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:
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. Due to its internal design it is more efficent, which can result in even faster transmission speeds.
`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.
##### STM32F103 Medium Density (C8-CB) [Datasheet](https://www.st.com/resource/en/datasheet/stm32f103c8.pdf)
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.
