UART driver refactor (#11637)

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diff --git a/docs/_summary.md b/docs/_summary.md
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       * [WS2812 Driver](ws2812_driver.md)
       * [EEPROM Driver](eeprom_driver.md)
       * ['serial' Driver](serial_driver.md)
+      * [UART Driver](uart_driver.md)
     * [GPIO Controls](internals_gpio_control.md)
     * [Keyboard Guidelines](hardware_keyboard_guidelines.md)
 
diff --git a/docs/uart_driver.md b/docs/uart_driver.md
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+# UART Driver
+
+The UART drivers used in QMK have a set of common functions to allow portability between MCUs.
+
+Currently, this driver does not support enabling hardware flow control (the `RTS` and `CTS` pins) if available, but may do so in future.
+
+## AVR Configuration
+
+No special setup is required - just connect the `RX` and `TX` pins of your UART device to the opposite pins on the MCU:
+
+|MCU          |`TX`|`RX`|`CTS`|`RTS`|
+|-------------|----|----|-----|-----|
+|ATmega16/32U2|`D3`|`D2`|`D7` |`D6` |
+|ATmega16/32U4|`D3`|`D2`|`D5` |`B7` |
+|AT90USB64/128|`D3`|`D2`|*n/a*|*n/a*|
+|ATmega32A    |`D1`|`D0`|*n/a*|*n/a*|
+|ATmega328/P  |`D1`|`D0`|*n/a*|*n/a*|
+
+## ChibiOS/ARM Configuration
+
+You'll need to determine which pins can be used for UART -- as an example, STM32 parts generally have multiple UART peripherals, labeled USART1, USART2, USART3 etc.
+
+To enable UART, modify your board's `halconf.h` to enable the serial driver:
+
+```c
+#define HAL_USE_SERIAL TRUE
+```
+
+Then, modify your board's `mcuconf.h` to enable the peripheral you've chosen, for example:
+
+```c
+#undef STM32_SERIAL_USE_USART2
+#define STM32_SERIAL_USE_USART2 TRUE
+```
+
+Configuration-wise, you'll need to set up the peripheral as per your MCU's datasheet -- the defaults match the pins for a Proton-C, i.e. STM32F303.
+
+|`config.h` override       |Description                                                    |Default Value|
+|--------------------------|---------------------------------------------------------------|-------------|
+|`#define SERIAL_DRIVER`   |USART peripheral to use - USART1 -> `SD1`, USART2 -> `SD2` etc.|`SD1`        |
+|`#define SD1_TX_PIN`      |The pin to use for TX                                          |`A9`         |
+|`#define SD1_TX_PAL_MODE` |The alternate function mode for TX                             |`7`          |
+|`#define SD1_RX_PIN`      |The pin to use for RX                                          |`A10`        |
+|`#define SD1_RX_PAL_MODE` |The alternate function mode for RX                             |`7`          |
+|`#define SD1_CTS_PIN`     |The pin to use for CTS                                         |`A11`        |
+|`#define SD1_CTS_PAL_MODE`|The alternate function mode for CTS                            |`7`          |
+|`#define SD1_RTS_PIN`     |The pin to use for RTS                                         |`A12`        |
+|`#define SD1_RTS_PAL_MODE`|The alternate function mode for RTS                            |`7`          |
+
+## Functions
+
+### `void uart_init(uint32_t baud)`
+
+Initialize the UART driver. This function must be called only once, before any of the below functions can be called.
+
+#### Arguments
+
+ - `uint32_t baud`  
+   The baud rate to transmit and receive at. This may depend on the device you are communicating with. Common values are 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200.
+
+---
+
+### `void uart_putchar(uint8_t c)`
+
+Transmit a single byte.
+
+#### Arguments
+
+ - `uint8_t c`  
+   The byte (character) to send, from 0 to 255.
+
+---
+
+### `uint8_t uart_getchar(void)`
+
+Receive a single byte.
+
+#### Return Value
+
+The byte read from the receive buffer.
+
+---
+
+### `bool uart_available(void)`
+
+Return whether the receive buffer contains data. Call this function to determine if `uart_getchar()` will return meaningful data.
+
+#### Return Value
+
+`true` if the receive buffer length is non-zero.