I’m operating an ATMega32 at 3v3 through USB and a LDO.
Is it possible to connect the AREF pin directly to the USB voltage of 5V so that I can read voltages between 0 and 5V when reading from the analog pins?
Electronic – arduino – AREF above operating voltage of the microcontroller
arduinoavr
Related Solutions
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.
Perhaps you are changing the A/D reference mode somewhere else in your code? You can double check this by adding a call to analogReference(DEFAULT) just before your call to analogRead(). This should select Vcc as the input to the A/D Mux on the ATTINY85 just as you expected.
Here is the ADMUX register from the ATTINY85 datasheet...
Here is the relevant code excerpted from the Arduino library....
#if defined(__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__) || defined(__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || defined(__AVR_ATtiny85__)
65 #define DEFAULT 0
66 #define EXTERNAL 1
67 #define INTERNAL 2
uint8_t analog_reference = DEFAULT;
void analogReference(uint8_t mode)
{
// can't actually set the register here because the default setting
// will connect AVCC and the AREF pin, which would cause a short if
// there's something connected to AREF.
analog_reference = mode;
}
int analogRead(uint8_t pin)
{
uint8_t low, high;
if (pin >= 14) pin -= 14; // allow for channel or pin numbers
// set the analog reference (high two bits of ADMUX) and select the
// channel (low 4 bits). this also sets ADLAR (left-adjust result)
// to 0 (the default).
ADMUX = (analog_reference << 6) | (pin & 0x07);
...
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Best Answer
Indeed this information will be in the datasheet, but since it might be difficult to find I'll help a bit:
The datasheet will state that the voltages at the uC's inputs/outputs cannot exceed the voltage on the VDD pins.
The reason for this is the ESD protection diodes. As soon as the voltage at AREF becomes higher than VDD + 0.5 V the AREF's ESD diode will start conducting.
So if you connect 5 V to AREF, the VDD supply of the uC will be pulled up to about 4.4 V ! But you supplied 3.3 V via an LDO. Yes, then the LDO will do nothing.
If you want a 5 V input range either power the uC with 5 V
or
use a voltage divider to lower the input voltage to the ADC.
To learn more about what issues the ESD diodes can cause, watch this EEVBlog video