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;
}