on a ATMEGA328p i am trying to set the internal 1.1V reference of the ADC by simply setting the ADMUX: ADMUX = 0xC0;
(Datasheet Table 24-3, internal 1.1V)
With this upper two bits set the internal reference should be set, but i cannot measure any voltage (0.01V) on the AREF pin.
But i should be capable to measure, since following the shematic in the data sheet this voltage is connected to the pin when the bits are set correctly.
Also they talk about to connect a capacitor to this pin for decoupling.
When i set the ADMUX register to: ADMUX = 0x40; i can measure almost the input voltage although it is about 0.1V less (AVCC = 3.240, ADMUX = 3.146)
So with the reference set to AVCC it seems to work, why not with the internal?
I also tried to enable the ADC by setting PRR |= 0x01; but this does not help. Do i need annother register to set correctly (nothing found)