adcavrvoltage-reference
If I use the internal 1.1V reference for the ADC, and my Analog input exceeds 1.1V, lets say 2.5V, is that harmful to my microcontroller? Or will ADC value simply clip (to 0x3FF) at 1.1V?
For convenience, I am including an excerpt block diagram from the ATMega328 datasheet.
I don't see any problem with applying an external voltage, through a 5 kohm resistor, to the Arduino reference input. Or better, with using a resistor divider, so that you turn 5 V into your desired AREF voltage, while at the same time exhibiting a source resistance of approximately 5 kohm. This second requirement does not have to be accurate. That is just to limit the current that will flow from AVCC to ground, through the external circuitry.
If you want to end up with 1.8 V at the AREF input of the MCU, just choose R1 and R2 so that \$ V_{AREF} = 5ยท\frac{R2||32000}{R1+(R2||32000)} =\$ 1.8 V and \$ R_{source} = R1||R2 \approx\$ 5 kohm.
When you need to work with the [0, 1.8] V range, disable the references internal to the ATMega, and when you need to work with the [0, 5] V, enable the internal AVCC reference (if that is 5 V). If the MOSFET shown in Fig. 24-1 (that connects the internal references to the AREF line) has an on resistance much lower than 5 kohm (which I suppose it has), the internal circuitry will see AVCC. In this second situation, the current drain from the internal AVCC (assumed 5 V) to your external resistor divider will be \$\leqslant\$ 1 mA, but that is not a problem.
In summary: it would be bad advice if something could get damaged, but 1 mA won't damage anything.
An ADC reads an analogue value via its input but that analogue value is dependent on: -
If one of the two above analogue voltages are known the other can be computed or inferred.
I wouldn't have thought it was very common because most folk will want to have a stable voltage reference (from a known voltage reference chip) so that they can make accurate voltage measurements of the various input signals going into their ADC (multiple channels are of course usually "served" by one stable reference).
However, if the inputs are a bunch of RTDs and the RTDs are excited via a resistor from the AVcc rail then ratiometric measurements are the most accurate method of using the ADC and, if AVcc is also needed to be known then a stable voltage reference on a spare ADC input allows you to make this measurement.
Best Answer
The datasheet has your answer in section 24.5.2:
A voltage reference in a microcontroller is not the same as a voltage limit. The voltage reference (in this case) is used by the ADC to perform a comparison, and the microcontroller appears to be designed to allow you to exceed this amount.
You never want to exceed the voltage limit (in this case, that would be AVcc, the ADC's analog power supply). Microcontrollers have built-in protection diodes to save your bacon if that happens, but generally speaking, one should design the external circuit so that it does not exceed the voltage limit. (This is a best practice; in some cases, one may have reasons to ignore it.)