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UART Driver :id=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 :id=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 :id=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 :id=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:

#undef STM32_SERIAL_USE_USART2
#define STM32_SERIAL_USE_USART2 TRUE

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

API :id=api

void uart_init(uint32_t baud) :id=api-uart-init

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

Arguments :id=api-uart-init-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) :id=api-uart-write

Transmit a single byte.

Arguments :id=api-uart-write-arguments

  • uint8_t data
    The byte to write.

uint8_t uart_read(void) :id=api-uart-read

Receive a single byte.

Return Value :id=api-uart-read-return

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) :id=api-uart-transmit

Transmit multiple bytes.

Arguments :id=api-uart-transmit-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) :id=api-uart-receive

Receive multiple bytes.

Arguments :id=api-uart-receive-arguments

  • 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) :id=api-uart-available

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

Return Value :id=api-uart-available-return

true if the receive buffer length is non-zero.