AREF pin on Arduino Uno R3

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edit

So then, AREF can only output a voltage when used by the ADC, but not when used by the AC.

That is what I sense all your answers are pointing at, yet none has it black on white. My questions remains unmodified, below:

On the Uno R3, is the AREF pin supposed to present any voltage when default / bandgap voltage is set for the analog comparator?

With this code (analog comparator):

void initAC()
{
  ADCSRA = ADCSRB = ACSR = DIDR1 = 0; // also enables comparator
  ADCSRB |= (1 << ACME); // AIN1 from ADC mux
  ADMUX |= (1 << MUX0); // ADC1 input pin selected (PIN_VSENSE)
  ACSR |= (1 << ACBG); // AIN0 set to bandgap reference
                                          // voltage 1.1V
  //ACSR |= (1 << ACIS0) | (1 << ACIS1); // enable interrupr on rising edge;
  ACSR |= (1 << ACIE); // interrupts enable
  DIDR1 |= (1 << AIN0D) | (1 << AIN1D); // disable digital input buffers
                                          // for power saving
  PRR &= ~(1 << PRADC);                   // no power reduction ADC in order for
                                          // the AC to be able to use MUX input
  EEPROM.put(0x155, 0x5501);
}

When keeping the DMM probes on AREF / GND as the Arduino reboots I read some 10 mV steadily.

When touching the probes while the Arduino is running, I can read a decreasing voltage as through an RC group.

Tried with or without ACBG, same observation.

With this other code (analog digital converter):

{
 //power_adc_enable();          nah-uh!
 //power_usart0_enable();
ADCSRA = ADCSRB = ADMUX = ACSR = DIDR0 = 0;
 ADCSRA |= (1 << ADEN);                  // power enable ADC
 ADCSRA |= (1 << ADIE);                  // enable interrupts
 ADCSRA |= (1 << ADPS2) | (1 << ADPS0);  // 250 kHz operation
 ADMUX |= (1 << REFS0);                  // 3.3V (AVCC) as voltage reference
 ADMUX |= (1 << ADLAR);                  // discard least significant 2 bits,
                                         // only read ADCH
 ADMUX |= (1 << MUX0);                   // ADC1 input pin selected (PIN_VSENSE)
 DIDR0 |= (1 << ADC1D) | (1 << ADC3D) | (1 << ADC5D);  // disable digital input
                                       // buffers for power saving, which is not
                                       // really needed given how the ADC is used
}

I was able to read a good 4.8V on the AREF pin. Swapped the Atmega328P chip on my Arduino board with another one - same behaviour.

Edgar Bonet · Answer 1 · 2024-06-14T14:52:29.857

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit 1: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.

Edit 2: Answering this new point:

AREF can only output a voltage when used by the ADC, but not when used by the AC.

Whether or not it is being used by the ADC and/or the AC is irrelevant to the behavior of the AREF pin. The only thing that matters is the value of the REFS0 bit of ADMUX. The pin outputs a voltage if and only if that bit is set. This can be clearly seen in this part of the schematic of the ADC, from the datasheet:

ADC reference selection

When REFS0 is 1, the transistor circled in red connects the AREF pin to either AVCC or the internal reference, as selected by the multiplexer controlled by the REFS1 bit.

Note that you can change this bit without ever really using the ADC, although that would probably be useless.

score 1 · Answer 2 · answered Jun 13 '24 at 11:36

1

You are supposed to supply an aref voltage to the aref pin. You supply your 5v reference to the aref pin, so that the ADC compares its input against the aref voltage.

answered Jun 13 '24 at 11:36

Hacky

154
4

score 0 · Answer 3 · answered Jun 14 '24 at 07:32

As stated in the datasheet, section 1.1.8, the AREF pin is a reference voltage for the ADC. In other words it is an input.

AREF is the analog reference pin for the A/D converter.

From the datasheet:

23.5.2 ADC Voltage Reference

The reference voltage for the ADC (V_REF) indicates the conversion range for the ADC. Single ended channels that exceed V_REF will result in codes close to 0x3FF. V_REF can be selected as either AV_CC, internal 1.1V reference, or external AREF pin.

AV_CC is connected to the ADC through a passive switch. The internal 1.1V reference is generated from the internal bandgap reference (V_BG ) through an internal amplifier. In either case, the external AREF pin is directly connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground. V_REF can also be measured at the AREF pin with a high impedance voltmeter. Note that V_REF is a high impedance source, and only a capacitive load should be connected in a system.

If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference voltage options in the application, as they will be shorted to the external voltage. If no external voltage is applied to the AREF pin, the user may switch between AV CC and 1.1V as reference selection.

As you can see from the above quote from the datasheet, once initialised, AREF is made available to you for measurement, and they recommend a capacitor to smooth out noise.

AREF pin on Arduino Uno R3

3 Answers3

23.5.2 ADC Voltage Reference