/*
 *  AX.25 protocol frame structure:
 *  -------------------------------
 *
 *  Flag Field: 8 bits (1 byte)
 *  Header Field: 128 bits (16 bytes)
 *      {
 *    Destination Address: 56 bits (7 bytes)
 *    Source      Address: 56 bits (7 bytes)
 *    Control Bits       : 8  bits (1 byte )
 *    Protocol Identifier: 8  bits (1 byte )
 *  }
 *  Data Field: 32-2048 bits (4-256 bytes)
 *  Frame Checksum Field: 16 bits (2 bytes)
 *  Flag Field: 8 bits (1 byte)
 *
 */

#include <Crc16.h>
Crc16 crc;

unsigned char AX25_Flag = 0x7E;     // flag = 0b01111110;
unsigned char AX25_Header[16]  = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0xF0,0xF0};
unsigned char AX25_Data[256]   = {0};
unsigned char AX25_CheckSum[2] = {0};

int inByte = 0;
unsigned short value = 0;

void setup()
{
  // Serial1 : TX pin 18, RX pin 19
  // Serial2 : TX pin 16, RX pin 17
  // Serial1 will receive data from a device and resend it on Serial2 using AX25 protocol format

  Serial1.begin(9600);
  Serial2.begin(9600);

}

void loop()
{

  // Wait until data is received on Serial1 port
  while ( !(Serial1.available()) );
  // Read received data
  inByte = Serial1.read();

  //calculate CRC
  crc.clearCrc();
  crc.updateCrc(inByte);
  value = crc.getCrc();

  // "value" is 2 bytes long
  // Get the lower byte and put it into AX25_CheckSum[1]
  // Get the higher byte and put it into AX25_CheckSum[0]
  AX25_CheckSum[0] = ( value >> 8 ) & 0xFF;
  AX25_CheckSum[1] = value & 0xFF;

  AX25_Data[0] = inByte;

  // Send the data using AX.25 protocol format
  Serial2.write(AX25_Flag);
  Serial2.write(AX25_Header, 16);
  Serial2.write(AX25_Data, 256);
  Serial2.write(AX25_CheckSum, 2);
  Serial2.write(AX25_Flag);

  delay(500);

    while(true);
}
//Check routine taken from
//http://web.mit.edu/6.115/www/miscfiles/amulet/amulet-help/xmodem.htm
int calcrc(char *ptr, int count)
{
    int  crc;
    char i;
    crc = 0;
    while (--count >= 0)
    {
        crc = crc ^ (int) *ptr++ << 8;
        i = 8;
        do
        {
            if (crc & 0x8000)
                crc = crc << 1 ^ 0x1021;
            else
                crc = crc << 1;
        } while(--i);
    }
    return (crc);
}