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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.


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


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:


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:


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 overrideDescriptionDefault Value
#define UART_DRIVERUSART peripheral to use - USART1 -> SD1, USART2 -> SD2 etc.SD1
#define UART_TX_PINThe pin to use for TXA9
#define UART_TX_PAL_MODEThe alternate function mode for TX7
#define UART_RX_PINThe pin to use for RXA10
#define UART_RX_PAL_MODEThe alternate function mode for RX7
#define UART_CTS_PINThe pin to use for CTSA11
#define UART_CTS_PAL_MODEThe alternate function mode for CTS7
#define UART_RTS_PINThe pin to use for RTSA12
#define UART_RTS_PAL_MODEThe alternate function mode for RTS7


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.


  • 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.


  • 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.


  • 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.


  • uint8_t *data
    A pointer to the 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 the receive buffer length is non-zero.