I2C Master library

Table of contents

1. What is this library?
2. Contents of this library
3. Supported devices
4. Functions in this library

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1. What is this library?

This library is an I2C master library which uses the TWI peripheral inside the AVR microcontroller to establish connections using the I2C protocol

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2. Contents of this library

The library contains the following files:

• main.c
• I2C_master.c
• I2C_master.h

main.c

This is a piece of example code which uses this library to read out an HMC5338L I2C magnetometer.

I2C_master.c

This file contains all the function declarations to setup and work with the TWI hardware peripheral inside the AVR.

Make sure you add this source file to your Makefile!

I2C_master.h

This file contains the function prototypes and the definition of the Read / Write bit (0 = read, 1 = write)

This file has to be included in your source file

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3. Supported devices

Though I have only tested this library on an ATmega328P it should be running on all major ATmega AVRs like:

• ATmega644
• ATmega32
• ATmega16
• ATmega328/P
• ATmega168/P
• ATmega88/P
• ATmega44/P
• ATmega8

If your device is not supported you can probably adapt this library easily to your needs by having a look at the your device's data sheet and changing the register names appropriately

The ATtiny range of microcontrolles is not supported as they only provide a USI peripheral which is completely different from the TWI peripheral used on the other controllers

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4. Functions in this library

• void i2c_init(void)
• uint8_t i2c_start(uint8_t address)
• uint8_t i2c_write(uint8_t data)
• uint8_t i2c_read_ack(void)
• uint8_t i2c_read_nack(void)
• uint8_t i2c_transmit(uint8_t address, uint8_t* data, uint16_t length)
• uint8_t i2c_receive(uint8_t address, uint8_t* data, uint16_t length)
• void i2c_stop(void)

void I2C_init(void)

This function needs to be called only once to set up the correct SCL frequency for the bus

uint8_t I2C_start(uint8_t address)

This function needs to be called any time a connection to a new slave device should be established. The function returns 1 if an error has occurred, otherwise it returns 0.

The syntax to start a operation write to a device is either: I2C_start(SLAVE_ADDRESS+I2C_WRITE); or I2C_start(SLAVE_WRITE_ADDRESS);

The syntax to start a read operation from a device is either: I2C_start(SLAVE_ADDRESS+I2C_READ); or I2C_start(SLAVE_READ_ADDRESS);

uint8_t I2C_write(uint8_t data)

This function is used to write data to the currently active device. The only parameter this function takes is the 8 bit unsigned integer to be sent. The function returns 1 if an error has occurred, otherwise it returns 0.

uint8_t I2C_read_ack(void)

This function is used to read one byte from a device and request another byte of data after the transmission is complete by sending the acknowledge bit. This function returns the received byte.

uint8_t I2C_read_nack(void)

This function is used to read one byte from a device an then not requesting another byte and therefore stopping the current transmission. This function returns the received byte.

uint8_t i2c_transmit(uint8_t address, uint8_t* data, uint16_t length);

This function is used to transmit [length] number of bytes to an I2C device with the given I2C address from [data]. The [address] passed to this function is the 7-bit slave address, left shifted by one bit (i.e. 7-bit slave address is 0x2F -> (0x2F)<<1 = 0x5E)

uint8_t i2c_receive(uint8_t address, uint8_t* data, uint16_t length);

This function is used to read [length] number of bytes from the I2C device with the given I2C address into the [data]. The [address] passed to this function is the 7-bit slave address, left shifted by one bit (i.e. 7-bit slave address is 0x2F -> (0x2F)<<1 = 0x5E)

void I2C_stop(void)

This function disables the TWI peripheral completely and therefore disconnects the device from the bus.