Read sensor data repeatedly after specified time?

Question

I want to read sensor data using analogRead() after every 5ms. I have read other answers to similar questions but what I'm confused about is:

• let's say, I made a loop to read sensor data using analogRead() at the end of which I write delay(50)

• it means it'll take some time to read data(adc) , then it'll wait for 50ms

• time to execute loop isn't same for every iteration so we get the pattern as: some time for loop execution, delay of 50ms, some time for next iteration........and so on I want to ask if I want arduino to read sensor data exactly every 5ms, what should I do? Thanks

• jsotola: do not use delay() ..... see BlinkWithoutDelay example sketch

Answer 1

You store the time (millis()) of the last measurement. Then wait till 5ms have past since the last measurement. Do your measurement, and update the time.

Something like:

unsigned long lastMeasurement = millis();
void loop()
{
 while( (millis()-lastMeasurement)<5 ){/* do nothing */}
 lastMeasurement = lastMeasurement + 5; //or use `lastMeasurement=millis();` depending on whether you want accuracy between measurement, or accuracy over time
 auto measurement = analogRead(A0);
}

• Talha Yousuf: It waited for 5ms, then last measurement=0+5=5 ms. Then it’ll do analogRead(). Let’s say analogRead() took 10ms, then mills()- last measurement() = 15-5 ms=10 ms so the loop will do another analogRead() although it’s not reading measurement exactly after 5 ms. So this will only work if loop execution time is <5ms
• Gerben: Indeed. It can't magically go back in time. Was there any question there?

Answer 2

You do something like this

unsigned long timerMesurement = 0;
void loop()
{
 if (millis() - timerMesurement > 5UL) { 
 timerMesurement = millis(); 
 int measurement = analogRead(A0); 
 }
}

• Jot: Sigma, it should be >= for the right interval. It should also be timermeasurement += 5UL; for a interval that is fixed in time without increasing delays, the return value of millis() could have increased since the previous line.