AFSKEncode.cpp

#include "AFSKEncode.h"
#include "Modem.h"

#define ACCUMULATOR_BITS 24 // This is 2^10 bits used from accum
#undef PROGMEM
#define PROGMEM __attribute__((section(".progmem.data")))
const uint8_t PROGMEM sinetable[256] = {
  128,131,134,137,140,143,146,149,152,156,159,162,165,168,171,174,
  176,179,182,185,188,191,193,196,199,201,204,206,209,211,213,216,
  218,220,222,224,226,228,230,232,234,236,237,239,240,242,243,245,
  246,247,248,249,250,251,252,252,253,254,254,255,255,255,255,255,
  255,255,255,255,255,255,254,254,253,252,252,251,250,249,248,247,
  246,245,243,242,240,239,237,236,234,232,230,228,226,224,222,220,
  218,216,213,211,209,206,204,201,199,196,193,191,188,185,182,179,
  176,174,171,168,165,162,159,156,152,149,146,143,140,137,134,131,
  128,124,121,118,115,112,109,106,103,99, 96, 93, 90, 87, 84, 81, 
  79, 76, 73, 70, 67, 64, 62, 59, 56, 54, 51, 49, 46, 44, 42, 39, 
  37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 18, 16, 15, 13, 12, 10, 
  9,  8,  7,  6,  5,  4,  3,  3,  2,  1,  1,  0,  0,  0,  0,  0,  
  0,  0,  0,  0,  0,  0,  1,  1,  2,  3,  3,  4,  5,  6,  7,  8,  
  9,  10, 12, 13, 15, 16, 18, 19, 21, 23, 25, 27, 29, 31, 33, 35, 
  37, 39, 42, 44, 46, 49, 51, 54, 56, 59, 62, 64, 67, 70, 73, 76, 
  79, 81, 84, 87, 90, 93, 96, 99, 103,106,109,112,115,118,121,124
};

#define AFSK_SPACE 0
#define AFSK_MARK  1

#define REFCLK 9600
//#define REFCLK 31372.54902
//#define REFCLK (16000000.0/510.0)
//#define REFCLK 31200.0
// 2200Hz = pow(2,32)*2200.0/refclk
// 1200Hz = pow(2,32)*1200.0/refclk
static const unsigned long toneStep[2] = {
  pow(2,32)*2200.0/REFCLK,
  pow(2,32)*1200.0/REFCLK
};

// Set to an arbitrary frequency
void AFSKEncode::setFreq(unsigned long freq, byte vol) {
  unsigned long newStep = pow(2,32)*freq/REFCLK;
  rStep = newStep; // Atomic? (ish)
}

// This allows a programmatic way to tune the output tones
static const byte toneVolume[2] = {
  255,
  255
};

#define T_BIT ((unsigned int)(REFCLK/1200))

void AFSKEncode::process() {
  // Check what clock pulse we're on
  if(bitClock == 0) {
    // We are onto our next bit timing
    
    // We're on the start of a byte position, so fetch one
    if(bitPosition == 0) {
      if(preamble) { // Still in preamble
        currentByte = HDLC_PREAMBLE;
        --preamble; // Decrement by one
      } else {
        if(!packet) { // We aren't on a packet, grab one
          // Unless we already sent enough
          if(maxTx-- == 0) {
            stop();
            lastTxEnd = millis();
            return;
          }
          packet = pBuf.getPacket();
          if(!packet) { // There actually weren't any
            stop(); // Stop transmitting and return
            lastTxEnd = millis();
            return;
          }
          lastTx = millis();
          currentBytePos = 0;
        }
        
        // We ran out of actual data, provide an HDLC frame (idle)
        if(currentBytePos++ == packet->len) {
          pBuf.freePacket(packet);
          packet = pBuf.getPacket(); // Get the next, if any
          currentBytePos = 0;
          currentByte = HDLC_FRAME;
          hdlc = true;
        } else {
          // Grab the next byte
          currentByte = packet->getByte(); //[currentBytePos++];
          if(currentByte == HDLC_ESCAPE) {
            currentByte = packet->getByte(); //[currentBytePos++];
            hdlc = true;
          } else {
            hdlc = false;
          }
        }
      }
    }
    
    // Pickup the last bit
    currentBit = currentByte & 0x1;    
  
    if(lastZero == 5) {
      currentBit = 0; // Force a 0 bit output
    } else {
      currentByte >>= 1; // Bit shift it right, for the next round
      ++bitPosition; // Note our increase in position
    }
    
    // To handle NRZI 5 bit stuffing, count the bits
    if(!currentBit || hdlc)
      lastZero = 0;
    else
      ++lastZero;

     // NRZI and AFSK uses toggling 0s, "no change" on 1
     // So, if not a 1, toggle to the opposite tone
     if(!currentBit)
       currentTone = !currentTone;
  }
  
  // Advance the bitclock here, to let first bit be sent early 
  if(++bitClock == T_BIT)
    bitClock = 0;
  
  accumulator += toneStep[currentTone];
  uint8_t phAng = (accumulator >> ACCUMULATOR_BITS);
  /*if(toneVolume[currentTone] != 255) {
    OCR2B = pwm * toneVolume[currentTone] / 255;
  } else {*/
    // No volume scaling required
    OCR2B = pgm_read_byte_near(sinetable + phAng);
  /*}*/
}

bool AFSKEncode::start() {
  if(!done || sending)
    return false;
    
  if(randomWait > millis())
    return false;
    
  accumulator = 0;
  // First real byte is a frame
  currentBit = 0;
  lastZero = 0;
  bitPosition = 0;
  bitClock = 0;
  preamble = 23; // 6.7ms each, 23 = 153ms
  done = false;
  hdlc = true;
  packet = 0x0; // No initial packet, find in the ISR
  currentBytePos = 0;
  maxTx = 3;
  sending = true;
  return true;
}

void AFSKEncode::stop() {
  randomWait = 0;
  sending = false;
  done = true;
}