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Using the Arduino Timers

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The use of timers in Arduino is an advanced method to execute code without disturbing the rest of the program. They allow to activate functions at specific time intervals. Timers are used in many libraries in a completely transparent way for the user (millis(), Servo.h, PWM, etc.)

In some cases, it is possible to use libraries that configure timers that will simplify the use.

Material

Description

Timers are registers in the microprocessor which increment according to clock pulses or external pulses. It is possible to configure them to modify their behavior.

The microprocessor of the Arduino UNO (ATmega328P) has 3 timers:

Timer configuration

The most complicated part of using the sensor is their configuration. A timer is configured by means of its control register. Here is a reminder of the registers.

Timer 0Timer 1Timer 2Rôle
TCNT0TCNT1LTCNT2Timer (bit 0 à 7)
TCNT1HTimer (bit 8 à 15)
TCCR0ATCCR1ATCCR2ARegistre de contrôle
TCCR0BTCCR1BTCCR2BRegistre de contrôle
TCCR1CRegistre de contrôle
OCR0AOCR1ALOCR2AOutput Compare (bit 0 à 7)
OCR1AHOutput Compare (bit 8 à 15)
OCR0BOCR1BLOCR2BOutput Compare (bit 0 à 7)
OCR1BHOutput Compare (bit 8 à 15)
ICR1LInput Capture (bit 0 à 7)
ICR1HInput Capture (bit 8 à 15)
TIMSK0TIMSK1TIMSK2Interrupt Mask
TIFR0TIFR1TIFR2Interrupt Flag

The TCNTx register contains the timer/counter value, the TCCRxA and TCCRxB register are the control registers. OCRxA and OCRxB contain the register values to be compared. TIMSKx contains the interrupt mask and TIFRx contains the enable flags.

Code using timer2

For this example, we use timer2 which is coded on 8bits (256). So we have to introduce an extra counter.

unsigned long elapsedTime, previousTime;

void onTimer() {
  static boolean state = HIGH;
  elapsedTime=millis()-previousTime;
  Serial.print(F("Set LED 13 : "));
  if(state){
    Serial.print(F("ON"));
  }else{
    Serial.print(F("OFF"));
  }
  Serial.print(F(" - "));Serial.print(elapsedTime);Serial.println(F("ms"));
  digitalWrite(13, state);
  state = !state;

  previousTime=millis();
}

void setup(){
  Serial.begin(9600);
  pinMode(13, OUTPUT);

  cli();           // disable all interrupts
  TCCR2A = (1<<WGM21)|(0<<WGM20); // Mode CTC
  TIMSK2 = (1<<OCIE2A); // Local interruption OCIE2A
  TCCR2B = (0<<WGM22)|(1<<CS22)|(1<<CS21); // Frequency 16Mhz/ 256 = 62500
  OCR2A = 250;       //250*125 = 31250 = 16Mhz/256/2
  sei();             // enable all interrupts

}
ISR(TIMER2_COMPA_vect){         // timer compare interrupt service routine
  static int counter=0;
  if (++counter >= 125) { // 125 * 4 ms = 500 ms
    counter = 0;
    onTimer();
  }
}

void loop(){
}

Code using timer1

For this example, we will use the timer1

unsigned long elapsedTime, previousTime;

void onTimer() {
  static boolean state = HIGH;
  elapsedTime=millis()-previousTime;
  Serial.print(F("Set LED 13 : "));
  if(state){
    Serial.print(F("ON"));
  }else{
    Serial.print(F("OFF"));
  }
  Serial.print(F(" - "));Serial.print(elapsedTime);Serial.println(F("ms"));
  digitalWrite(13, state);
  state = !state;

  previousTime=millis();
}

void setup(){
  Serial.begin(9600);
  pinMode(13, OUTPUT);

  cli();           // disable all interrupts
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1  = 0;
  OCR1A = 31250;            // compare match register 16MHz/256/2 = 31250
  TCCR1B = (1 << WGM12);   // CTC mode
  TCCR1B = (1 << CS12);    // // Frequency 16Mhz/ 256 = 62500 
  TIMSK1 = (1 << OCIE1A);  // Local interruption OCIE1A
  sei();             // enable all interrupts
}

ISR(TIMER1_COMPA_vect){          // timer compare interrupt service routine
  onTimer();
}

void loop(){
}

N.B.: We have added an elapsed time measurement for illustration purposes, but care must be taken when using interrupt-based functions in interrupt-call functions. There may be interferences.

Result

Once the program is started, the on-board LED turns on and off every 500ms

Sources

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