31

There are some pins on the Arduino which I haven't been able to find out anything about:

  • IOREF
  • AREF
  • An unlabeled one next to IOREF

What are they?

31

AREF:

This is the voltage reference analog-to-digital converter (ADC). It can be used instead of the standard 5V reference for the top end of the analog spectrum – for example, if you wanted to use the ADC to monitor a signal that had a 0-1.5 volt range you could get the full scale of the ADC by connect AREF to a 1.5V signal. DO NOT CONNECT A SIGNAL OUTSIDE THE 0V TO 5V RANGE!

Note that in order for this to work, you must run analogReference(EXTERNAL); before using analogRead(). Also:

After changing the analog reference, the first few readings from analogRead() may not be accurate.

Fore more information, see AnalogReference.

IOREF:

This is a voltage corresponding to the i/o of that board, for example an Uno would supply 5v to this pin, but a Due would supply 3.3v. Sending a signal to this pin does nothing.

Unlabeled pin:

This pin is unused, but is there to ensure compatibility with future products. It is not connected to anything on any R3 boards I have seen.

  • I agree with possibly using this pin directly from the 5V pin. My Nano boards do not measure the same on the ADCs as the UNO boards. It is somewhere below 5V that I get the 1023 reading. I assumed they were already referenced to 5V but apparently that was wrong. Thank you for the tip. – SDsolar Dec 31 '16 at 4:52
12

Concerning the AREF pin, you have to remember that the Arduino comes with a 10bit ADC (Analog-Digital-Converter), which converts incoming voltages between 0V and 5V to integer values between 0 and 1023. This results in a resolution of roughly 4.8 mV.

If a sensor only delivers a lower maximum voltage, it is resonable to apply this voltage to the AREF pin, just in order to obtain a higher resolution.

  • 3
    analogReference(); – jippie Feb 13 '14 at 5:51
  • I agree with possibly using this pin directly from the 5V pin. My Nano boards do not measure the same on the ADCs as the UNO boards. It is somewhere below 5V that I get the 1023 reading. I assumed they were already referenced to 5V but apparently that was wrong. Thank you for the tip. Upvote. – SDsolar Dec 31 '16 at 4:53

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