GPSのPPS信号でOCXOの周波数調整をDACで制御するための適応フィードバックの実装抜粋。see http://ttrftech.tumblr.com/post/131004700971/

gpsdo-dac-control-snippet.c

short phase_err;
short phase_err_int;
short duration_count;
short duration_exp;

#define REFERENCE 10000000L

static void
dac_control_init()
{
    phase_err = 0;
    phase_err_int = 0;
    duration_count = 0;
    duration_exp = 0;
}

static void
control_dac()
{
     long d = counter_delta - REFERENCE;
     if (d > 10 || d < -10) {
         phase_err = 0;
         phase_err_int = 0;
         duration_exp = 0;
         duration_count = 0;
         return;
     }

     phase_err += d;
     phase_err_int += phase_err;
     if (duration_count == 0) {
         short error = phase_err;
         short emod;
         if (duration_exp > 0 && duration_exp < 8) {
             if (phase_err_int < 0)
                 error -= (-phase_err_int) >> (duration_exp-1);
             else
                 error += phase_err_int >> (duration_exp-1);
         }

         if (duration_exp < 3)
             emod = error * 16;
         else if (duration_exp < 5)
             emod = error * 2;
         else if (duration_exp < 6)
             emod = error;
         else
             emod = error > 0 ? 1 : -1;

         if (error == 0) {
            if (duration_exp <= 12)
                duration_exp++;
         } else if (error > 1) {
             set_dac_value(dac_value - emod);
             if (error >= 4 && duration_exp > 0)
                 duration_exp--;
             phase_err = 0;
             phase_err_int = 0;
         } else if (error < -1) {
             set_dac_value(dac_value - emod);
             if (error <= -4 && duration_exp > 0)
                 duration_exp--;
             phase_err = 0;
             phase_err_int = 0;
         }
         duration_count = 1L << duration_exp;
    }
    duration_count--;
}