This is a "3rd order polynomial ?", and it is used to linearize or
compensate a curve.  Handheld instrumentation, mostly calibrators,
should use several of those polynomials to calculate correct values from
a physical sensor, as for example thermocouples.  Unfortunately the
precision is the king, it will rule all above you. If you want a great
precision, will need to have a great table or a great math routine.  

Math routine is not a big deal, just a multiple byte, lets say 64 to 128
bits is ok. It will require some keyboard work, but you are lucky, it
needs only 3 calculation blocks. For certain sensors I already saw more
than 12 blocks processed in a 68HC705.

Ok, you can be lucky again, as this is just a sine formula, just
convince yourself that 5 digits sine value is enough and 628 entries
(PI*200) should be enough, so a 1256 table entries can give you a
resolution of 0.588¡.  

Wagner

"Phu T. Van" wrote:
> 
> Hi. I've got a problem I don't quite know how to tackle.
> I need to program the following function for a 16F84 :
> 
> P(n) = x - (x^3/6) + (x^5/120) - (x^7/5040)
> 
> I don't know what this approximation is called or if it really works the
> way it's supposed to. I got it from a local math whiz; supposedly it
> will approximate sine of x where x is the angle in radians.
> 
> Anyhow, I need a reliable, fast and efficient way to calculate sine in
> realtime to do navigation. I've heard about lookup tables but apparently
> that is said to consume a huge amount of memory and processing power.
> I'd like to stick with the PIC if possible.
> 
> Any/all inputs appreciated.
> 
> --Phu T. Van