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88 lines
3.5 KiB
Markdown
88 lines
3.5 KiB
Markdown
# TWI/I2C Library for AVR
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## Description
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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:
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```c
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#define TW_SCL_PIN PORTC5
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#define TW_SDA_PIN PORTC4
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```
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## Usage
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There are 3 functions available for this library:
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```c
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void tw_init(twi_freq_mode_t twi_freq, bool pullup_en);
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```
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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.
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```c
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ret_code_t tw_master_transmit(uint8_t slave_addr, uint8_t* p_data, uint8_t len, bool repeat_start);
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```
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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.
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`ret_code_t` is return to the user to handle the state of transmission such as transmission success, arbitration lost, slave acknowledge, etc.
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```c
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ret_code_t tw_master_receive(uint8_t slave_addr, uint8_t* p_data, uint8_t len);
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```
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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`.
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## Example
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The example provided with this library is tested with MPU6050 to read the value of acceleration in X, Y and Z axis. In order to activate MPU6050, we have to write `0` to `PWR_MGMT_1` register as shown below:
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```c
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void mpu_init(void)
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{
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ret_code_t error_code;
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puts("Write 0 to PWR_MGMT_1 reg to wakeup MPU.");
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uint8_t data[2] = {PWR_MGMT_1, 0};
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error_code = tw_master_transmit(MPU6050_ADDR, data, sizeof(data), false);
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ERROR_CHECK(error_code);
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}
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```
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In this situation, we want to write the value but don't read back the response; therefore, `repeat_start` bit is set to `false`.
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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:
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```c
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void mpu_get_accel_raw(mpu_data_t* mpu_data)
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{
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ret_code_t error_code;
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/* 2 registers for each of accel x, y and z data */
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uint8_t data[6];
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data[0] = ACCEL_XOUT_H;
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error_code = tw_master_transmit(MPU6050_ADDR, data, 1, true);
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ERROR_CHECK(error_code);
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error_code = tw_master_receive(MPU6050_ADDR, data, sizeof(data));
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ERROR_CHECK(error_code);
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/* Default accel config +/- 2g */
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mpu_data->x = (int16_t)(data[0] << 8 | data[1]) / 16384.0;
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mpu_data->y = (int16_t)(data[2] << 8 | data[3]) / 16384.0;
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mpu_data->z = (int16_t)(data[4] << 8 | data[5]) / 16384.0;
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}
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```
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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.
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## Debugging I2C Transaction
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Found in `tw_master.h`, you can set `DEBUG_LOG` variable to `1` or `0` to enable or disable debug functionality of the library. However, this requires that you have UART library included in your project. |