Arduino source code: Adjust AD9850 DDS frequency offset to zero-beat with GPS-locked oscillator

gistfile1.txt

// Zero-beat oscillator to 10 MHz GPS-locked reference

  printf("Zero beat DDS oscillator against GPS\n");

  TempFreq.fx_64 = CALFREQ;

  u8x8.clearDisplay();
  byte ln = 0;
  u8x8.drawString(0,ln++,"10 MHz Zero Beat");
  u8x8.drawString(0,ln++,"<-  Joystick  ->");
  u8x8.drawString(0,ln++,"  Button = set  ");

  int32_t OldOffset = OscOffset;

  while (analogRead(PIN_JOYBUTTTON) > 500) {

    int ai = analogRead(PIN_JOY_Y) - 512;                 // totally ad-hoc axes
    if (ai < -100) {
      OscOffset += 1;
    }
    else if (ai > 100) {
      OscOffset -= 1;
    }

    if (OscOffset != OldOffset) {
      ln = 4;
      sprintf(Buffer,"Offset %8ld",OscOffset);
      u8x8.drawString(0,ln++,Buffer);

      CalcOscillator(OscOffset);                            // recalculate constants

      TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz);  // recalculate delta phase count

      WriteDDS(TempCount.fx_32.high);                       // should be 10 MHz out!

      OldOffset = OscOffset;
    }

    Wire.requestFrom(LM75_ADDR,2);
    Temperature.fx_32.high = Wire.read();
    Temperature.fx_32.low = (uint32_t)Wire.read() << 24;
    PrintFixedPtRounded(Buffer,Temperature,3);
    ln = 7;
    u8x8.drawString(0,ln,"DDS Temp");
    u8x8.drawString(16-strlen(Buffer),ln++,Buffer);

    delay(100);
  }

  printf("Oscillator offset: %ld\n",OscOffset);

More details on my blog at http://wp.me/poZKh-6Sz