I'm designing a device that uses an ATmega328 @ 8MHz with Arduino bootloader that I would power from a 1 cell lithium ion battery (4.2V fully charged). The device contains 2 potentiometers, 3 buttons and a small oled display that is controlled through I2C (pins A4-A5). I'm not using a voltage regulator but powering it direct from the battery to keep things simple.
I would like to monitor the battery voltage through an analog input. So I can display a low battery warning on the oled display. But since I'm not using a voltage regulator I don't have a reference voltage. I thought about using the internal voltage reference (1.1V). But how will this effect the I2C pins and the 2 analog inputs I'm using for the potentiometer?
Maybe I should use a low dropout 2.8V regulator?