refclk.c

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
typedef unsigned int uint;

#ifndef PICO_PLL_VCO_MIN_FREQ_MHZ
#define PICO_PLL_VCO_MIN_FREQ_MHZ 750
#endif

#ifndef PICO_PLL_VCO_MAX_FREQ_MHZ
#define PICO_PLL_VCO_MAX_FREQ_MHZ 1600
#endif

#define clock_get_hz(arg) (12000000) // reference clock is 12MHz
bool check_sys_clock_khz(uint32_t freq_khz, uint *vco_out, uint *postdiv1_out,
                         uint *postdiv_out) {
    uint crystal_freq_khz = clock_get_hz(clk_ref) / 1000;
    for (uint fbdiv = 320; fbdiv >= 16; fbdiv--) {
        uint vco = fbdiv * crystal_freq_khz;
        if (vco < PICO_PLL_VCO_MIN_FREQ_MHZ * 1000 ||
            vco > PICO_PLL_VCO_MAX_FREQ_MHZ * 1000)
            continue;
        for (uint postdiv1 = 7; postdiv1 >= 1; postdiv1--) {
            for (uint postdiv2 = postdiv1; postdiv2 >= 1; postdiv2--) {
                uint out = vco / (postdiv1 * postdiv2);
                if (out == freq_khz && !(vco % (postdiv1 * postdiv2))) {
                    *vco_out = vco * 1000;
                    *postdiv1_out = postdiv1;
                    *postdiv_out = postdiv2;
                    return true;
                }
            }
        }
    }
    return false;
}

int main() {
    int s = 0, f = 0;
    uint vco, p1, p2;
    for (int i = 1000; i < 133000; i += 1000) {
        if (check_sys_clock_khz(i, &vco, &p1, &p2)) {
            printf("%dkHz\n", i);
            s++;
        } else {
            printf("!%dkHz\n", i);
            f++;
        }
    }
    printf("%d successes, %d failures (%d%%)\n", s, f, s / (s + f));
}