-// must be 0x40, but on my cpu - included divider is optimal
-// 0x20 - 1 ms / bit
-// 0x40 - 2 ms / bit
-
- AT91C_BASE_RTTC->RTTC_RTMR = AT91C_RTTC_RTTRST + 0x001D; // was 0x003B
+ // This timer is based on the slow clock. The slow clock frequency is between 22kHz and 40kHz.
+ // We can determine the actual slow clock frequency by looking at the Main Clock Frequency Register.
+ uint16_t mainf = AT91C_BASE_PMC->PMC_MCFR & 0xffff; // = 16 * main clock frequency (16MHz) / slow clock frequency
+ // set RealTimeCounter divider to count at 1kHz:
+ AT91C_BASE_RTTC->RTTC_RTMR = AT91C_RTTC_RTTRST | ((256000 + (mainf/2)) / mainf);
+ // note: worst case precision is approx 2.5%