Scott Daniels has published an interesting tutorial on making use of the ability of an AVR to measure the internal 1.1V reference voltage. The resulting value can then be used to determine the voltage at Vcc - which is ideal for monitoring the project supply voltage, and also used to improve the accuracy of analog input readings and therefore any measurements following this. Very useful indeed - as when running from 5V the analogRead range falls below 5V and accuracy can suffer.
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