Re: [gpsd-dev] 1PPS on the Pi - let's pool our knowledge.

[Frank]

Few quick comments below

> On Apr 19, 2016, at 7:06 AM, Eric S. Raymond <address@hidden> wrote:
> 
> The next logical thing to add to the draft HOWTO is a description of
> how to configure the Pi+HAT so that 1PPS is visible to our test programs - I'm
> thinking mainly of gpsmon, ppstest, and GPSD's contrib/ppscheck.  This
> seems like a good preparatory step for making it visible to NTPsec.
> 
I have a spare GPS & Pi.  I can test & provide some feedback, but I’m not sure 
if I can get to it before this weekend.
> 
> Gary refers to GPIO pin 4.  His "dig deeper" source is David Taylor's,
> which says GPIO pin 18. Examining the traces on my Pi 3 causes me to
> suspect that it uses pin 4.  Did this vary by Pi model?  If so, we
> need to be quite clear about which pin it is on which Pi.

All Raspberry Pi’s have 2 x headers for GPIO.  The larger one, always 
populated, except on the Pi Zero is referred to “P1”.  There is a smaller, 
alway unpopulated (I think), referred to as “P5”.  See these diagrams for a 
pinouts of all versions: http://www.panu.it/raspberry/ I believe the Raspberry 
Pi 2 & 3 follow the same general pinout as the B+ (at least I KNOW the lower 
GPIO pins do as I am using the B+, 2 & 3 for the same purpose with the same 
pins.)  More reference:
https://www.raspberrypi.org/documentation/usage/gpio-plus-and-raspi2/
http://pinout.xyz
https://www.element14.com/community/docs/DOC-73950/l/raspberry-pi-3-model-b-gpio-40-pin-block-pinout

The GPIO pins are sometimes referenced by physical pin location on the header 
(1-40 on the current rev), and sometimes as the GPIO line connected to the CPU. 
 This can cause confusion.  They are best referred to as P1-[1-40], p5-[1-8] or 
as GPIO [2-31].  Note that some are typically pre-configured for specific 
functions (serial, i2c, etc.)

Earlier Raspberry Pi’s (original A & B) had a smaller P1 (26 pins), with fewer 
pins.  However, the newer Pi’s maintained backward compatibility with the 
original Pi’s, with just additional pins.

The pps-gpio module used to allow you to specify which GPIO pin to use, when 
loading the module.  I’m not sure if current versions still supports this, or 
if it’s all done in the overlay line.  I am glad to test this, but it may not 
be until this weekend.

Note that GPIO pins function can be changed (as you learned with the Raspberry 
Pi 3 serial/BT/bad) issue.  The function change can only be done at boot up 
with the overlay files.  The overlay files are relatively new, after the 
Raspberry Pi 2 was released. You might want to mention in the How To, that if 
they are using any other overly files, the function of their GPIO pins might be 
different.

I would recommend we always target the current hardware/OS distribution and use 
the latest reference/design.  Users looking at the HowTo should be instructed 
to be sure they are using the latest Raspian version at:  
https://www.raspberrypi.org/downloads/