LPC1768 RTC frequency check with WDT. Compare RTC against MCU's microsecond Timer.
Check LPC1768 RTC crystal frequency using WDT. Measure with MCU's microsecond Timer. The MCU's crystal may be off a bit as well. I measured the RTC frequency error at 22 ppm. The spec for the RTC crystal is 20 ppm. Using NTP and a GPS, I had measured the MCU's crystal frequency error at -2 ppm.
Frequency measurements for other MCU/RTC cyrstals at