AT91ST_DIVIDER
WRITE_ST(ST_PIMR, AT91ST_DIVIDER);
WRITE_ST(ST_PIMR, AT91ST_DIVIDER);
tvp->tv_usec += at91st_count_to_usec(AT91ST_DIVIDER - current_count);
#define USEC_PER_TICK (1000000 / (AT91_SCLK / AT91ST_DIVIDER))