adrian_h:
So, as long as the voltage is higher than 1.1V, even if the rest of the system has dropped below 5V or 3.3V (depending on hardware), it will get an accurate reading for 1.1V? Does the chip have an on board voltage regulator to ensure an accurate 1.1V over a range of voltages so it can be done just using the bare chip?
The band-gap voltage can be considered a 'regulated' source not effected by Vcc variation. However while it's a pretty constant value for any specific AVR chip the actual value of the band-gap is subject to a device tolerance specification. So one does have to derive what their specific band-gap voltage is if they are going to use it for 'calibration' purposes.
Lefty