Timer issue when writing to SD card

Community General Discussion
1

I wanted to validate a 1ms timer, but I got a very strange result.
Briefly about the project:
I'm using mega328p avr in a nano shield, I read in value from analog pin, then save it into sd card.
Previously I was using polling; when the program has reached the proper code line, then the data has saved to the SD card. It caused me oversampling, so I wanted to make the writing slower, so I set a 1ms timer, like this:

ISR(TIMER2_COMPA_vect)
{
    flag.dataWrite = true;
}

void setup()
{
    /* Timer 2 128 prescale, CTC mode */
    TCCR2A = (1U << WGM21);
    TCCR2B = (1U << CS22) | (1U << CS20);/
    TIMSK2 = (1U << OCIE2A);
    OCR2A = 125; /* F_CPU / 128 -->  1 ms */
    sei();
    ...
}
void loop()
{
    if(flag.dataWrite)
    {
      counter += 1;
      sprintf(buf, "%u\t%u", analogRead(sensorPin), counter);
      myFile.println(buf);
      flag.dataWrite = false;
     }
     ....
}

I set up a stopwatch, and I measure 1 minute. In that case, I would expect, that in 1 minute the counter value on the SD card should be somewhere around 60.000. But, the counter only reaches around 54.000.

Then I rearranged the program, now I am not writing the data to the SD card every milisec, only at the end of the measurement. In that case I get the proper value, the counter is around 60.000.

My question is, why is the timer runs slower at the first time?

2

Snippets of code are rarely useful. You need to post your entire sketch. Writing to the SD card takes some time. If it takes more than a millisecond, then your code will miss an update cycle.

If you want to test this, make your flag.dataWrite a byte and increment it in your ISR and then check the value in loop(). If it is 1, then you are good. If it is 2 or greater, you've missed something.

3

I did not want to past to entire code, because it is way to big, I wanted to point out the timer relevant part.
My idea is identical to you, but without the 1 ms timer, the amount of data written to the SD card is a lot more. In addition, if I double the timer (from 1ms to 2ms by changing OCR2A to 250), the counter is still less then it should.
The suggestion of the counter is useful, thanks.

4

I tried the counter, looks like there is no dataloss.

5

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