This library is tested on ATMega328p, but it should work with any AVR compatible microcontroller. To use with other type of ATMega, change the I2C pin definition in twi_master.h according the datasheet. In ATMega328p, I2C pins are describe as follows:

#define TW_SCL_PIN			PORTC5
#define TW_SDA_PIN			PORTC4

There are 3 functions available for this library:

void tw_init(twi_freq_mode_t twi_freq, bool pullup_en);

Initialize I2C with predefined frequency and enable internal pull-up resistors. There are 3 mode of frequency to choose from: TW_FREQ_100K, TW_FREQ_250K and TW_FREQ_400K. Then set pullup_en bit to true to enable internal pull-up resistors on SCL and SDA pin, or set it to false if external pull-up is used.

ret_code_t tw_master_transmit(uint8_t slave_addr, uint8_t* p_data, uint8_t len, bool repeat_start);

This function transmits data bytes to a desired slave address. If repeat_start bit is set to true, I2C module will send REPEATED START condition instead of STOP condition. This is useful in multi-master environment where one Master sends a command and wait for response from slave without having to release the Bus, thus preventing the other Master from taking control of the Bus in between the transaction.

ret_code_t is return to the user to handle the state of transmission such as transmission success, arbitration lost, slave acknowledge, etc.

ret_code_t tw_master_receive(uint8_t slave_addr, uint8_t* p_data, uint8_t len);

This functions receives the data bytes from the desired slave address and return ret_code_t for error handling as have been discussed in tw_master_transmit.

The example provided with this library is tested with MPU6050 to read the value of acceleration in X, Y and Z axis. This use this UART Library to print the result to the terminal but you can use any library and modified the library accordingly.

In order to activate MPU6050, we have to write 0 to PWR_MGMT_1 register as shown below:

void mpu_init(void)
{
	ret_code_t error_code;
	puts("Write 0 to PWR_MGMT_1 reg to wakeup MPU.");
	uint8_t data[2] = {PWR_MGMT_1, 0};
	error_code = tw_master_transmit(MPU6050_ADDR, data, sizeof(data), false);
	ERROR_CHECK(error_code);
}

In this situation, we want to write the value but don't read back the response; therefore, repeat_start bit is set to false.

To read the value of accelerometer, we have to transmit the register that we want to read, which is ACCEL_XOUT_H then wait for the response using tw_master_receive as shown below:

void mpu_get_accel_raw(mpu_data_t* mpu_data)
{
	ret_code_t error_code;
	/* 2 registers for each of accel x, y and z data */
	uint8_t data[6];
	
	data[0] = ACCEL_XOUT_H;
	error_code = tw_master_transmit(MPU6050_ADDR, data, 1, true);
	ERROR_CHECK(error_code);
	
	error_code = tw_master_receive(MPU6050_ADDR, data, sizeof(data));
	ERROR_CHECK(error_code);
	
	/* Default accel config +/- 2g */
	mpu_data->x = (int16_t)(data[0] << 8 | data[1]) / 16384.0;
	mpu_data->y = (int16_t)(data[2] << 8 | data[3]) / 16384.0;
	mpu_data->z = (int16_t)(data[4] << 8 | data[5]) / 16384.0;
}

In this situation, since we write and then want to wait for the response from MPU, repeat_start bit is set to true to prevent other Master from pulling the Bus in between write and read process. This step is, however, not necessary if there is only one Master on the Bus.

Found in tw_master.h, you can set DEBUG_LOG variable to 1 or 0 to enable or disable debug functionality of the library. This will print each and every I2C transaction and current state of I2C Bus.