I'm building a ultra-low powered temperature logger intended to run for months on just 3 AA batteries.
I want it to log the temperature with fairly precise intervals, something like 10 minutes or so. So the internal oscillator is ruled out, at least for driving the timers, the temperature will vary a lot!
So to save battery the µC needs to be in sleep mode 99% of the time, but from what I read in the datasheet I'm a little unsure if I can use the power-saving modes from which the CPU will wake up on timer interrupt!
In the datasheet p. 33 it says "Note: 1. Standby mode is only available with external crystals or resonators." - Nowhere it mentions which of the others (if any) will work with crystal. Looking at the typical characteristics on p. 253 it looks like "idle" mode should work, but I was hoping to get somewhat below 1mA at 2 MHz I'm planning to run it at.
Does anyone have any tips of how to save as much power as possible? Is there another way to keep track of time so I can use the internal oscillator? I've bought a Atmega8L. Also there's going to be a DS1624 temperature sensor, a 24AA64 EEPROM and a MAX3222 on the board, all have very low standby currents in the nA-µA range.