AT91_SCLK
AT91_SCLK / 1000U, AT91_SCLK % 1000U,
result = (1000000 * count) / AT91_SCLK;
if ((result * AT91_SCLK) != (count * 1000000))
result = (AT91_SCLK * usec) / 1000000;
if ((result * 1000000) != (usec * AT91_SCLK))
usec = (usec * 1000 + AT91_SCLK - 1) / AT91_SCLK + 1;
#define AT91ST_DIVIDER (AT91_SCLK / HZ)
#define USEC_PER_TICK (1000000 / (AT91_SCLK / AT91ST_DIVIDER))
result = (AT91_SCLK * usec) / 1000000;
if ((result * 1000000) != (usec * AT91_SCLK))