ARM split - Add uart half duplex transport support (#7987)

* ARM split - Add uart half duplex transport support

* Fix for f103

* initial full duplex pass

* partially remove full duplex

* Correct speeds within driver docs

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

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

1 files changed, 59 insertions, 0 deletions

diff --git a/docs/serial_driver.md b/docs/serial_driver.md
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+++ b/docs/serial_driver.md
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+# '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.
+
+All drivers in this category have the following characteristics:
+* Provides data and signaling over a single conductor
+* Limited to single master, single slave
+
+## Supported Driver Types
+
+|                   | AVR                | ARM                |
+|-------------------|--------------------|--------------------|
+| bit bang          | :heavy_check_mark: | Soon™              |
+| USART Half-duplex |                    | :heavy_check_mark: |
+
+## Driver configuration
+
+### Bitbang
+Default driver, the absence of configuration assumes this driver. To configure it, add this to your rules.mk:
+
+```make
+SERIAL_DRIVER = bitbang
+```
+
+Configure the driver via your config.h:
+```c
+#define SOFT_SERIAL_PIN D0  // or D1, D2, D3, E6
+#define SELECT_SOFT_SERIAL_SPEED 1 // or 0, 2, 3, 4, 5
+                                   //  0: about 189kbps (Experimental only)
+                                   //  1: about 137kbps (default)
+                                   //  2: about 75kbps
+                                   //  3: about 39kbps
+                                   //  4: about 26kbps
+                                   //  5: about 20kbps
+```
+
+### 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:
+
+```make
+SERIAL_DRIVER = usart
+```
+
+Configure the hardware via your config.h:
+```c
+#define SOFT_SERIAL_PIN B6  // USART TX pin
+#define SELECT_SOFT_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 SD1 // USART driver of TX pin. default: SD1
+#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