Murray Greenman wrote: > In the light of the latest posts on driving clocks from 1pps, it sounds > as though I'd better rethink what I was planning! > > I am in the middle of the design of a micro which uses a 10MHz crystal > to provide a digital clock, but the time is kept in line via GPS using a > 1pps NCO, which is steered digitally, rather than altering the 10MHz > oscillator in GPSDO fashion. > > The plan was to provide two outputs (biphase) at 1pps to drive slave > clocks, but in the light of the notes from Brooke and Chuck, I would be > better off just providing a single output, and use a series cacacitor. > > Chuck, I would expect that the 1pps would need to be about 50% duty > cycle, or at least have a pulse width of 100ms or so. I can imagine a > clock driven from 1pps with a low duty cycle would sound quite > different.
Yes, for a simple circuit like that, you need a 50% 1/2pps duty cycle, otherwise the clock will go: tictoc...................tictoc...............tictoc..... The simple ubiquitous quartz clock that runs on a single AA cell has one most important requirement and that is to keep the coil current off except when switching seconds. The motor has a permanent magnet, so it is a latching stepper motor. And the actual waveform used to drive the clock's coil is 1/2 Hz, not 1pps as I showed. If you have 1pps, you will need to use a toggle FF to cut the frequency in 1/2 to 1/2pps with a 50 percent duty cycle. If you want to do something like divide down from 10MHz, most any modern PIC can be used to make a one chip, zero extra part solution, as was described by TVB earlier. -Chuck Harris _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
