[time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Fri Dec 7 01:54:51 UTC 2012


PSoC's are another attractive possibility that suffers from the same basic "re-clock everything" flaw. Lots of time down the drain there….


On Dec 6, 2012, at 8:22 PM, SAIDJACK at aol.com wrote:

> David,
> The NXP LPC932 processor series are very cheap and small, and we got very  
> excited to see timers running at up to 32MHz internally if I remember  
> correctly.
> Then setting up a test system we noted that the timer can capture with  
> 32MHz resolution which is good enough for a low-cost GPSDO implementation, but  
> that they gated the input pin through a flip-flop running at CPU core 
> speed,  which was around 6MHz if I remember correctly.
> DAAHHHH. So all that fast timer resolution goes out the door by gating the  
> input pin instead of using non-gated inputs for the timer functions.
> It does work however, in the end we made that processor do the chores in  
> our quite old and discontinued FireFox GPSDO circuit. TVB has some plots  on 
> his website for that unit I think, and its quite surprising what type of  
> stability we achieved with that little 8 bit bugger back then.
> bye,
> Said
> In a message dated 12/6/2012 16:00:27 Pacific Standard Time,  
> davidwhess at gmail.com writes:
> You can  use the ATmega328 16 bit timer/counter in input capture mode
> to count the  number of 10 MHz OCXO cycles per pulse per second period
> to a resolution of  100ns but there are some problems:
> The ATmega328 16 bit timer/counter  external clock is limited to 1/4 of
> the CPU frequency with an asynchronous  source so the 10 MHz OCXO would
> need to be divided down which would further  limit performance and
> require an external divider.  Modifying the  Aruino board to use the 10
> MHz OCXO in place of the CPU clock solves that  problem.
> Then operating the counter/timer in input capture mode with  the GPS
> pulse per second signal connected to the input capture pin would  allow
> almost Shera like performance.  The timing resolution would be  2.4
> times lower (and not asynchronous) limiting performance over  short
> time spans.
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