//waveform generator

// hacked from https://www.instructables.com/Arduino-Waveform-Generator-1/

#include <Adafruit_MCP4728.h>
#include <Wire.h>

Adafruit_MCP4728 mcp;

#define nsamp 256
#define dacmax 256

const byte nclk = 200; // a guess
long int freq; //frequency in Hz
long unsigned int phase;
long unsigned int phase_inc;

void setup() {

//   // Set a timer interrupt for the phase accumulation
  cli();//stop interrupts


//set timer1 interrupt at 44Hz
  TCCR1A = 0;// set entire TCCR1A register to 0
  TCCR1B = 0;// same for TCCR1B
  TCNT1  = 0;//initialize counter value to 0
  // set compare match register for 1hz increments
  OCR1A = 500; // = (16 * 10**6) / (44000) - 1
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS10 and CS12 bits for 1024 prescaler
  TCCR1B |= (1 << CS12) | (1 << CS10);  
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);

  sei();//allow interrupts

  Serial.begin(115200);
  while (!Serial)
    delay(10); // will pause Zero, Leonardo, etc until serial console opens


  // Try to initialize!

  if (!mcp.begin(0x64)) {
    Serial.println("Failed to find MCP4728 chip");
    while (1) {
      delay(10);
    }
  } else {
    Serial.println("Adafruit MCP4728 initialised");
  }

  Wire.setClock(3400000L);

  freq=6600000;
  phase=0;
  calc_phase_inc();

  setwave();
}

const float pi=3.14159265;
byte waveform[nsamp];
void setwave(){
  for (int isamp=0; isamp<nsamp; ++isamp){
    float phip=(isamp+0.5)/nsamp;
    float phi=2*pi*phip;
    int val=0;

    //saw
    //val = dacmax * isamp / nsamp;
    //val = ( isamp < nsamp / 2 ) ? 0 : dacmax - 1;
        //sine
    val=(sin(phi)+1.0)*dacmax/2;
    //val=((sin(phi)+0.333*sin(3*phi))/0.943+1)*dacmax/2;

    val=max(val,0);
    val=min(val,dacmax-1);
    waveform[isamp]=val;
  }
}


//calculate the phase increment. 
void calc_phase_inc(){
  phase_inc=0.268435456*nclk*freq;
}


ISR(TIMER1_COMPA_vect){// timer1 interrupt
   phase += phase_inc;
}


//regular running: 
void loop() {
  int redphase = phase >> 24;
  mcp.fastWrite(waveform[redphase] << 4, 0, 0, 0);
  //Serial.println(redphase);
}