[time-nuts] F9T, was Re: Ublox F9P PPS output

[Achim Gratz]

Bob kb8tq writes:
> It’s just like the other uBlox modules (and just like the F9T …) the external 
> input is a once a second time stamper with an 8 ns resolution (just like the 
> +/- 4 ns on the sawtooth ….).

A minor nit: If it works as the M8 series does (which is very likely),
the timestamp facility gives you the last positive and the last negative
going transitions w.r.t. the configured message rate, plus the count of
positive going transistions.  This means you can get both higher
timestamp rates by configuring a message rate of more than one per
second (which does have some other implications, though) and can use
(much) higher frequencies than just simply PPS (but get only the count +
measurement of the last transitions).  Also, if the hardware doesn't use
the second EXTINT pin for something else (like power management), there
actually should be two of these timestamp inputs useable.

> One would *guess* they are using the same 
> free running / TCXO clock for both the PPS out and the timestamp coming in. 

It's not confirmed in any documentation that I've found, but the above
characteristic hints at the timestamp facility being some counters and
registers in hardware, so the system clock is the only thing that could
be used for this.  On a proper timing module with FTS firmware you could
of course inject your own cleaned up clock to use as the timebase, plus
you can use it to discipline an external frequency source.

Btw, if anybody is thinking about getting the ardusimple F9P-RTK module,
somewhere in their forum it is mentioned that a bug on the current board
revision has the PPS output at 0.7V instead of the documented 3.3V.


Regards,
Achim.
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