Stange behavior of micros() between atmega328p and atmega168

Development Boards Arduino
5

skimask:
Let’s see the code…

void onchange(){
  _change=!_change;
  if (!_change) onup();else ondown();
}
void onup(){}
void ondown(){
  long itime=micros();
  timeus=itime-time; //get a full periode on down:_┌─┐_______┌─┐_
  time=itime;
  // avoid overflow
  if (usi<SAMPLES){
    us[usi]=(timeus)/1000.0;
    usi++;
  }
  ...
}
...
void setup(){
   ...
   // bind ardware interuption on pin 2
   attachInterrupt(0,onchange,CHANGE);  
  ....
}

void loop(){ 
  //clean buffer
  usi=0;
  time=0;
  //measure 1s
  _sample=true;
  digitalWrite(P_SENS, HIGH);  
  delay(1000);
  digitalWrite(P_SENS, LOW);  
  delay(2);
#ifdef CC_DEBUG
  // vref 5V=5.0/1024.0=0.0048828, 3.3/1024.0=0.0032226 
  Serial.print(" SAMPLES: ");
  for (int i=2;i<(usi);i++){
    Serial.print(us[i],4);
    Serial.print(" ");
  }
  Serial.println();
#endif
....

Mee_n_Mac:
What’s the standard deviation of timing for each processor ?

The average of 200[microseconds] is in fact the sum of 8 samples that composes the 250[ms]. I took a picture of the deviation between the 168 and the 328p. One other thing interresting, the deviation is about 200[us] [u]but there is no relevant deviation between two 328p board[/u].

Philba:
At first I thought it was because of resonators but both boards appear to use crystals. Still, there is always going to be some error and .12% is a bit out of spec for typical crystals (actually, it could be 0.0625%, one high and the other low). It would be strange if they both agreed to the microsecond. 50 ppm crystals could yield up to a difference of 25 microseconds.

What is the formula to get the delta from the ppm?

thank you very much for helping!!!

– olivier