// 2 Wire Bus initialization
// Generate Acknowledge Pulse: On
// 2 Wire Bus Slave Address: TWI_ADDRESS
// General Call Recognition: On
// Bit Rate: 400,000 kHz
TWSR=0x00;
TWBR=0x0C;
TWAR=(TWI_ADDRESS<<1)|0x01;
TWCR=0x45;

// 2 Wire bus interrupt service routine
interrupt [TWI] void twi_isr(void) {

  switch((TWSR&0xF8)) {
    
    // START has been transmitted
    case 0x08:
    // Repeated START has been transmitted
    case 0x10:
      // Set TWI busy flag
      twi_flag_busy=1;
      // Write slave address and W to data field
      TWDR=(twi_message_t[twi_message_index][0x00]<<1)&0xFE;
      // Send address
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
    
    // Arbitration lost
    case 0x38:
      // Set TWI transmit flag
        twi_flag_t=1;
      // Abort this try 
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE  
    break;
    
    // SLA+W has been tramsmitted and ACK received
    case 0x18:
      // Reset message part
      twi_message_part=0x01;
      // Write data to data field
      TWDR=twi_message_t[twi_message_index][twi_message_part];
      // Send data
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
    
    // Data byte has been tramsmitted and ACK received
    case 0x28:      
      // Continue to send message if necessary
      if((twi_message_part<TWI_MESSAGE_SIZE-0x01)&&(twi_message_t[twi_message_index][0x00]<0x20)) {
        twi_message_part++;
        // Write data to data field
        TWDR=twi_message_t[twi_message_index][twi_message_part];
        // Send data
        TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
      }
      // Message was sent
      else {
        // Delete message from TWI buffer
        twi_message_t[twi_message_index][1]=0xFF;
        // Reset TWI busy flag
        twi_flag_busy=0;
        // Set TWI transmit flag
        twi_flag_t=1;
        // Send STOP
        TWCR=0xD5; // TWINT, TWEA, TWSTO, TWEN, TWIE 
      }    
    break;
    
    // Own SLA+W has been received and ACK has been returned
    case 0x60:
    // Arbitration lost in SLA+R/W as Master, own SLA+W has been received and ACK has been returned
    case 0x68:
    // General call address has been received and ACK has been returned
    case 0x70:
    // Arbitration lost in SLA+R/W as Master, general call address has been received and ACK has been returned
    case 0x78:
      // Set TWI busy flag
      twi_flag_busy=1;
      // Reset message part
      twi_message_part=0x00;
      // Reset TWINT
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
    
    // Previously addressed with own SLA+W, data has been received and ACK has been returned
    case 0x80:
    // Previously addressed with general call, data has been received and ACK has been returned
    case 0x90:
      // Set TWI busy flag
      twi_flag_busy=1;
      // Save data in message
      twi_message_temp_r[twi_message_part]=TWDR;
      if(twi_message_part<TWI_MESSAGE_SIZE-2)
        twi_message_part++;
      // Reset TWINT
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
    
    // A STOP condition or repeated START has been received while still addressed as slave
    case 0xA0:
      twi_message_part=0x00;
      if(twi_attach_r(twi_message_temp_r)) {
        // Set TWI receive flag
        twi_flag_r=1;
      }
      else
        PORTC.7=0;
      // Reset TWI busy flag
      twi_flag_busy=0;
      // Set TWI transmit flag
      twi_flag_t=1;
      // Reset TWINT
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
    
    // Bus error due to an illegal START or STOP
    //case 0x00:
      PORTC.7=0;
      // Reset TWINT
      TWCR=0xC5; // TWINT, TWEA, TWEN, TWIE
    break;
  
  }
}

// Send messages in TWI buffer
void twi_transmit(void) {
  
  // Local variables
  unsigned char index;
  
  if(!twi_flag_busy) {
    for(index=0x00;index<TWI_BUFFER_SIZE;index++) {
      if(twi_message_t[index][1]!=0xFF) {
        // Set index to message index in buffer
        twi_message_index=index;
        // Send START
        TWCR=0xE5; // TWINT, TWEA, TWSTA, TWEN, TWIE
        break;
      } 
    }
  }
  // Reset TWI transmit flag
  twi_flag_t=0;  

}