External crystal clock. A precision quartz crystal tuning fork provides a stable clock to drive all device timing. The crystal’s 32.768 kHz oscillations are counted into the PIC12 16-bit incrementing counter so that one timer rollover occurs every 2 s. This rollover initiates a wake from the microcontroller sleep state (wake every 2 sec), and all device timing (e.g. measurement interval) is a multiple of 16-bit timer cycles. The 200 ppm accuracy of the quartz crystal leads to timing accuracy of 6 ms h-1 (4.3 s per 30 days).