Re: [gpsd-dev] SemPiTernal - Bounding PPS uncertainty

[Hal Murray]

address@hidden said:
> If you recall, we had an issue with connecting the Garmin GPS hockey puck to
> a serial port - even after converting the 0 to 5VDC data lines to RS-232's
> -10 to +10VDC, we still couldn't read it, because all the data was inverted.
>  That's why I built that little PC board which fits in the blue plastic case
> - to convert NMEA GPS data lines to RS-232 with the PPS line included.  It's
> got _two_ chips - an inverter, and a voltage level shifter. 

It also works if you just wire it up without either chip.


address@hidden said:
> The issue here seems to be not so much what the GPS devices themselves do,
> nor what the kernel does - they're both well defined.  What's kind of random
> is how things get connected together - does everyone who's making these
> hats, coats, shields, etc. pay attention to making sure the PPS signal
> always hits the CPU in such a way that the edge the GPS chip thinks is top
> of second is the edge the kernel thinks is the top of second?

In general, all sorts of scrambling happens in there.

GPSD does the right thing automagically.  ntpd requires you to tell it which 
edge to use.  That assumes the kernel supports both edges.

The broken kernel is not a problem for ntpd with either of the 2 HATs that I 
know about because they got it right.


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