AVR-Chat

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...

Thanks!

You need to construct a voltage divider with the thermistor
and a second fixed resistor. It is connected with the fixed
resistor between ADC input and AVref and the variable
resistor between ADC input and ground (usually). If you
used AVcc for Vref, then connect the fixed resistor to Vcc.


Jim




On Thu, 19 Apr 2007 20:03:43 -0000
 "jasonparkerhendrix" <hendrix.jason@gmail.com> 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...
> 
> Thanks!
> 

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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

Thanks for your help!  I made very good use of the epcos tool and 
everything is working great now.

--- In AVR-Chat@yahoogroups.com, Ralph Hilton <ralph@...> wrote:
>
> 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
>

If you want accuracy, you can run the hart-steinhart equation.