ref: 6076b4f9367729f60efe5a729070caaa5e38d29f
dir: /sys/src/cmd/aux/vga/tvp3025clock.c/
#include <u.h>
#include <libc.h>
#include <bio.h>
#include "pci.h"
#include "vga.h"
#define SCALE(f) ((f)/1) /* could be /10 */
static void
init(Vga* vga, Ctlr* ctlr)
{
int f;
ulong d, dmax, fmin, fvco, n, nmax, p;
if(ctlr->flag & Finit)
return;
if(vga->f[0] == 0)
vga->f[0] = vga->mode->frequency;
/*
* Look for values of n, d and p that give
* the least error for
* Fvco = RefFreq*((n+2)*8)/(d+2)
* Fpll = Fvco/2**p
* N and d are 7-bits, p is 2-bits. Constraints:
* RefFreq/(n+2) > 1MHz
* 110MHz <= Fvco <= 220MHz
* n, d >= 1
* Should try to minimise n, d.
*
* There's nothing like brute force and ignorance.
*/
fmin = vga->f[0];
vga->d[0] = 6;
vga->n[0] = 5;
vga->p[0] = 2;
dmax = (RefFreq/1000000)-2;
for(d = 1; d < dmax; d++){
/*
* Calculate an upper bound on n
* to satisfy the condition
* Fvco <= 220MHz
* This will hopefully prevent arithmetic
* overflow.
*/
nmax = ((220000000+RefFreq)*(d+2))/(RefFreq*8) - 2;
for(n = 1; n < nmax; n++){
fvco = SCALE(RefFreq)*((n+2)*8)/(d+2);
if(fvco < SCALE(110000000) || fvco > SCALE(220000000))
continue;
for(p = 1; p < 4; p++){
f = SCALE(vga->f[0]) - (fvco>>p);
if(f < 0)
f = -f;
if(f < fmin){
fmin = f;
vga->d[0] = d;
vga->n[0] = n;
vga->p[0] = p;
}
}
}
}
ctlr->flag |= Finit;
}
Ctlr tvp3025clock = {
"tvp3025clock", /* name */
0, /* snarf */
0, /* options */
init, /* init */
0, /* load */
0, /* dump */
};