AVR-Chat

On Thu, 19 Apr 2007 20:03:43 -0000 you wrote:

>Does anyone know where I might find some example Winavr code for 
>reading the temperature from an NTC Thermistor Probe 
>(http://www.epcos.com/inf/50/db/ntc_06/ProbeAss__B57020__M2020.pdf) 
>connected to one of the adc pins of an avr (atmega32)?
>
>I would also like to know how the 2 wire probe should be connected.  
>Should one wire go to ground and the other to an adc pin?  Or should 
>one go to ground and the other go to an adc pin with voltage connected 
>in line via a resistor? etc...

The butterfly code was, I seem to remember, ported. I would check AVRfreaks.

Basically one has a lookup table like
flash unsigned int ntclookup[69]=
{774,765,755,745,735,724,714,703,692,
682,671,659,648,637,626,614,603,592,580,
569,557,546,535,523,512,501,490,479,468,
457,446,436,425,415,405,395,385,375,365,
356,347,338,329,320,311,303,295,287,279,
271,264,256,249,242,235,229,222,216,210,
204,198,193,187,182,177,171,167,162,157};

And use it like
  ntc2data=read_adc(2); 
  roomtemp=0;
  while ((ntc2data < ntclookup[roomtemp])&&(roomtemp<70)) roomtemp++;

Epcos have a tool on their site somewhere for calculating all the resistance
values at different temperatures.

The connection is from the NTC to ground and an ADC pin and a resistor from the
ADC pin to V+ .The value of the resistor would be around the NTC resistance at
the average operating temperature or the middle of the desired temperature
range.

The Epcos tool puts the resistance values in an MSAccess database.
You can get the values for the lookup table with an Access query like

SELECT Result.t, Result.r_soll, Int(([r_soll]*1024)/([r_soll]+10000)+0.5) AS
adcval INTO adcvals
FROM Result
ORDER BY Result.t;

Replace 10000 by the resistance value connected ADC/V+
--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net