I2C Master Driver

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

Usage

In most cases, the I2C Master driver code is automatically included if you are using a feature or driver which requires it, such as OLED.

However, if you need to use the driver standalone, add the following to your rules.mk:

I2C_DRIVER_REQUIRED = yes

You can then call the I2C API by including i2c_master.h in your code.

I2C Addressing

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

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

TIP

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

ChibiOS/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, then modify your board's mcuconf.h to enable the peripheral you've chosen:

halconf.h

#pragma once

#define HAL_USE_I2C TRUE

#include_next <halconf.h>

mcuconf.h

#pragma once

#include_next <mcuconf.h>

#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

• 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

• 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

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) {
    gpio_set_pin_input(B6); // Try releasing special pins for a short time
    gpio_set_pin_input(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_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 I2C 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_write_register(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Writes to a register with an 8-bit address 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_write_register16(uint8_t devaddr, uint16_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Writes to a register with a 16-bit address (big endian) on the I2C device.

Arguments

• uint8_t devaddr
  The 7-bit I2C address of the device.
• uint16_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_read_register(uint8_t devaddr, uint8_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Reads from a register with an 8-bit address 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_read_register16(uint8_t devaddr, uint16_t regaddr, uint8_t* data, uint16_t length, uint16_t timeout)

Reads from a register with a 16-bit address (big endian) on the I2C device.

Arguments

• uint8_t devaddr
  The 7-bit I2C address of the device.
• uint16_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_ping_address(uint8_t address, uint16_t timeout)

Pings the I2C bus for a specific address.

On ChibiOS a "best effort" attempt is made by reading a single byte from register 0 at the requested address. This should generally work except for I2C devices that do not not respond to a register 0 read request, which will result in a false negative result (unsucessful response to ping attempt).

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

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.