[time-nuts] STM32 based thing (was GPSDO Alternatives)
lists at rtty.us
Fri Dec 7 12:22:39 UTC 2012
I believe your second counter (the one that starts on one edge and stops on the other) will have trouble. It is unlikely you will get your 14 ns resolution out of it.
The analog TDC is a fine idea. Cheap, easy, and requires a few parts that are not built into the micro controller. The simple answer is to use a cheap CPLD (possibly sub $2) for a bunch of this stuff and move on.
On Dec 7, 2012, at 4:26 AM, Fabio Eboli <fabioeb at quipo.it> wrote:
> Michael Tharp <gxti at partiallystapled.com> ha scritto:
>> On 12/6/2012 4:26 AM, Fabio Eboli wrote:
>>> Are you using the pll to obtain 72MHz (x9) for the clock?
>> Yes, the crystal oscillator is multiplied up to 72MHz which then drives the timer. Even though the particular timer peripheral I chose happens to be on the APB1 bus which is restricted to 36MHz, the timer itself is still fed with the 72MHz clock. Both PPS signals (generated from OCXO and received from GPS) are then independently timestamped.
> Ok, now I understand.
> How is faring the pll clock on 1s intervals? The datasheet reports
> 300pS jitter, that should be good.
> My plan for now is this:
> (sorry if my terms are not exact), i will start with a Rb instead
> of an OCXO. I want to try to measure the ratio between the GPS and
> the Rb, to do so I will use the Rb 10MHz clock a timebase for the pps.
> The first counter will be setup as input capture with external clock,
> the clock is the Rb 10MHz and the captured will be the rising edges
> of the GPS.
> This will give a count of around 10^7. Then I will use a second
> counter with internal clock that will start on the pps edge and stop
> on the Rb edge, the result will go from zero to 100nS/13.9ns=7
> This count will give some interpolation.
> As far as I understood all this could be done in the internal hw, but
> it remains to be seen if there are problems with the actual hardware
> (bugs, silicon errata etc).
> The external hw for now will be a normal XO for the micro, and some
> logic to square the inputs, perhaps I will setup also a FF
> to have a pulse that starts with the PPS and stops with the
> first 10MHz edge, this will be useful for testing with the
> real counter and to eventually implement an analogue TAC.
> Just now I was simulating on ltspice some TAC interpolators,
> if the previous system will go I'm thinking to add an analog
> interpolator that will convert the 0 to 100nS edge difference
> between the Rb clock and the pps in analog value, to increment
> the single shot resolution. I was also thinking that for the
> disciplining I dont need to have the interpolator giving the
> exact time interval, but I need it only be sensitive and stable
> The control loop is entirely another problem :)
> Ok now I need to start burning some flash to check if
> the thing can go somewhere.
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