Add Full-duplex driver to docs

docs/serial_driver.md

@@ -3,16 +3,18 @@ This driver powers the [Split Keyboard](feature_split_keyboard.md) 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
+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 initiator and single target communication scheme
 
 ## Supported Driver Types
 
 |                   | AVR                | ARM                |
 |-------------------|--------------------|--------------------|
 | bit bang          | :heavy_check_mark: | :heavy_check_mark: |
 | USART Half-duplex |                    | :heavy_check_mark: |
+| USART Full-duplex |                    | :heavy_check_mark: |
 
 ## Driver configuration
 
@@ -42,7 +44,7 @@ Configure the driver via your config.h:
 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. `SERIAL_PIN_TX` 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:
+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**. To configure it, add this to your rules.mk:
 
 ```make
 SERIAL_DRIVER = usart
@@ -67,3 +69,34 @@ You must also enable the ChibiOS `SERIAL` feature:
 * 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 instead of one conductor like the Half-duplex driver, but it is more efficent as it uses DMA transfers, which can result in even faster transmission speeds. To use this driver the usart peripherals `TX` and `RX` pins must be configured. `SERIAL_PIN_TX` is the USART `TX` pin, `SERIAL_PIN_RX` is the USART `RX` pin. No external pull-up resistors are needed as the `TX` pin operates in push-pull mode.
+
+To use the driver, add this to your rules.mk:
+
+```make
+SERIAL_DRIVER = usart_duplex
+```
+
+Next configure the hardware via your config.h:
+
+```c
+#define SERIAL_PIN_TX B6  // USART TX pin
+#define SERIAL_PIN_RX B7  // USART RX pin
+#define SELECT_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 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
+```
+
+You must also enable the ChibiOS `UART` with blocking api feature:
+* In your board's halconf.h: `#define HAL_USE_USAR 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.