At 08:37 PM 12/10/97 -0500, you wrote:
>Good evening,
>
>        Can anyone point me in the direction of an example showing the
>implementation of a logarithmic calculation on the PIC16C84.  I am trying
>to convert a voltage (1.5-8.7 volts) from a vacuum gauge into a pressure
>in Torr using the relationship (P=10^(V-11.00).  Any help/suggestions
>will be greatly appreciated.
>
>Sincerely,
>Aaron Hickman
>

Aaron,

It would seem to me that you are looking for an exponential function rather
than a log. If you have voltage and wish to calculate pressure, then you
simply want to raise 10 to the power of the Voltage - 11.00

Take a look at www.microchip.com I took a quick look at the Embedded
Control Handbook which has many Microchip App notes, including floating
point math routenes, but no exponentiation functions. As Andrew Mayo
suggested, you might try a lookup table, but I would also suggest using a
polynomial approximation to fill in the gaps among values from the lookup
table. I just tried constructing a taylor series for 10^x and it works, but
it requires alot of terms to be accurate over the range you are talking
about. You might try expanding around some value other than the center of
your range. I used 5.1, your center.

Microchip does publish code for floating point multiplication, division,
square roots, etc.

Good luck,

Sean




+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
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