16 MHz blink

Projects Discussion and Showcase Science and Measurement
1

Hello! I need to build a 2 channel signal generator with delay from 500 ns to 10 us between the signals. I tried the next program code for blink something (led on 13 pin) with 16 MHz frequency, but oscilloscope shows period about 25 us. What is it? Why isn't 125 ns (62.5*2)?

#define ledPin 13

int timer1_counter;
void setup()
{
  pinMode(ledPin, OUTPUT);

 /* // initialize timer1 
  noInterrupts();           // disable all interrupts
  TCCR1A = 0;
  TCCR1B = 0;

  // Set timer1_counter to the correct value for our interrupt interval
  timer1_counter = 65535;   // preload timer 65536-16MHz/16Mhz
  
  TCNT1 = timer1_counter;   // preload timer
  TCCR1B |= (1 << CS10);    // 1 prescaler
  TIMSK1 |= (1 << TOIE1);   // enable timer overflow interrupt
  interrupts();*/             // enable all interrupts
}

ISR(TIMER1_OVF_vect)        // interrupt service routine 
{
  TCNT1 = timer1_counter;   // preload timer
  digitalWrite(ledPin, digitalRead(ledPin) ^ 1);
}

void loop()
{
    // your program here...
}

Thanx.

2

Because this is a 16MHz processor and the timers can not output something at twice the clock rate which it would have to do to get one cycle, that is a high and a low, to get a 16MHz output.

3

Also it takes time to call an interrupt, and also quite a long time for the digital write call, so you are never going to get anywhere even close using that code.

4 
ISR(TIMER1_OVF_vect)        // interrupt service routine 
{
  TCNT1 = timer1_counter;   // preload timer
  PINB |= _BV(7);                                // 169.309 kHz,  2.9 uS
//digitalWrite(ledPin, digitalRead(ledPin) ^ 1); //  26.778 kHz, 18.8 uS
}

void setup() { 
  pinMode(13, OUTPUT); }

void loop() {
    while(1)
      PINB |= _BV(7);  //  2 MHz
}

void loop() {
      PINB |= _BV(7);  //  533 kHz
}
5

I think is better you use Arduino to control external electronic. You may use an external oscillator (oscillators) and a clock divider such as HEF4521B.

Other logic components might be used to address the HEF4521B channels: outputs and input. In this way Arduino could be used to select input and output source.

I hope you find this hint intersting.

6

digitalRead / digitalWrite each take about 4us to execute. ISR will take a us or two to
enter.

If you want 16MHz you could try and derive it from the Xtal oscillator, but the highest
frequency the chip can produce is 8MHz because that's one transition per master clock.
The timers can do that unaided, just set with MAX=1, OCR=0, fast mode - the counter
will clock 0,1,0,1,0,1 etc.

Perhaps describe in more detail what waveforms you want to generate and we can
figure out if there's a way to do this with the timer hardware...