qmk_firmware/docs/uart_driver.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.

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 halconf.h to enable the serial driver:

#define HAL_USE_SERIAL TRUE

Then, 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 SERIAL_DRIVER	USART peripheral to use - USART1 -> SD1, USART2 -> SD2 etc.	SD1
#define SD1_TX_PIN	The pin to use for TX	A9
#define SD1_TX_PAL_MODE	The alternate function mode for TX	7
#define SD1_RX_PIN	The pin to use for RX	A10
#define SD1_RX_PAL_MODE	The alternate function mode for RX	7
#define SD1_CTS_PIN	The pin to use for CTS	A11
#define SD1_CTS_PAL_MODE	The alternate function mode for CTS	7
#define SD1_RTS_PIN	The pin to use for RTS	A12
#define SD1_RTS_PAL_MODE	The alternate function mode for RTS	7

Functions

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_putchar(uint8_t c)

Transmit a single byte.

Arguments

• uint8_t c
  The byte (character) to send, from 0 to 255.

---

uint8_t uart_getchar(void)

Receive a single byte.

Return Value

The byte read from the receive buffer.

---

bool uart_available(void)

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

Return Value

true if the receive buffer length is non-zero.