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Beacon of Wierd
Beacon of Wierd
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I have set up Timer2 to count every tick at 16MHz, or 62.5ns per tick. Then I run the following code:

int t = 0;
    for (int i=0;i<64;i++){
  data[4*i] = TCNT2;
  data[4*i+1] = TCNT2;
  byte now = data[4*i+1];
  byte last = data[4*i];
  t += now > last? (long)(now-last) : 256 -(long)(last-now);
  data[4*i+2] = TCNT2;
  data[4*i+3] = TCNT2;    
  }
}
for (int i=0;i<255;i++){
  Serial.println(data[i]-data[i+1]);
}

and I get the output: "4 4 4 11" (repeated) which makes sense, 4 clock cycles to read TCNT2 and write it to the correct place in data and 11 cycles when also evaluating the loop condition. However, I can't understand when the variable t is updated. I have tried running with and without t and both give the same timings. From my (obviously lacking) understanding updating a variable is at minimum one instruction, so how can it do it under one clock cycle? Am I misunderstanding what one instruction is or does the TCNT2 counter for the timer not update accurately here?

I have set up Timer2 to count every tick at 16MHz, or 62.5ns per tick. Then I run the following code:

int t = 0;
    for (int i=0;i<64;i++){
  data[4*i] = TCNT2;
  data[4*i+1] = TCNT2;
  byte now = data[4*i+1];
  byte last = data[4*i];
  t += now > last? (long)(now-last) : 256 -(long)(last-now);
  data[4*i+2] = TCNT2;
  data[4*i+3] = TCNT2;    
  }
}
for (int i=0;i<255;i++){
  Serial.println(data[i]-data[i+1]);
}

and I get the output: "4 4 4 11" (repeated) which makes sense, 4 clock cycles to read TCNT2 and write it to data and 11 cycles when also evaluating the loop condition. However, I can't understand when the variable t is updated. I have tried running with and without t and both give the same timings. From my (obviously lacking) understanding updating a variable is at minimum one instruction, so how can it do it under one clock cycle? Am I misunderstanding what one instruction is or does the TCNT2 counter for the timer not update accurately here?

I have set up Timer2 to count every tick at 16MHz, or 62.5ns per tick. Then I run the following code:

int t = 0;
    for (int i=0;i<64;i++){
  data[4*i] = TCNT2;
  data[4*i+1] = TCNT2;
  byte now = data[4*i+1];
  byte last = data[4*i];
  t += now > last? (long)(now-last) : 256 -(long)(last-now);
  data[4*i+2] = TCNT2;
  data[4*i+3] = TCNT2;    
  }
}
for (int i=0;i<255;i++){
  Serial.println(data[i]-data[i+1]);
}

and I get the output: "4 4 4 11" (repeated) which makes sense, 4 clock cycles to read TCNT2 and write it to the correct place in data and 11 cycles when also evaluating the loop condition. However, I can't understand when the variable t is updated. I have tried running with and without t and both give the same timings. From my (obviously lacking) understanding updating a variable is at minimum one instruction, so how can it do it under one clock cycle? Am I misunderstanding what one instruction is or does the TCNT2 counter for the timer not update accurately here?

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Beacon of Wierd
Beacon of Wierd
  • 123
  • 7

Missing time when using timer

I have set up Timer2 to count every tick at 16MHz, or 62.5ns per tick. Then I run the following code:

int t = 0;
    for (int i=0;i<64;i++){
  data[4*i] = TCNT2;
  data[4*i+1] = TCNT2;
  byte now = data[4*i+1];
  byte last = data[4*i];
  t += now > last? (long)(now-last) : 256 -(long)(last-now);
  data[4*i+2] = TCNT2;
  data[4*i+3] = TCNT2;    
  }
}
for (int i=0;i<255;i++){
  Serial.println(data[i]-data[i+1]);
}

and I get the output: "4 4 4 11" (repeated) which makes sense, 4 clock cycles to read TCNT2 and write it to data and 11 cycles when also evaluating the loop condition. However, I can't understand when the variable t is updated. I have tried running with and without t and both give the same timings. From my (obviously lacking) understanding updating a variable is at minimum one instruction, so how can it do it under one clock cycle? Am I misunderstanding what one instruction is or does the TCNT2 counter for the timer not update accurately here?