On Wed, 10 May 2000, Jim Ham wrote:

> The routine below works mostly OK - it seems to have about a 20% error. If
> anyone sees the problem please drop me a note.

Yeah the problem is that the PIC is getting bored taking such a long time
calculating the square root and is forgetting the answer before the algorithm
completes :). But seriously, if you need a square root routine I'd suggest
looking at James' pic pages  (what's the latest URL James?) or

http://www.dattalo.com/technical/software/pic/picsqrt.html

I've haven't been paying attention to the thread so I'm not sure what your
equation looks like.

Scott

>
>
> /***********************************************************************\
> *
> * return indicated airspeed (kts)(assume std density at sea level
> * also assume 80mv FS in A2D
> * returns airspeed in 10ths of knots
> *
> \***********************************************************************/
> int SpdLookup( ulong raw )
> {
> /*
>       return (int)sqrt( raw * 200 )/10 ;
> */
>
>    byte iteration ;
>    unsigned long guess ;
>    unsigned long last_guess ;
>
>    // 1 psi = 2.13e6 counts from A/D
>    // rho 2.378e-3 slugs/cu.ft - density of air at sea level
>    // p(psf) = 1/2 rho(slugs/cu.ft) V^2(ft/sec)
>    // p(psf) = raw / 14789 ;
>    // V(ft/sec) = sqrt( 2 * p /rho )
>    // 1 kt = 0.5925 ft/sec
>    // V(kts) = 0.5925*sqrt( 2 * p /rho )
>    // V(kts*100) = 59.25*sqrt( 2 * p /rho )
>    // V(kts*100) = sqrt( (59.25^2*2*raw) /(2.378e-3*14.789e3) )
>    raw *= 200ul ;
>   //***********************************************************************
>   //*
>   //* Take a square root - Newton's method
>   //*
>   //***********************************************************************
>    iteration = 20 ;
>    last_guess = guess = raw / 128  ;
>    do {
>       if (guess == 0l ) guess = 1l ;
>       guess += (raw / guess) ;
>       guess /= 2 ;
>       if (last_guess == guess) break ;
>       last_guess = guess ;
>    } while ( --iteration ) ;
>    return MakeInt(guess/10l) ;
> }
>
> Jim Ham, Porcine Associates
> (650)326-2669 fax(650)326-1071
> "http://www.porcine.com"
>