I2C Master Driver :id=i2c-master-driver

The I2C Master drivers used in QMK have a set of common functions to allow portability between MCUs.

I2C Addressing :id=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.

AVR Configuration :id=avr-configuration

The following defines can be used to configure the I2C master driver:

`config.h` Override	Description	Default
`F_SCL`	Clock frequency in Hz	`400000`

No further setup is required - just connect the SDA and SCL pins of your I2C devices to the matching pins on the MCU:

MCU	`SCL`	`SDA`
ATmega16/32U4	`D0`	`D1`
AT90USB64/128	`D0`	`D1`
ATmega32A	`C0`	`C1`
ATmega328/P	`C5`	`C4`

?> The ATmega16/32U2 does not possess I2C functionality, and so cannot use this driver.

ChibiOS/ARM Configuration :id=arm-configuration

You'll need to determine which pins can be used for I2C -- a an example, STM32 parts generally have multiple I2C peripherals, labeled I2C1, I2C2, I2C3 etc.

To enable I2C, modify your board's halconf.h to enable I2C:

#define HAL_USE_I2C TRUE

Then, modify your board's mcuconf.h to enable the peripheral you've chosen, for example:

#undef STM32_I2C_USE_I2C2
#define STM32_I2C_USE_I2C2 TRUE

`mcuconf.h` Setting	Description	Default
`STM32_I2C_BUSY_TIMEOUT`	Time in milliseconds 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`

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` Overrride	Description	Default
`I2C_DRIVER`	I2C peripheral to use - I2C1 -> `I2CD1`, I2C2 -> `I2CD2` etc.	`I2CD1`
`I2C1_SCL_PIN`	The pin definition for SCL	`B6`
`I2C1_SCL_PAL_MODE`	The alternate function mode for SCL	`4`
`I2C1_SDA_PIN`	The pin definition for SDA	`B7`
`I2C1_SDA_PAL_MODE`	The alternate function mode for SDA	`4`

The following configuration values depend on the specific MCU in use.

I2Cv1 :id=arm-configuration-i2cv1

STM32F1xx
STM32F2xx
STM32F4xx
STM32L0xx
STM32L1xx

See this page for the I2Cv1 configuration structure.

`config.h` Override	Default
`I2C1_OPMODE`	`OPMODE_I2C`
`I2C1_CLOCK_SPEED`	`100000`
`I2C1_DUTY_CYCLE`	`STD_DUTY_CYCLE`

I2Cv2 :id=arm-configuration-i2cv2

STM32F0xx
STM32F3xx
STM32F7xx
STM32L4xx

See this page for the I2Cv2 configuration structure.

`config.h` Override	Default
`I2C1_TIMINGR_PRESC`	`0U`
`I2C1_TIMINGR_SCLDEL`	`7U`
`I2C1_TIMINGR_SDADEL`	`0U`
`I2C1_TIMINGR_SCLH`	`38U`
`I2C1_TIMINGR_SCLL`	`129U`

API :id=api

`void i2c_init(void)` :id=api-i2c-init

Initialize the I2C driver. This function must be called only once, before any of the below functions can be called.

This function is weakly defined, meaning it can be overridden if necessary for your particular use case:

void i2c_init(void) {
    setPinInput(B6); // Try releasing special pins for a short time
    setPinInput(B7);
    wait_ms(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
}

`i2c_status_t i2c_start(uint8_t address, uint16_t timeout)` :id=api-i2c-start

Start an I2C transaction.