less than the setting, it raises tx_watermark interrupt.
0txlvl12 characters
1txlvl44 characters
2txlvl88 characters
3txlvl1616 characters

uart.FIFO_STATUS @ + 0x20
UART FIFO status register
Reset default = 0x0, mask 0x3f003f
5:0ro0x0TXLVLCurrent fill level of TX fifo
21:16ro0x0RXLVLCurrent fill level of RX fifo

uart.OVRD @ + 0x24
TX pin override control. Gives direct SW control over TX pin state
Reset default = 0x0, mask 0x3
0rw0x0TXENEnable TX pin override control
1rw0x0TXVALWrite to set the value of the TX pin

uart.VAL @ + 0x28
UART oversampled values
Reset default = 0x0, mask 0xffff
15:0ro0x0RXLast 16 oversampled values of RX. Most recent bit is bit 0, oldest 15.

uart.TIMEOUT_CTRL @ + 0x2c
UART RX timeout control
Reset default = 0x0, mask 0x80ffffff
EN   VAL...
23:0rw0x0VALRX timeout value in UART bit times
31rw0x0ENEnable RX timeout feature