Timer 2 interrupt with ATmega328p and make measurement in ISR

Question

I am working in my project and i want to use Timer2 interrupt every 100ms to get new measurement from the output of my 5 sensors which connected to pins A1,A2,A3,A4,A5.

I read in some article that using delay instruction is not as good as using timers'Interruptions so, I tried this code put it didn't work properly :

void setup(){
cli();
 TCCR2A = 0;
 TCCR2B = 0;
 TCNT2 = 0;
 OCR2A = 99;
 TCCR2A |= (1 << WGM21);
 TCCR2B |= (1 << CS21); 
 TIMSK2 |= (1 << OCIE2A);
sei();
Serial.begin(9600); 
}
void loop()
{
Serial.print(digitalRead(1));
Serial.print(' ');
Serial.print(digitalRead(2));
Serial.print(' ');
Serial.print(digitalRead(3));
Serial.print(' ');
Serial.print(digitalRead(4));
Serial.print(' ');
Serial.print(digitalRead(5));
Serial.println(' ');
}

• Ignacio Vazquez-Abrams: Where is your ISR?
• omran: in fact I dont know where i put the ISR interupt service routine where i have to put measurement instruction .
• Ignacio Vazquez-Abrams: Enabling the interrupt without writing a ISR is pointless.
• omran: i know that but where in the code i put the ISR?I think i have to put it in the loop
• Gerben: ISR(TIMER2_COMPA_vect){ ..code here.. }. However with your current setting the ISR gets called once every 0.5ms.

Answer 1

Well let's start from the start ...

---

Analog readings

to get new measurement from the output of my 5 sensors which connected to pins A1,A2,A3,A4,A5.

Serial.print(digitalRead(1));

That is not reading analog pin A1, it is reading digital pin 1. If you want to read A1 you need:

Serial.print(analogRead(1));

---

Check the time

By far the simplest thing would be to simply check the time and take a reading when 100 ms is up.

Example:

void setup()
{
 Serial.begin(115200); 
} // end of setup
unsigned long lastReading;
const unsigned long INTERVAL = 100; // ms
void loop()
{
 // is time up?
 if (millis () - lastReading >= INTERVAL)
 {
 lastReading = millis (); // when we took this reading
 Serial.print(analogRead(1));
 Serial.print(" ");
 Serial.print(analogRead(2));
 Serial.print(" ");
 Serial.print(analogRead(3));
 Serial.print(" ");
 Serial.print(analogRead(4));
 Serial.print(" ");
 Serial.print(analogRead(5));
 Serial.println();
 } // end of if time up
 // do other things here
} // end of loop

I increased the baud rate to 115200. Why use 9600 when you are trying to print 5 x 4 characters every 100 ms?

---

Using timers

If you really want to use timers (and I don't see what they achieve here) I have a page about timers that describes using them, and interrupts, in some depth.

The very least you would need is a timer 2 "compare A" interrupt like this:

ISR (TIMER2_COMPA_vect) 
 {
 // Timer 2 has reached its comparison value
 }