what effects millis() ?

Projects Programming
6

John,
Try replacing this:

       void loop (){   
    currentMillis = millis()  ;
     if(currentMillis > previousMillis)   
     {        
       previousMillis = currentMillis  +1000;

with

       void loop (){   
    currentMicros = micros()  ;
    elapsedMicros = currentMicros - previousMicros
     if(elapsedMicros > duration)   
     {        
       previousMicros = previousMicros  + duration;
       hundredths = hundreths +1;
       (if hundredths == 10){ 
           hundredths = 0;
           tenths = tenths +1;
           if (tenths == 10){
               tenths = 0;
               secU = secU +1;
           } 
        }
     }

with
unsigned long currentMicros;
unsigned long previousMicros;
unsigned long elapsedMicros;
unsigned long duration = 1000000UL;
byte tenths;
byte hundredths;

I usually keep track of 0.01S, setting duration to 10000UL and having a
hundreths and tenths counter.

The key is here tho:
previousMicros = previousMicros + duration;

So the next time event is always set the same, and not varying a little by how long the code might take in between loop passes.