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[Docs] Update to I2C docs: Clarify address expectation and return values (#8413)

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* Clarifying docs for i2c

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* Update docs/i2c_driver.md

* Update docs/i2c_driver.md
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The I2C Master drivers used in QMK have a set of common functions to allow portability between MCUs. The I2C Master drivers used in QMK have a set of common functions to allow portability between MCUs.
## An important note on I2C Addresses
All of the addresses expected by this driver should be pushed to the upper 7 bits of the address byte. Setting
the lower bit (indicating read/write) will be done by the respective functions. Almost all I2C addresses listed
on datasheets and the internet will be represented as 7 bits occupying the lower 7 bits and will need to be
shifted to the left (more significant) by one bit. This is easy to do via the bitwise shift operator `<< 1`.
You can either do this on each call to the functions below, or once in your definition of the address. For example if your device has an address of `0x18`:
`#define MY_I2C_ADDRESS (0x18 << 1)`
See https://www.robot-electronics.co.uk/i2c-tutorial for more information about I2C addressing and other technical details.
## Available functions ## Available functions
|Function |Description | |Function |Description |
|------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------| |------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|`void i2c_init(void);` |Initializes the I2C driver. This function should be called once before any transaction is initiated. | |`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, uint16_t timeout);` |Starts an I2C transaction. Address is the 7-bit slave address without the direction bit. | |`i2c_status_t i2c_start(uint8_t address, uint16_t timeout);` |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. Returns status of transaction. | |`i2c_status_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. | |`i2c_status_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. | |`i2c_status_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. | |`i2c_status_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(void);` |Ends an I2C transaction. | |`i2c_status_t i2c_stop(void);` |Ends an I2C transaction. |
### Function Return ### Function Return
All the above functions, except `void i2c_init(void);` return the following truth table: All the above functions, except `void i2c_init(void);` return the following truth table:
|Return Value |Description | |Return Constant |Value|Description |
|---------------|---------------------------------------------------| |--------------------|-----|--------------------------------|
|0 |Operation executed successfully. | |`I2C_STATUS_SUCCESS`|0 |Operation executed successfully.|
|-1 |Operation failed. | |`I2C_STATUS_ERROR` |-1 |Operation failed. |
|-2 |Operation timed out. | |`I2C_STATUS_TIMEOUT`|-2 |Operation timed out. |
## AVR ## AVR