Example of bit-banging the I²C Master protocol
Example of bit-banging the I²C Master protocol
Below is an example of bit-banging the I²C protocol as an I²C master. The example is written in pseudo C. It illustrates all of the I²C features described before (clock stretching, arbitration, start/stop bit, ack/nack)
// Hardware-specific support functions that MUST be customized: #define I2CSPEED 100 void I2C_delay() { volatile int v; int i; for (i=0; i < I2CSPEED/2; i++) v; } bool read_SCL(void); // Set SCL as input and return current level of line, 0 or 1 bool read_SDA(void); // Set SDA as input and return current level of line, 0 or 1 void clear_SCL(void); // Actively drive SCL signal low void clear_SDA(void); // Actively drive SDA signal low void arbitration_lost(void); bool started = false; // global data void i2c_start_cond(void) { if (started) { // if started, do a restart cond // set SDA to 1 read_SDA(); I2C_delay(); while (read_SCL() == 0) { // Clock stretching // You should add timeout to this loop } // Repeated start setup time, minimum 4.7us I2C_delay(); } if (read_SDA() == 0) { arbitration_lost(); } // SCL is high, set SDA from 1 to 0. clear_SDA(); I2C_delay(); clear_SCL(); started = true; } void i2c_stop_cond(void){ // set SDA to 0 clear_SDA(); I2C_delay(); // Clock stretching while (read_SCL() == 0) { // add timeout to this loop. } // Stop bit setup time, minimum 4us I2C_delay(); // SCL is high, set SDA from 0 to 1 if (read_SDA() == 0) { arbitration_lost(); } I2C_delay(); started = false; } // Write a bit to I2C bus void i2c_write_bit(bool bit) { if (bit) { read_SDA(); } else { clear_SDA(); } I2C_delay(); while (read_SCL() == 0) { // Clock stretching // You should add timeout to this loop } // SCL is high, now data is valid // If SDA is high, check that nobody else is driving SDA if (bit && read_SDA() == 0) { arbitration_lost(); } I2C_delay(); clear_SCL(); } // Read a bit from I2C bus bool i2c_read_bit(void) { bool bit; // Let the slave drive data read_SDA(); I2C_delay(); while (read_SCL() == 0) { // Clock stretching // You should add timeout to this loop } // SCL is high, now data is valid bit = read_SDA(); I2C_delay(); clear_SCL(); return bit; } // Write a byte to I2C bus. Return 0 if ack by the slave. bool i2c_write_byte(bool send_start, bool send_stop, unsigned char byte) { unsigned bit; bool nack; if (send_start) { i2c_start_cond(); } for (bit = 0; bit < 8; bit++) { i2c_write_bit((byte & 0x80) != 0); byte <<= 1; } nack = i2c_read_bit(); if (send_stop) { i2c_stop_cond(); } return nack; } // Read a byte from I2C bus unsigned char i2c_read_byte(bool nack, bool send_stop) { unsigned char byte = 0; unsigned bit; for (bit = 0; bit < 8; bit++) { byte = (byte << 1) | i2c_read_bit(); } i2c_write_bit(nack); if (send_stop) { i2c_stop_cond(); } return byte; }
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