Arduino analogreference()

[metrologist]

Is this reference worth anything? I find it varies ~20% depending on supply voltage to the chip. I'm using a pro micro ko on various USB ports/battery banks, and it's unusable.

Is it my junk ko or should it perform as an actual reference? I don't see much diff if just using the 5v range.

I have two functions, one that uses an ntc divider with analogReference and the other with an lm35 without, and the lm35 is half a mag better when just swapping supply.

basic code example:

Code: [Select]

void ntc() {
  analogReference(INTERNAL);
  analogRead(ThermistorPin0);
  for (int i = 0; i <= avg; i++) {
    Vntc = Vntc + analogRead(ThermistorPin0);
    delay(1);
  }
  Vntc = Vntc / avg;
  analogReference(DEFAULT);
  analogRead(ThermistorPin0);
}


[rstofer]

I'm skeptical of your calculation of the average.  You take 'avg'+1 samples and divide the total by just 'avg'.  I would think the code would be:

for (i = 0; i < avg; i++)

[sokoloff]

In addition to the off by one that rstofer notes, you need to be concerned with integer overflow, depending on the value of avg and the type of Vntc (neither of which is shown in the snippet above).

[ledtester]

Continuing the code review, I would add:

Vntc = 0;

just before the for loop. Perhaps you've set it to 0 elsewhere, but just before the for loop is the most natural and appropriate place to do this initialization.

[metrologist]

I found this about the analog reference: https://forum.arduino.cc/index.php?topic=193177.0

All valid points about the coding. Even more ignorant was that I was sourcing my divider from the supply. 

[iMo]

Around 1.1V and maybe 100ppm/C.

[OwO]

Actually sourcing the thermistor supply from Vcc *is* the correct thing to do, but in that case you must not use a precision voltage reference for the ADC; you must use Vcc as the voltage reference.

[rstofer]

I don't often see multiple variables initialized in the first parameter of the 'for' statement but it is a perfectly logical place to initialize 'vntc'.
 

Code: [Select]

void loop() {
  uint16_t vntc;
  const uint16_t avg = 8;

  for (uint16_t i = 0, vntc = 0; i < avg; i++){
    vntc += 1; // just a place holder
  }
  vntc /= avg;
}
[/font]

I picked a 16 bit unsigned integer, choose wisely...

[SiliconWizard]

I don't often see multiple variables initialized in the first parameter of the 'for' statement but it is a perfectly logical place to initialize 'vntc'.

I do that all the time. When the additional variables are all to be initialized at the beginning of the for loop, it's a pretty natural place to do that indeed.

[metrologist]

Thanks all for the programming help. I think I'm also having breadboard connection problems.

[jackthomson41]

Breadboard always makes the things worst, always use PCB or at least wero board.  https://www.theengineeringprojects.com/2018/09/introduction-to-arduino-due.html