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serial_driver.md

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'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.

Drivers in this category have the following characteristics:

  • bit bang and USART Half-duplex provide data and signaling over a single conductor
  • USART Full-duplex provide data and signaling over two conductors
  • They are all limited to single master and single slave communication scheme

Supported Driver Types

AVR ARM
bit bang ✔️ ✔️
USART Half-duplex ✔️
USART Full-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 over bitbang is that this provides fast and accurate timings. SERIAL_PIN_TX for this driver is the configured USART TX pin. As this Pin is configured in open-drain mode an external pull-up resistor is needed to keep the line high (resistor values of 1.5k to 8.2k are known to work). 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 USART1_REMAP             // Remap USART TX and RX pins on STM32F103 MCUs, see table below.
#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
#define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100

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.

USART Full-duplex

Targeting STM32 boards where communication is offloaded to a USART hardware device. The advantage over bitbang is that this provides fast and accurate timings. USART Full-Duplex requires two conductors without pull-up resistors instead of one conductor with a pull-up resistor unlike the Half-duplex driver, but it is more efficent as it uses DMA transfers, which can result in even faster transmission speeds.

Pin configuration

SERIAL_USART_TX_PIN is the USART TX pin, SERIAL_USART_RX_PIN is the USART RX pin. No external pull-up resistors are needed as the TX pin operates in push-pull mode. To use this driver the usart peripherals TX and RX pins must be configured with the correct Alternate-functions. If you are using a Proton-C everything is already setup, same is true for STM32F103 MCUs. For MCUs which are using a modern flexible GPIO configuration you have to specify these by setting SERIAL_USART_TX_PAL_MODE and SERIAL_USART_RX_PAL_MODE. Refeer to the corresponding datasheets of your MCU or find those settings in the table below.

Connecting the halves and Pin Swap

Please note that TX of the master half has to be connected with the RX pin of the slave half and RX of the master half has to be connected with the TX pin of the slave half! Usually this pin swap has to be done outside of the MCU e.g. with cables or on the pcb. Some MCUs like the STM32F303 used on the Proton-C allow this pin swap directly inside the MCU, this feature can be enabled using #define SERIAL_USART_PIN_SWAP in your config.h.

Setup

To use the driver, add this to your rules.mk:

SERIAL_DRIVER = usart_duplex

Next configure the hardware via your config.h:

#define SERIAL_USART_TX_PIN B6     // USART TX pin
#define SERIAL_USART_RX_PIN B7     // USART RX pin
//#define USART1_REMAP             // Remap USART TX and RX pins on STM32F103 MCUs, see table below.
//#define SERIAL_USART_PIN_SWAP    // Swap TX and RX pins if keyboard is master halve.
                                   // Check if this feature is necessary with your keyboard design and available on the mcu.
#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
                                   //  0: 460800 baud
                                   //  1: 230400 baud (default)
                                   //  2: 115200 baud
                                   //  3: 57600 baud
                                   //  4: 38400 baud
                                   //  5: 19200 baud
#define SERIAL_USART_DRIVER UARTD1 // USART driver of TX and RX pin. default: UARTD1
#define SERIAL_USART_TX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
#define SERIAL_USART_RX_PAL_MODE 7 // Pin "alternate function", see the respective datasheet for the appropriate values for your MCU. default: 7
#define SERIAL_USART_TIMEOUT 100 // USART driver timeout. default 100

You must also enable the ChibiOS UART with blocking api feature:

  • In your board's halconf.h: #define HAL_USE_UART TRUE and #define UART_USE_WAIT TRUE
  • In your board's mcuconf.h: #define STM32_UART_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 UART peripheral, not the SERIAL peripheral.

Pins for USART Peripherals with Alternate Functions for selected STM32 MCUs

STM32F303 / Proton-C Datasheet

Pin Swap available: ✔️

Pin Function Mode
USART1
PA9 TX AF7
PA10 RX AF7
PB6 TX AF7
PB7 RX AF7
PC4 TX AF7
PC5 RX AF7
PE0 TX AF7
PE1 RX AF7
USART2
PA2 TX AF7
PA3 RX AF7
PA14 TX AF7
PA15 RX AF7
PB3 TX AF7
PB4 RX AF7
PD5 TX AF7
PD6 RX AF7
USART3
PB10 TX AF7
PB11 RX AF7
PC10 TX AF7
PC11 RX AF7
PD8 TX AF7
PD9 RX AF7
STM32F072 Datasheet

Pin Swap available: ✔️

Pin Function Mode
USART1
PA9 TX AF1
PA10 RX AF1
PB6 TX AF0
PB7 RX AF0
USART2
PA2 TX AF1
PA3 RX AF1
PA14 TX AF1
PA15 RX AF1
USART3
PB10 TX AF4
PB11 RX AF4
PC4 TX AF1
PC5 RX AF1
PC10 TX AF1
PC11 RX AF1
PD8 TX AF0
PD9 RX AF0
USART4
PA0 TX AF4
PA1 RX AF4
STM32F103 Medium Density (C8-CB) Datasheet

Pin Swap available: N/A

TX Pin is always Alternate Function Push-Pull, RX Pin is always regular input pin for any USART peripheral. For STM32F103 no additional Alternate Function configuration is necessary. QMK is already configured.

Pin remapping:

The pins of USART Peripherals use default Pins that can be remapped to use other pins using the AFIO registers. Default pins are marked bold. Add the appropriate defines to your config.h file.

Pin Function Mode USART_REMAP
USART1
PA9 TX AFPP
PA10 RX IN
PB6 TX AFPP USART1_REMAP
PB7 RX IN USART1_REMAP
USART2
PA2 TX AFPP
PA3 RX IN
PD5 TX AFPP USART2_REMAP
PD6 RX IN USART2_REMAP
USART3
PB10 TX AFPP
PB11 RX IN
PC10 TX AFPP USART3_PARTIALREMAP
PC11 RX IN USART3_PARTIALREMAP
PD8 TX AFPP USART3_FULLREMAP
PD9 RX IN USART3_FULLREMAP