I2C driver docs (#4298)

* I2C driver docs

- Added I2C driver docs.
- Changed arm master i2c driver to have a weak i2c_init as it is impossible to account for all possible port configuration in the STM32 world.

* Update docs/i2c_driver.md

Co-Authored-By: yiancar <yiangosyiangou@cytanet.com.cy>

* Update docs/i2c_driver.md

Co-Authored-By: yiancar <yiangosyiangou@cytanet.com.cy>

* Update docs/i2c_driver.md

Co-Authored-By: yiancar <yiangosyiangou@cytanet.com.cy>

* Update docs/i2c_driver.md

Co-Authored-By: yiancar <yiangosyiangou@cytanet.com.cy>

* GPIO release fix

3 files changed, 85 insertions, 0 deletions

diff --git a/docs/_sidebar.md b/docs/_sidebar.md
index 2c57380122..67e81422a1 100644
--- a/docs/_sidebar.md
+++ b/docs/_sidebar.md
@@ -79,6 +79,7 @@
   * [Hand Wiring Guide](hand_wire.md)
   * [ISP Flashing Guide](isp_flashing_guide.md)
   * [ARM Debugging Guide](arm_debugging.md)
+  * [I2C Driver](i2c_driver.md)
 
 * For a Deeper Understanding
   * [How Keyboards Work](how_keyboards_work.md)
diff --git a/docs/_summary.md b/docs/_summary.md
index 2c57380122..67e81422a1 100644
--- a/docs/_summary.md
+++ b/docs/_summary.md
@@ -79,6 +79,7 @@
   * [Hand Wiring Guide](hand_wire.md)
   * [ISP Flashing Guide](isp_flashing_guide.md)
   * [ARM Debugging Guide](arm_debugging.md)
+  * [I2C Driver](i2c_driver.md)
 
 * For a Deeper Understanding
   * [How Keyboards Work](how_keyboards_work.md)
diff --git a/docs/i2c_driver.md b/docs/i2c_driver.md
new file mode 100644
index 0000000000..f69eb5a9da
--- /dev/null
+++ b/docs/i2c_driver.md
@@ -0,0 +1,83 @@
+# I2C Master Driver
+
+The I2C Master drivers used in QMK have a set of common functions to allow portability between MCUs.
+
+## Available functions
+
+|Function                                                                                                          |Description                                                                                                                                                                  |
+|------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|`void i2c_init(void);`                                                                                            |Initializes the I2C driver. This function should be called once before any transaction is initiated.                                                                         |
+|`uint8_t i2c_start(uint8_t address);`                                                                             |Starts an I2C transaction. Address is the 7-bit slave address without the direction bit.                                                                                     |
+|`uint8_t i2c_transmit(uint8_t address, uint8_t* data, uint16_t length, uint16_t timeout);`                        |Transmit data over I2C. Address is the 7-bit slave address without the direction.                                                                                            |
+|`uint8_t i2c_transmit(uint8_t address, uint8_t* data, uint16_t length, uint16_t timeout);`                        |Transmit data over I2C. Address is the 7-bit slave address without the direction. Returns status of transaction.                                                             |
+|`uint8_t i2c_receive(uint8_t address, uint8_t* data, uint16_t length, uint16_t timeout);`                         |Receive data over I2C. Address is the 7-bit slave address without the direction. Saves number of bytes specified by `length` in `data` array. Returns status of transaction. |
+|`uint8_t i2c_writeReg(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout);`       |Same as the `i2c_transmit` function but `regaddr` sets where in the slave the data will be written.                                                                          |
+|`uint8_t i2c_readReg(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout);`        |Same as the `i2c_receive` function but `regaddr` sets from where in the slave the data will be read.                                                                         |
+|`uint8_t i2c_stop(uint16_t timeout);`                                                                             |Stops the I2C driver.                                                                                                                                                        |
+
+### Function Return
+
+All the above functions, except `void i2c_init(void);` return the following truth table:
+
+|Return Value   |Description                                        |
+|---------------|---------------------------------------------------|
+|0              |Operation executed successfully.                   |
+|-1             |Operation failed.                                  |
+|-2             |Operation timed out.                               |
+
+
+## AVR
+
+### Configuration
+
+The following defines can be used to configure the I2C master driver.
+
+|Variable          |Description                                        |Default|
+|------------------|---------------------------------------------------|-------|
+|`#F_SCL`          |Clock frequency in Hz                              |400KHz |
+|`#Prescaler`      |Divides master clock to aid in I2C clock selection |1      |
+
+AVRs usually have set GPIO which turn into I2C pins, therefore no further configuration is required.
+
+## ARM
+
+For ARM the Chibios I2C HAL driver is under the hood.
+This section assumes an STM32 MCU.
+
+### Configuration
+
+The configuration for ARM MCUs can be quite complex as often there are multiple I2C drivers which can be assigned to a variety of ports.
+
+Firstly the `mcuconf.h` file must be setup to enable the necessary hardware drivers.
+
+|Variable                      |Description                                                                        |Default|
+|------------------------------|------------------------------------------------------------------------------------|-------|
+|`#STM32_I2C_USE_XXX`          |Enable/Disable the hardware driver XXX (each driver should be explicitly listed)    |FALSE  |
+|`#STM32_I2C_BUSY_TIMEOUT`     |Time in ms until the I2C command is aborted if no response is received              |50     |
+|`#STM32_I2C_XXX_IRQ_PRIORITY` |Interrupt priority for hardware driver XXX (THIS IS AN EXPERT SETTING)              |10     |
+|`#STM32_I2C_USE_DMA`          |Enable/Disable the ability of the MCU to offload the data transfer to the DMA unit  |TRUE   |
+|`#STM32_I2C_XXX_DMA_PRIORITY` |Priority of DMA unit for hardware driver XXX (THIS IS AN EXPERT SETTING)            |1      |
+
+Secondly, in the `halconf.h` file, `#define HAL_USE_I2C` must be set to `TRUE`. This allows ChibiOS to load its I2C driver.
+
+Lastly, we need to assign the correct GPIO pins depending on the I2C hardware driver we want to use.
+
+By default the I2C1 hardware driver is assumed to be used. If another hardware driver is used,  `#define I2C_DRIVER I2CDX` should be added to the `config.h` file with X being the number of hardware driver used. For example is I2C3 is enabled, the `config.h` file should contain `#define I2C_DRIVER I2CD3`. This aligns the QMK I2C driver with the Chibios I2C driver.
+
+STM32 MCUs allows a variety of pins to be configured as I2C pins depending on the hardware driver used. By default B6 and B7 are set to I2C.
+
+This can be changed by declaring the `i2c_init` function which intentionally has a weak attribute. Please consult the datasheet of your MCU for the available GPIO configurations. The following is an example initialization function:
+
+```C
+void i2c_init(void)
+{
+  setPinInput(B6); // Try releasing special pins for a short time
+  setPinInput(B7);
+  chThdSleepMilliseconds(10); // Wait for the release to happen
+
+  palSetPadMode(GPIOB, 6, PAL_MODE_ALTERNATE(4) | PAL_STM32_OTYPE_OPENDRAIN | PAL_STM32_PUPDR_PULLUP); // Set B6 to I2C function
+  palSetPadMode(GPIOB, 7, PAL_MODE_ALTERNATE(4) | PAL_STM32_OTYPE_OPENDRAIN | PAL_STM32_PUPDR_PULLUP); // Set B7 to I2C function
+}
+```
+
+