millis(); not updating with the same loop

im trying to determine how much time has elapsed for a few lines of code:

//Read Flow Meter
  for(int xCount=0; xCount<10001; xCount++){
    int signalA = digitalRead(rpm_rd3); //detects tachometer signal, flow pin goes high every time the magnet spins past the sensor. NOTE: must be int.
  FLOWCALC = countA/((pTime-cTime)/1000)*60;

but cTime and pTime are returning the same time.

this is a portion of code. i know the values are the same as i have them reading out to a 20x4 LCD.

i know i could use interrupts for this but i am using a Arduino UNO IDE 1.0.3 and pins 2 and 3 are being used for capturing RPMs from a pump and a fan using interrupts 0 and 1.

original code idea from

It's quite possible that your 10000 cycle loop takes less than a millisecond to execute

Instead of millis() try using micros() as a 16mhz AVR chip may run code a lot faster then you appreciate. :wink:


thanks i'll give that a try.

also wondering if serial needs to be running as the examples i have seen have it running at 9600 or 19200.
not sure if that makes a difference.

What do you mean by "serial is running" ? It's not like Serial.begin() starts some kind of background process :slight_smile:

  FLOWCALC = countA/((pTime-cTime)/1000)*60;

Notice that this is ALL integer arithmetic. Unless the loop takes more than a second to execute (highly unlikely), (pTime - cTime)/1000 will always be 0. (pTime - cTime)/1000.0 would not.

digitalread takes in the order of 1-4 uSec the loop should be above 10 millisecs at least. using micros() is OK but won’t solve the error I expect…

Note you are doing 10001 iterations, and divide by 1000
for(int xCount=0; xCount<10001; xCount++){

FLOWCALC = countA/((pTime-cTime)/1000)*60;

I don’t know the datatypes of the vars used but if ptime and ctime are int/long and ãpprox 40 apart, dividing by 1000 will return zero * 60 is still zero.
Integer math is not forgiving.

Try this formula instead (make ptime and ctime and flowcalc unsigned long )

FLOWCALC = (countA * 60000L)/(pTime-cTime);

Style remark:
It is quite common in C/C++ to write constants/#defines only in upper case and variables not. So FLOWCALC is misleading because of its style.