Electronic – arduino – Measuring Arduino internal voltage reference with respect to Vcc

arduinovoltage measurement

How do I read the internal reference voltages of the Arduino (Ardino Mega, ATmega2560, specifically the 2.56v reference) with respect to the input voltage?

I'm trying to measure the remaining battery capacity of a device through a voltage divider. The Vcc going into the Arduino is also hooked up to the battery and thus varies as well. I've measured the internal 2.56v reference so I know approximately what it should be regardless of the varying Vcc voltage but I can't find any simple documentation on exactly how to measure that particular internal reference voltage with respect to the Vcc.

Best Answer

You're trying to measure the battery voltage, but you need a stable reference point. This is precisely what you use the internal reference for.

Let's assume that your battery pack is a LiPo pack and has a nominal voltage of 3.7V and you want to shut things down when you get to 3.1V.

Now, the internal reference you have is 2.56V, so even at the cutoff point, 3.1V is too high. Let's chop this in half with the voltage divider you already have.

Now, the nominal voltage you'll see through the divider is 1.85V. Perfect! You can now use your internal reference for your ADC readings. In this case, 1.85V is what you'll see at the ADC for a battery voltage of 3.7V and 1.55V at the ADC is your cut off of 3.1V.

Related Solutions

Voltage limitations on AREF for Arduino

Not sure which Arduino you have but on page 377 the datasheet for the atmega2560 says Vref min for the adc is 1.0V. So unfortunately that probably won't help you.

Verify changed analog reference voltage on Arduino

With a sketch, read and print a known test voltage of a little bit less than your reference voltage, say 1.5V in this case. The printed result will be the same fraction of 1024 that your test voltage is of the reference voltage, or:

V_test     x
------ =  ----
V_ref     1024


         1024
V_ref =  ---- V_test
          x

Edit: The intent here was to supply a presumed 2.0V external analog reference, say it's your 20K resistor. Now supply a known voltage of ~80% the 2V reference to one of the MCU's analog inputs. Run a sketch to print what the MCU reads as that voltage as. Calculate the actual analog reference voltage as I did above. If it is 2.0V, you're done.

This is how I'd make a 2.0V ARef. An AtMega328p's ARef only draws ~100μA at Vcc = 3.3V, so this should still be pretty close:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

As above, get A/D reading of the known voltage input and calculate what the reference voltage must be, to confirm it.

Best Answer

Related Solutions

Voltage limitations on AREF for Arduino

Verify changed analog reference voltage on Arduino

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