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

[Phil Salkie]

Hey Eric,

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.  (BTW, if anyone needs to convert NMEA to
RS-232 with PPS, drop me a line, I've got boards and cases to put them
in - Garmin 18X-LVC direct connection, as well as 0.1" header pins.
One LED for power, one for PPS.  Power from USB-Micro or header pins.)

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?

If PPS signals are always less than 500ms wide, it would be pretty
straightforward for the kernel driver to determine the leading edge by
looking at the width of a pulse, and inverting its logic if it sees
the pulse width is >500ms.  This obviously wouldn't work for an exact
square wave output, but that would seem like a weird thing for a
vendor to build, just because it's harder to determine which edge one
should be looking at.  (Or trigger on either edge, then ignore edges
for the next 550ms, then trigger on either edge, and repeat - that'd
work, too.)

(For the curious, the scope is a Hantek DSO-8060, dual channel 60MHz
storage scope, color 5.7"LCD, arbitrary waveform generator, with full
independent and isolated DMM functions.  It was a big $408 new on
ebay, and replaces both the 30 year old Fluke DVM and the Seeed
Studios DSO-Nano in my field service tools kit.)

Cheers,

Phil