Why its not 1,000ms but arround 1,000.01ms

Using Arduino Programming Questions
#1 

unsigned long myTime = 0;
unsigned long myInterval = 1000;

void setup() {

  Serial.begin (9600);
}

void loop() {  

  if ( millis () - myTime >= myInterval) {
    myTime = millis ();
 
  analogRead (A0);
  Serial.println (analogRead (A0));
  }
  
}

the input not exactly every 1000ms

i am using uno

#2

Try:
myTime = myTime + myInterval;

Also, you are dealing with a low grade ceramic oscillator on the Arduino.

2 Likes
#3 
myTime = millis ();

Should be

myTime += myInterval;
#4

this one not work...

#5

Not work in what way ?

#6

this one not work.. the timer keep adding arround 100ms at every 1 minutes

#8

thanks larry.. maybe because of the low grade ceramic oscilator.. that error cant repair

#9

DO NOT post pictures of Serial monitor output. Copy and paste the output here in code tags. This makes it so much easier to read and examine in detail

1 Like
#10

I'm not sure what you used to measure at 10ns precision

So you are looking at accuracy and precision for the clock. The difference between 1000ms and 1000.01ms is 0,001%.. Clearly beyond the low grade ceramic oscilator that's builtin

You could try with micro and a faster baud rate see if you get better results

unsigned long lastTime = 0;
const unsigned long myInterval = 1000000ul;

void setup() {
  Serial.begin (115200); // don't go slow. interrupts do have impact. even go for 1000000 or 2000000 
}

void loop() {  
  uint32_t now = micros();
  if ( now - lastTime >= myInterval) {
    lastTime += myInterval;
    Serial.println (analogRead (A0));
  }
}
#11

same result here...
i want to use the motor stepper to follow earth rotation, arround 4,300,800 step for 1 full rotation for 86,124 minutes + 0.916 seconds

i think just reduce the speed arround 0.001%

#12

and your physical gears are precise to 0.001%?

#13

my Gear designed for 1 rotation every 4,300,800 step.. if use less step for 1 rotation, maybe the image not good for star tracking.. not try yet

the worm gear 96:1
the gear 3.5:1
the motor 400:1
the microstepper 32:1

#14

DS3231 real time clock module generating a 1 Hz square wave (or interrupt) would keep more accurate time.

#15

it takes about 0.1 ms to read an analog input and OP is doing it twice, so I wonder what's the purpose of being that precise in the first place anyway...

#16

telescope for star tracking... but i think i found another way