ref: 75d6267a5f788162d92e7a5ae126cd8b0770aa8a
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 */ };