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Attiny85 timer programming using Timer1

This Arduino sketch uses Timer1 to drive the LED blinker:

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/*
 * Program ATTiny85 to blink LED connected to PB1 at 1s interval.
 * Assumes ATTiny85 is running at 1MHz internal clock speed.
 */

#include <avr/io.h>
#include <avr/wdt.h>
#include <avr/sleep.h>
#include <avr/interrupt.h>

bool timer1 = false, led = true;

// Interrupt service routine for timer1
ISR(TIMER1_COMPA_vect)
{
  timer1 = true;
}

void setup() {

  // Setup output pins
  pinMode(1, OUTPUT);
  digitalWrite(1, led);
  set_sleep_mode(SLEEP_MODE_IDLE);

  // Setup timer1 to interrupt every second
  TCCR1 = 0; // Stop timer
  TCNT1 = 0; // Zero timer
  GTCCR = _BV(PSR1); // Reset prescaler
  OCR1A = 243; // T = prescaler / 1MHz = 0.004096s; OCR1A = (1s/T) - 1 = 243
  OCR1C = 243; // Set to same value to reset timer1 to 0 after a compare match
  TIMSK = _BV(OCIE1A); // Interrupt on compare match with OCR1A
  
  // Start timer in CTC mode; prescaler = 4096; 
  TCCR1 = _BV(CTC1) | _BV(CS13) | _BV(CS12) | _BV(CS10);
  sei();
}

void loop() {
  if (timer1) {
    timer1 = false;
    led = !led;
    digitalWrite(1, led);
  }
  sleep_enable();
  sleep_cpu(); // CPU goes to sleep here; will be woken up by timer1 interrupt
}

The internal RC oscillator can have a factory variance of up to 10%. I observe that my blinker pulsed slightly faster than 1Hz.

Thankfully, unlike the watchdog oscillator, the internal RC oscillator can be calibrated by using the OSCCAL register, or adjusting the value of OCR1A. I believe it should not be too difficult to automatically calibrate the oscillator between NTP calls by taking the difference in actual time and clock time (in seconds) and adjust OCR1A accordingly to get as close to 1Hz as possible.

So on initial startup, the ESPCLOCK may not be pulsing at 1Hz, but after 15 or 30 minutes, it should be fully calibrated.

The downside with this approach is we can only use SLEEP_MODE_IDLE to keep Timer1 running during sleep, which is not as power efficient  as using the Watchdog Timer.
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Comments

  1. Unknown20 October 2017 at 03:41

    Hi .help me to change the function from arduino codes at attyni85 usb .please help with .thanks
    void setup() {
    // put your setup code here, to run once:

    //timer2
    OCR2A = 127; //50% duty cycle
    TCCR2A = _BV(COM2A0) | _BV(WGM21) | _BV(WGM20); //toggle output, fast PWM mode
    TCCR2B = _BV(WGM22) | _BV(CS21) | _BV(CS20); // fast PWM mode, prescale 1:64
    pinMode(11, OUTPUT);

    }

    void loop()
    {

    delay(200); //duration of each frequency
    OCR2A += 10; //step size of frequency change
    if(OCR2A >= 245){OCR2A = 10; } //range of sweep, 245 is max.

    }

    ReplyDelete
  2. ian1235 July 2021 at 15:02

    there are only 2 timers on attiny85, timer0 and timer1

    ReplyDelete

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