Arduino timer interrupt experiment

main.ino

#include "waveform.h"
#include <SPI.h>
//#include "TimerOne.h"

#define MOSI 11
#define SCK  13
#define LD0   8
#define CS0   9
#define CLR  10

int freq = FREQ;

volatile unsigned long sine_point = 0;
volatile unsigned long sine_data = 0;
volatile boolean toggle0 = 0;

void setup() {
  pinMode(MOSI, OUTPUT); // master output slave input
  pinMode(SCK, OUTPUT);  // serial clock (aka CLK, SCLK)
  pinMode(LD0, OUTPUT);  // load strobe; active low outputs value from DAC
  pinMode(CS0, OUTPUT);  // chip select (aka slave select, SS)
  pinMode(CLR, OUTPUT);  // clears DAC register, active low

  digitalWrite(CS0, HIGH);
  digitalWrite(LD0, HIGH);

  SPI.begin();  
  SPI.setBitOrder(MSBFIRST);  // most significant bit first
  SPI.setDataMode(SPI_MODE0); // clock polarity and phase
  SPI.setClockDivider(SPI_CLOCK_DIV2); // half of system clock (16 MHz / 2 = 8 MHz)

  noInterrupts();

  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 50khz increments
  OCR1A = 319;// = (16*10^6) / (1*50000) - 1 (must be <65536)
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS12 and CS10 bits for 1024 prescaler
  TCCR1B |= (1 << CS10);  
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);
  interrupts();
}

ISR(TIMER1_COMPA_vect){//timer0 interrupt 2kHz toggles pin 8
  
  // uncomment for sine
    
  digitalWrite(LD0, LOW);
  digitalWrite(LD0, HIGH);

  sine_point++;
  if (sine_point >= PHI_STEPS)
    sine_point = 0;

  setValue(sine_wave[sine_point]);
    
  // uncomment for square wave
  /*
  if (toggle0){
    setValue(2048);
    toggle0 = 0;
  }
  else{
    setValue(0);
    toggle0 = 1;
  }
  */

  //should I use it?
  //TCNT1 = 0; 
}

void loop() {
  
}

// packages an int as 2 bytes and sends it
// over SPI to a 12 bit DAC (DAC8512)
void setValue(unsigned int value) { 
  //noInterrupts();
  //digitalWrite(CLR, HIGH);
  //digitalWrite(CLR, LOW);
  
  int ctrl_reg = 0; // 0000
  int level = value; // 111010111000

  // Shift 'ctrl_reg' left by 4 bits
  int shifted_ctrl_reg = ctrl_reg << 4; // 00010000

  // Shift 'level' 8 bits to the right
  int shifted_right_level = level >> 8; // 00001110

  // Create byte 1 by doing a bitwise or of 'shifted_ctrl_reg' and 'shifted_right_level'
  int byte1 = shifted_ctrl_reg;
  byte1 = byte1 | shifted_right_level; // 00011110

  // Now in byte2 I only want the last 8 bits of 'level' as the first 4 were in byte1 so I just delete the first 4 bits from 'level' by making them 0
  int byte2 = level;
  bitWrite(byte2, 8, 0);
  bitWrite(byte2, 9, 0);
  bitWrite(byte2, 10, 0);
  bitWrite(byte2, 11, 0);

  SPI.transfer(byte1);
  SPI.transfer(byte2);
  
  // Load strobe to output value from DAC analog output
  // moved to ISR

  //digitalWrite(LD0, LOW);
  //digitalWrite(LD0, HIGH);

  //interrupts();
}