Your approach is probably possible on a PIC, although you don't say
which one you're thinking of using, but on small micros I would think
looking up the temperature in a table would be more practical.

Tim McDonough -- timmed@cencom.net


On Mon, 13 Mar 1995, James L. Johnson wrote:

> Hello,
>
>   I am going to try to implement the solution to the Steinhart-Hart
> equation using a PIC.  This equation gives the relationship between
> the resistance of a negative-temperature-coefficient thermistor and
> its temperature.  It is an empirically determined equation, so is
> only an approximation to the relationship between resistance and
> temperature.  But, it is quite accurate over a large range as long
> as the coefficients are accurate.  The thermistor manufacturer gives
> you the coefficients so all you have to do is plug in the numbers.
>
>   This is a no-brainer in any high level language and we have it running
> in BASIC here in our lab.  However, it's probably rocket science to get
> it to work in a PIC.  At least it seems like it to someone who has no
> experience in programming microcontrollers (me).
>
>   Can someone give me some advice on how to do this?  Are there pieces
> of code already written which can do parts of this (natural log, for
> example)?  Are there pitfalls which make it harder than it looks or am
> I over-estimating the effort here?
>
>   The equation is:  1/T = a + b(ln R) + c(ln R)^3  where T is in Kelvin,
>   a,b, and c are the coefficients provided by the manufacturer, R is the
>   resistance in ohms, and the notation "ln" is the natural logarithm
>   (base e).  I assume the "cubed" term can be done by doing three
>   "multiplies" but I don't know about the log and the reciprocal.
>
>   Thanks, in advance, to anyone who can get me started on this!
>
>   Jim Johnson
>   jjohnson@hpl.hp.com
>