The Serial driver powers the [Split Keyboard](../features/split_keyboard) feature. Several implementations are available that cater to the platform and capabilites of MCU in use. Note that none of the drivers support split keyboards with more than two halves.
| [Bitbang](#bitbang) | :heavy_check_mark: | :heavy_check_mark: | Single wire communication. One wire is used for reception and transmission. |
| [USART Half-duplex](#usart-half-duplex) | | :heavy_check_mark: | Efficient single wire communication. One wire is used for reception and transmission. |
| [USART Full-duplex](#usart-full-duplex) | | :heavy_check_mark: | Efficient two wire communication. Two distinct wires are used for reception and transmission. |
This is the Default driver, absence of configuration assumes this driver. It works by [bit banging](https://en.wikipedia.org/wiki/Bit_banging) a GPIO pin using the CPU. It is therefore not as efficient as a dedicated hardware peripheral, which the Half-duplex and Full-duplex drivers use.
On ARM platforms the bitbang driver causes connection issues when using it together with the bitbang WS2812 driver. Choosing alternate drivers for both serial and WS2812 (instead of bitbang) is strongly recommended.
One GPIO pin is needed for the bitbang driver, as only one wire is used for receiving and transmitting data. This pin is referred to as the `SOFT_SERIAL_PIN` (SSP) in the configuration. A TRS or USB cable provides enough conductors for this driver to function.
Targeting ARM boards based on ChibiOS, where communication is offloaded to a USART hardware device that supports Half-duplex operation. The advantages over bitbanging are fast, accurate timings and reduced CPU usage. Therefore it is advised to choose Half-duplex over Bitbang if MCU is capable of utilising Half-duplex, and Full-duplex can't be used instead (e.g. lack of available GPIO pins, or imcompatible PCB design).
Only one GPIO pin is needed for the Half-duplex driver, as only one wire is used for receiving and transmitting data. This pin is referred to as the `SERIAL_USART_TX_PIN` in the configuration. Ensure that the pin chosen for split communication can operate as the TX pin of the contoller's USART peripheral. A TRS or USB cable provides enough conductors for this driver to function. As the split connection is configured to operate in open-drain mode, an **external pull-up resistor is needed to keep the line high**. Resistor values of 1.5kΩ to 8.2kΩ are known to work.
For STM32 MCUs several GPIO configuration options can be changed as well. See the section ["Alternate Functions for selected STM32 MCUs"](#alternate-functions-for-selected-stm32-mcus).
Targeting ARM boards based on ChibiOS where communication is offloaded to an USART hardware device. The advantages over bitbanging are fast, accurate timings and reduced CPU usage; therefore it is advised to choose this driver over all others where possible. Due to its internal design Full-duplex is slightly more efficient than the Half-duplex driver, but Full-duplex should be primarily chosen if Half-duplex operation is not supported by the controller's USART peripheral.
Two GPIO pins are needed for the Full-duplex driver, as two distinct wires are used for receiving and transmitting data. The pin transmitting data is the `TX` pin and refereed to as the `SERIAL_USART_TX_PIN`, the pin receiving data is the `RX` pin and refereed to as the `SERIAL_USART_RX_PIN` in this configuration. Please note that `TX` pin of the master half has to be connected with the `RX` pin of the slave half and the `RX` pin 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. A TRRS or USB cable provides enough conductors for this driver to function.
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 development board 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`. Refer to the corresponding datasheets of your MCU or find those settings in the section ["Alternate Functions for selected STM32 MCUs"](#alternate-functions-for-selected-stm32-mcus).
For STM32 MCUs several GPIO configuration options, including the ability for `TX` to `RX` pin swapping, can be changed as well. See the section ["Alternate Functions for selected STM32 MCUs"](#alternate-functions-for-selected-stm32-mcus).
The `SERIAL` Subsystem is supported for the majority of ChibiOS MCUs and should be used whenever supported. Follow these steps in order to activate it:
1. In your keyboards `halconf.h` add:
```c
#define HAL_USE_SERIAL TRUE
```
2. In your keyboards `mcuconf.h`: activate the USART peripheral that is used on your MCU. The shown example is for an STM32 MCU, so this will not work on MCUs by other manufacturers. You can find the correct names in the `mcuconf.h` files of your MCU that ship with ChibiOS.
Just below `#include_next <mcuconf.h>` add:
```c
#include_next <mcuconf.h>
#undef STM32_SERIAL_USE_USARTn
#define STM32_SERIAL_USE_USARTn TRUE
```
Where 'n' matches the peripheral number of your selected USART on the MCU.
3. In you keyboards `config.h`: override the default USART `SERIAL` driver if you use a USART peripheral that does not belong to the default selected `SD1` driver. For instance, if you selected `STM32_SERIAL_USE_USART3` the matching driver would be `SD3`.
```c
#define SERIAL_USART_DRIVER SD3
```
### The `SIO` driver
The `SIO` Subsystem was added to ChibiOS with the 21.11 release and is only supported on selected MCUs. It should only be chosen when the `SERIAL` subsystem is not supported by your MCU.
Follow these steps in order to activate it:
1. In your keyboards `halconf.h` add:
```c
#define HAL_USE_SIO TRUE
```
2. In your keyboards `mcuconf.h:` activate the USART peripheral that is used on your MCU. The shown example is for an STM32 MCU, so this will not work on MCUs by other manufacturers. You can find the correct names in the `mcuconf.h` files of your MCU that ship with ChibiOS.
Just below `#include_next <mcuconf.h>` add:
```c
#include_next <mcuconf.h>
#undef STM32_SIO_USE_USARTn
#define STM32_SIO_USE_USARTn TRUE
```
Where 'n' matches the peripheral number of your selected USART on the MCU.
3. In the keyboard's `config.h` file: override the default USART `SIO` driver if you use a USART peripheral that does not belong to the default selected `SIOD1` driver. For instance, if you selected `STM32_SERIAL_USE_USART3` the matching driver would be `SIOD3`.
The `PIO` subsystem is a Raspberry Pi RP2040 specific implementation, using an integrated PIO peripheral and is therefore only available on this MCU. Because of the flexible nature of PIO peripherals, **any** GPIO pin can be used as a `TX` or `RX` pin. Half-duplex and Full-duplex operation modes are fully supported with this driver. Half-duplex uses the built-in pull-ups and GPIO manipulation of the RP2040 to drive the line high by default, thus an external pull-up resistor **is not required**.
There are several advanced configuration options that can be defined in your keyboards `config.h` file:
### Baudrate
If you're having issues or need a higher baudrate with serial communication, you can change the baudrate which in turn controls the communication speed for serial. You want to lower the baudrate if you experience failed transactions.
Alternatively you can specify the baudrate directly by defining `SERIAL_USART_SPEED`.
### Timeout
This is the default time window in milliseconds in which a successful communication has to complete. Usually you don't want to change this value. But you can do so anyways by defining an alternate one in your keyboards `config.h` file:
If you're having issues withe serial communication, you can enable debug messages that will give you insights which part of the communication failed. The enable these messages add to your keyboards `config.h` file:
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.
