fasterinterrupt.ino

#define LEDPIN   13
#define LATCHPIN 12

volatile uint32_t i = 0;
volatile uint32_t j = 0;
volatile uint32_t tablesize = 1000;
volatile uint32_t tablesize2 = 1000;

uint8_t table[1000], table2[1000];

void setup()
{
  pinMode(LEDPIN, OUTPUT);  // PORTB5
  pinMode(LATCHPIN, OUTPUT);  // PORTB4

  noInterrupts();                       // disable all interrupts
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1  = 0;

  OCR1A = 1;                            // compare match register. this divides by 2.
  TCCR1B |= (1 << WGM12);               // CTC mode (compare match)
  TCCR1B |= (1 << CS10) | (1 << CS11);  // 64 prescaler
  TIMSK1 |= (1 << OCIE1A);              // enable timer compare interrupt
  interrupts();                         // enable all interrupts
}

// freq = 16MHz / 64 / 2 = 125 kHz (or 62.5 kHz on 8MHz)
ISR(TIMER1_COMPA_vect)          // timer compare interrupt service routine
{
  i++;
  j++;
  
  // the following if statement is slower than j % tablesize
  //  if (j > tablesize) {
  //    j = 0;
  //  }
  
  // write to output 1 then switch to output 2 then write to output 2
  // (this probably won't work in practice since we instantly write after we toggle latch
  PORTD = table[j % tablesize];  
  PORTB |= (1 << PORTB4);  // latch on
  PORTD = table2[j % tablesize2]; 
  PORTB &= ~(1 << PORTB4);  // latch off
  
  // toggle LED at 250 / 2 / 62.5 Hz = 2 Hz. i.e LED blinks at 1 Hz
  if (i > 62500) {
    PORTB ^= (1 << PORTB5);  // toggle led
    i = 0;
  }
}

void loop() {}