How does the AVR clock trigger on the 1 PPS signal?
By default the AVR clock uses a 1PPS signal generated by the on-board RTC. This is a negative going pulse so the AVR clock triggers on the negative going edge. Most GPS units have a TIMEKEEP signal that is normally low and pulses high for 200 mSecs every second; by default, the AVR clock triggers on the negative edge at the end of this pulse. You can use the Serial Command ‘M’ to toggle the trigger mode of the AVR clock. Please note the following. The AVR Clock has an internal pull-up resistor enabled on this pin so that the pin is HIGH when not connected (e.g., when the shunt is removed from JP6). If your 1 PPS signal is normally low, it will have to PULL DOWN the signal to LOW, and a small current will run from the AVR to your signal source.
