qmk-firmware/docs/i2c_driver.md

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. 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(void);	Ends an I2C transaction.

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. You can use these defines to set your i2c pins:

Variable	Description	Default
I2C1_SCL_BANK	The bank of pins (GPIOA, GPIOB, GPIOC) to use for SCL	GPIOB
I2C1_SDA_BANK	The bank of pins (GPIOA, GPIOB, GPIOC) to use for SDA	GPIOB
I2C1_SCL	The pin number for the SCL pin (0-9)	6
I2C1_SDA	The pin number for the SDA pin (0-9)	7
I2C1_BANK (deprecated)	The bank of pins (GPIOA, GPIOB, GPIOC), superceded by I2C1_SCL_BANK, I2C1_SDA_BANK	GPIOB

STM32 MCUs allow for different timing parameters when configuring I2C. These can be modified using the following parameters, using https://www.st.com/en/embedded-software/stsw-stm32126.html as a reference:

Variable	Default
I2C1_TIMINGR_PRESC	15U
I2C1_TIMINGR_SCLDEL	4U
I2C1_TIMINGR_SDADEL	2U
I2C1_TIMINGR_SCLH	15U
I2C1_TIMINGR_SCLL	21U

STM32 MCUs allow for different "alternate function" modes when configuring GPIO pins. These are required to switch the pins used to I2C mode. See the respective datasheet for the appropriate values for your MCU.

Variable	Default
I2C1_SCL_PAL_MODE	4
I2C1_SDA_PAL_MODE	4

You can also overload the void i2c_init(void) function, which has a weak attribute. If you do this the configuration variables above will not be used. Please consult the datasheet of your MCU for the available GPIO configurations. The following is an example initialization function:

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
}