Objective
How to read a NTC thermistor via ADC to get the current temperature.
Problem
- NTCs have a negative exponential behavior [1,2,3].
- When trying to read with the ADC on an AVR, you somehow need to fit the voltage into AVcc, for example using the internal reference at 2.56V.
Solution
[2] suggests combining a resistor with the NTC in parallel to create an approximate linear response for a given temperature range.
I decided to do the math first, and it turns out a voltage divider works like a charm. Set the ADC reference voltage to 2.56V. Then wire the divider up this way:
5V ----------+
|
NTC 10k@25C
|
+----------o ADC
|
3.6k
|
__|__
___
-
The 3.6k value can be calculated from [2] and a bit of experimentation.
When you read the ADC (say, 8bit ADC), you'll have to post-process the reading. BUT! It is already linear. That means we can easily compute T (in degC):
T = (ADC - 40) >> 2;
Of course, you can decide not to shift and instead have .25 degC resolution.
The resulting temperature reading can be seen in [4] (analysis).
[1] https://secure.wikimedia.org/wikipedia/en/wiki/Thermistor
[2] https://www.avx.com/docs/Catalogs/ntc-general.pdf
[3] 
[4] 