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:

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

?> 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

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.

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

I2Cv1 :id=i2cv1

• STM32F1xx

• STM32F2xx

• STM32F4xx

• STM32L0xx

• STM32L1xx

See this page for the I2Cv1 configuration structure.

I2Cv2 :id=i2cv2

• STM32F0xx

• STM32F3xx

• STM32F7xx

• STM32L4xx

See this page for the I2Cv2 configuration structure.

Functions :id=functions

void i2c_init(void)

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)

Start an I2C transaction.

Arguments

• uint8_t address

  The 7-bit I2C address of the device (ie. without the read/write bit - this will be set automatically).

• uint16_t timeout

  The time in milliseconds to wait for a response from the target device.

Return Value

I2C_STATUS_TIMEOUT if the timeout period elapses, I2C_STATUS_ERROR if some other error occurs, otherwise I2C_STATUS_SUCCESS.

i2c_status_t i2c_transmit(uint8_t address, uint8_t *data, uint16_t length, uint16_t timeout)

Send multiple bytes to the selected I2C device.

Arguments

• uint8_t address

  The 7-bit I2C address of the device.

• uint8_t *data

  A pointer to the data to transmit.

• uint16_t length

  The number of bytes to write. Take care not to overrun the length of data.

• uint16_t timeout

  The time in milliseconds to wait for a response from the target device.

Return Value

I2C_STATUS_TIMEOUT if the timeout period elapses, I2C_STATUS_ERROR if some other error occurs, otherwise I2C_STATUS_SUCCESS.

i2c_status_t i2c_receive(uint8_t address, uint8_t* data, uint16_t length, uint16_t timeout)

Receive multiple bytes from the selected SPI device.

Arguments

• uint8_t address

  The 7-bit I2C address of the device.

• uint8_t *data

  A pointer to the buffer to read into.

• uint16_t length

  The number of bytes to read. Take care not to overrun the length of data.

• uint16_t timeout

  The time in milliseconds to wait for a response from the target device.

Return Value

I2C_STATUS_TIMEOUT if the timeout period elapses, I2C_STATUS_ERROR if some other error occurs, otherwise I2C_STATUS_SUCCESS.

i2c_status_t i2c_writeReg(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Writes to a register on the I2C device.

Arguments

• uint8_t devaddr

  The 7-bit I2C address of the device.

• uint8_t regaddr

  The register address to write to.

• uint8_t *data

  A pointer to the data to transmit.

• uint16_t length

  The number of bytes to write. Take care not to overrun the length of data.

• uint16_t timeout

  The time in milliseconds to wait for a response from the target device.

Return Value

I2C_STATUS_TIMEOUT if the timeout period elapses, I2C_STATUS_ERROR if some other error occurs, otherwise I2C_STATUS_SUCCESS.

i2c_status_t i2c_readReg(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Reads from a register on the I2C device.

Arguments

• uint8_t devaddr

  The 7-bit I2C address of the device.

• uint8_t regaddr

  The register address to read from.

• uint16_t length

  The number of bytes to read. Take care not to overrun the length of data.

• uint16_t timeout

  The time in milliseconds to wait for a response from the target device.

Return Value

I2C_STATUS_TIMEOUT if the timeout period elapses, I2C_STATUS_ERROR if some other error occurs, otherwise I2C_STATUS_SUCCESS.

i2c_status_t i2c_stop(void)

Stop the current I2C transaction.