Below is some code I cobbled together to produce GPS lock a Sillicon Labs DDS. This was inspired by a need George M1GEO had for a frequency stable DDS. This work is a derivative of the code found here, and addapted to suit our needs and the DDS we had to hand.

#include <Wire.h>
#include <si5351.h>
#include <avr/interrupt.h>
#include <avr/io.h>

// Set up MCU pins
#define ppsPin                   2
#define Resolution               6
#define Offset                  A2

unsigned int tcount = 2;
unsigned long MeasuredFreq = 250000000;
unsigned long TestFreq= 250000000;
unsigned long mult = 0, Freq_1, Freq_2;
static bool s_GpsOneSecTick = false;

int32_t currentCal = 0;

Si5351 si5351;


void PPSinterrupt() {
  s_GpsOneSecTick = true;        // New second by GPS.
  tcount++;
  if (tcount == 4)               // Start counting the 2.5 MHz $
  {
    TCCR1B = 7;                   //Clock on rising edge of pin 5
  }
  else if (tcount == 14)         //The 40 second gate time$
  {
    Serial.println("mult = "+mult);
    TCCR1B = 0;                                  //Turn off counter
    MeasuredFreq = mult * 0x10000 + TCNT1; //Calculate correction factor
    MeasuredFreq *= 10;
    TCNT1 = 0;                                   //Reset count to zero
    mult = 0;
    tcount = 0;                                  //Reset the seconds counter
  }
}



// Timer 1 overflow intrrupt vector.
ISR(TIMER1_OVF_vect)
{
  mult++;                                          //Increment multiplier
  TIFR1 = (1 << TOV1);                             //Clear overlow flag
}



void setup() {


  Serial.begin(9600);
  Serial.println("Initialising");

  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
  si5351.set_freq(TestFreq, SI5351_CLK0);

  //Set up Timer1 as a frequency counter - input at pin 5
  TCCR1B = 0;                                    //Disable Timer5 during setup
  TCCR1A = 0;                                    //Reset
  TCNT1  = 0;                                    //Reset counter to zero
  TIFR1  = 1;                                    //Reset overflow
  TIMSK1 = 1;                                    //Turn on overflow flag

  // Inititalize GPS 1pps input
  pinMode(ppsPin, INPUT);

  // Set 1PPS pin 2 for external interrupt input
  attachInterrupt(digitalPinToInterrupt(ppsPin), PPSinterrupt, RISING);


}

void loop() {
  int64_t offset = (MeasuredFreq - TestFreq) * 1e3;
  int32_t ppb = TestFreq/1e6;
  ppb = (int32_t) offset / ppb;
  if (s_GpsOneSecTick) {
    s_GpsOneSecTick = false;

    if (tcount == 1) {

      Serial.println("Test Freq:");
      Serial.println(TestFreq);
      Serial.println("Xtal Measured at:");
      Serial.println(MeasuredFreq);

      Serial.println("ppb");
      Serial.println(ppb);

      // Update the SI5351A after every GPS correction
      currentCal += ppb;
      si5351.set_correction(currentCal, SI5351_PLL_INPUT_XO);
      si5351.set_freq(TestFreq, SI5351_CLK0);

    }
  } else {
    
  }

}