Re: [gpsd-dev] Very basic PPS question:

[Hal Murray]

> When a GPS asserts PPS, is it top of second for the in-stream data
> *preceding* or *following*?   

If there is a standard, at least one device out there gets it wrong.  You may 
have to make a table.

-----------

address@hidden said:
> I am not an expert.  I have heard that it can go either way, it can be
> device-dependent, and there is a window around the PPS edge (which can be
> the leading or trailing edge) where NTP will "associate" the NMEA sentence
> with the PPS signal.  The window can be fudged to be earlier or later than
> its default position. 

All the device manufacturers that I'm familiar with use the leading/rising 
edge of the PPS pulse.

The confusion comes when people connect it to a modem control pin rather than 
a normal TTL/CMOS level logic receiver.  Almost all the TTL/CMOS to RS-232 
converter chips include an inverter and the ntpd drivers that handle PPS have 
an option to use the other edge.  So does the kernel interface.

----------

Eric, for your PPS trivia collection...

Most high end GPS gear uses a narrow PPS pulse, typically 10-20 microseconds. 
 That's narrow enough that some PCs can't/don't catch it, and sometimes they 
only get it some of the time.

Most low end gear has longer pulse so this isn't a problem.  100 milliseconds 
is typical.

TAPR has a pulse stretching gizmo: FatPPS
  http://tapr.org/kits_fatpps.html
It says "Price is TBD", so maybe that should be "had" rather than "has".
I expect they will build another batch if they get enough interest.  It 
didn't use any parts that are hard to find.



This might be a bug/oversight in the kernel(s).  I haven't looked at the 
detailed specs for any of the current modem chips.

I don't see how to fix this if all you can do is read the current status.  
But I think I've seen some chips that had interrupt status bits, so if you 
asked for an interrupt when it went low-high, and it went low-high-low, the 
interrupt status bit for the low-high interrupt would be set and tell you 
that it was high at one point in the past even if it is currently low.





-- 
These are my opinions.  I hate spam.