qmk_firmware/docs/drivers/uart.md

UART Driver

The UART drivers used in QMK have a set of common functions to allow portability between MCUs.

Currently, this driver does not support enabling hardware flow control (the RTS and CTS pins) if available, but may do so in future.

Usage

In most cases, the UART driver code is automatically included if you are using a feature or driver which requires it.

However, if you need to use the driver standalone, add the following to your rules.mk:

UART_DRIVER_REQUIRED = yes

You can then call the UART API by including uart.h in your code.

AVR Configuration

No special setup is required - just connect the RX and TX pins of your UART device to the opposite pins on the MCU:

MCU	TX	RX	CTS	RTS
ATmega16/32U2	D3	D2	D7	D6
ATmega16/32U4	D3	D2	D5	B7
AT90USB64/128	D3	D2	n/a	n/a
ATmega32A	D1	D0	n/a	n/a
ATmega328/P	D1	D0	n/a	n/a

ChibiOS/ARM Configuration

You'll need to determine which pins can be used for UART -- as an example, STM32 parts generally have multiple UART peripherals, labeled USART1, USART2, USART3 etc.

To enable UART, modify your board's mcuconf.h to enable the peripheral you've chosen, for example:

#pragma once

#include_next <mcuconf.h>

#undef STM32_SERIAL_USE_USART2 // [!code focus]
#define STM32_SERIAL_USE_USART2 TRUE // [!code focus]

Configuration-wise, you'll need to set up the peripheral as per your MCU's datasheet -- the defaults match the pins for a Proton-C, i.e. STM32F303.

config.h Override	Description	Default
UART_DRIVER	USART peripheral to use - USART1 -> SD1, USART2 -> SD2 etc.	SD1
UART_TX_PIN	The pin to use for TX	A9
UART_TX_PAL_MODE	The alternate function mode for TX	7
UART_RX_PIN	The pin to use for RX	A10
UART_RX_PAL_MODE	The alternate function mode for RX	7
UART_CTS_PIN	The pin to use for CTS	A11
UART_CTS_PAL_MODE	The alternate function mode for CTS	7
UART_RTS_PIN	The pin to use for RTS	A12
UART_RTS_PAL_MODE	The alternate function mode for RTS	7

API

void uart_init(uint32_t baud)

Initialize the UART driver. This function must be called only once, before any of the below functions can be called.

Arguments

• uint32_t baud
  The baud rate to transmit and receive at. This may depend on the device you are communicating with. Common values are 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200.

---

void uart_write(uint8_t data)

Transmit a single byte.

Arguments

• uint8_t data
  The byte to write.

---

uint8_t uart_read(void)

Receive a single byte.

Return Value

The byte read from the receive buffer. This function will block if the buffer is empty (ie. no data to read).

---

void uart_transmit(const uint8_t *data, uint16_t length)

Transmit multiple bytes.

Arguments

• const uint8_t *data
  A pointer to the data to write from.
• uint16_t length
  The number of bytes to write. Take care not to overrun the length of data.

---

void uart_receive(char *data, uint16_t length)

Receive multiple bytes.

Arguments

• uint8_t *data
  A pointer to a buffer to read into.
• uint16_t length
  The number of bytes to read. Take care not to overrun the length of data.

---

bool uart_available(void)

Return whether the receive buffer contains data. Call this function to determine if uart_read() will return data immediately.

Return Value

true if there is data available to read.