[time-nuts] raspberry pi, adafruit gps & ntp

[David J Taylor]

Check this picture:
http://jeffskinnerbox.files.wordpress.com/2012/11/raspberry-pi-rev-1-gpio-pin-out1.jpg

GPIO pin 8 is in the upper right block with SPI in it.
GPIO 8 (CE0) is written above it. By the numbering on that diagram it is
physical pin 24.
[]
If I remember correctly, the kernel patch uses GPIO 18 (PCM_CLK), so
using GPIO 8 will work fine with the patched kernel.
[]
Maybe, but I'm not an electronics expert (not at all in fact) you can
feed both pins?
[]
Ok, I've uploaded version 0.2 which can do so.
This version can also, in debug mode, show the offset of the local clock
to the PPS timestamp.


Folkert van Heusden
====================================

Fine, Folkert.  The GPIO pins are numbered in your program exactly as I 
would expect, and the diagram just confirms that.  Yes, the patched kernel 
can remain, and PPS operation should be unaffected.  I was able to drive the 
two pins (GPIO 18 and GPIO 8) in parallel, and I have a signal level of 
about 2.5 V from a resistive divider.  I'm actually driving it from a 
Trimble SMT module where the serial is over the USB, and gpsd see the GPS 
data perfectly.

Thanks for the monitoring output.  The tie delay between the PPS leading 
edge and the change of the monitoring line varies between 270 to 390 
microseconds, so despite the jitter being reported in the 3-5 microsecond 
range, the "internal clock" is perhaps running about 1/3 millisecond behind 
"real" time.  You will be able to see the performance compared to kernel-PPS 
here:

  http://www.satsignal.eu/mrtg/performance_raspi-2.php

and a comparison with other stratum-1 Raspberry Pi cards here:

  http://www.satsignal.eu/mrtg/performance_ntp.php

The software compiled and installed without any issues.  I've yet to figure 
out how to get it to auto-start - you might like to add that to your 
instructions.  I may add a section to my Web page with a step-by-step guide, 
as this is so much easier than have to use a non-standard kernel.

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk