Update ARM split keyboard docs (#9160)

* Update ARM split keyboard docs

* Update docs/serial_driver.md

Co-authored-by: Nick Brassel <nick@tzarc.org>

Co-authored-by: Nick Brassel <nick@tzarc.org>

docs/feature_split_keyboard.md

@@ -8,9 +8,20 @@ QMK Firmware has a generic implementation that is usable by any board, as well a
 
 For this, we will mostly be talking about the generic implementation used by the Let's Split and other keyboards. 
 
-!> ARM is not yet supported for Split Keyboards.  Progress is being made, but we are not quite there, yet. 
+!> ARM is not yet fully supported for Split Keyboards and has many limitations. Progress is being made, but we have not yet reached 100% feature parity.
 
 
+## Compatibility Overview
+
+| Transport                    | AVR                | ARM                |
+|------------------------------|--------------------|--------------------|
+| ['serial'](serial_driver.md) | :heavy_check_mark: | :white_check_mark: <sup>1</sup> |
+| I2C                          | :heavy_check_mark: |                    |
+
+Notes:
+
+1. Both hardware and software limitations are detailed within the [driver documentation](serial_driver.md).
+
 ## Hardware Configuration
 
 This assumes that you're using two Pro Micro-compatible controllers, and are using TRRS jacks to connect to two halves. 

docs/proton_c_conversion.md

@@ -36,7 +36,7 @@ These are defaults based on what has been implemented for ARM boards.
 | [RGB Lighting](feature_rgblight.md) | Disabled                                                                                                         |
 | [Backlight](feature_backlight.md)   | Forces [task driven PWM](feature_backlight.md#software-pwm-driver) until ARM can provide automatic configuration |
 | USB Host (e.g. USB-USB converter)   | Not supported (USB host code is AVR specific and is not currently supported on ARM)                              |
-| [Split keyboards](feature_split_keyboard.md) | Not supported yet                                                                                       |
+| [Split keyboards](feature_split_keyboard.md) | Partial - heavily dependent on enabled features                                                         |
 
 ## Manual Conversion
 

docs/serial_driver.md

@@ -1,7 +1,7 @@
 # 'serial' Driver
 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.
+?> 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
@@ -11,7 +11,7 @@ All drivers in this category have the following characteristics:
 
 |                   | AVR                | ARM                |
 |-------------------|--------------------|--------------------|
-| bit bang          | :heavy_check_mark: | Soon™              |
+| bit bang          | :heavy_check_mark: | :heavy_check_mark: |
 | USART Half-duplex |                    | :heavy_check_mark: |
 
 ## Driver configuration
@@ -35,6 +35,12 @@ Configure the driver via your config.h:
                                    //  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 is that this provides fast and accurate timings. `SOFT_SERIAL_PIN` 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:
 
@@ -56,4 +62,8 @@ Configure the hardware via your config.h:
 #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 turn on the SERIAL feature in your halconf.h and mcuconf.h
+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.