qmk_firmware/docs/serial_driver.md

'serial' Driver

This driver powers the Split Keyboard feature.

?> Serial in this context should be read as sending information one bit at a time, rather than implementing UART/USART/RS485/RS232 standards.

All drivers in this category have the following characteristics:

• Provides data and signaling over a single conductor
• Limited to single master, single slave

Supported Driver Types

	AVR	ARM
bit bang	✔️	✔️
USART Half-duplex		✔️

Driver configuration

Bitbang

Default driver, the absence of configuration assumes this driver. To configure it, add this to your rules.mk:

SERIAL_DRIVER = bitbang

Configure the driver via your config.h:

#define SOFT_SERIAL_PIN D0  // or D1, D2, D3, E6
#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
                                   //  0: about 189kbps (Experimental only)
                                   //  1: about 137kbps (default)
                                   //  2: about 75kbps
                                   //  3: about 39kbps
                                   //  4: about 26kbps
                                   //  5: about 20kbps

ARM

!> The bitbang driver causes connection issues with bitbang WS2812 driver

Along with the generic options above, you must also turn on the PAL_USE_CALLBACKS feature in your halconf.h.

USART Half-duplex

Targeting STM32 boards where communication is offloaded to a USART hardware device. The advantage is that this provides fast and accurate timings. SOFT_SERIAL_PIN for this driver is the configured USART TX pin. The TX pin must have appropriate pull-up resistors. To configure it, add this to your rules.mk:

SERIAL_DRIVER = usart

Configure the hardware via your config.h:

#define SOFT_SERIAL_PIN B6  // USART TX pin
#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
                                   //  0: about 460800 baud
                                   //  1: about 230400 baud (default)
                                   //  2: about 115200 baud
                                   //  3: about 57600 baud
                                   //  4: about 38400 baud
                                   //  5: about 19200 baud
#define SERIAL_USART_DRIVER SD1 // USART driver of TX pin. default: SD1
#define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7

You must also enable the ChibiOS SERIAL feature:

• In your board's halconf.h: #define HAL_USE_SERIAL TRUE
• In your board's mcuconf.h: #define STM32_SERIAL_USE_USARTn TRUE (where 'n' matches the peripheral number of your selected USART on the MCU)

Do note that the configuration required is for the SERIAL peripheral, not the UART peripheral.