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

[Hal Murray]

address@hidden said:
> 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. 

That looks like a mixup between pin numbers in the chip and pin numbers on 
the connector.

The pin numbering on modern chips using PGA (Pin Grid Array) packaging is 
usually two dimensional like A9 so that won't match either 4 or 18.

The individual I/O pins on a SoC chip like this usually have several possible 
uses.  You have to pick one.  The I/O device behind the other usage is 
ignored.  (except sometimes a usage goes to 2 pins)  Sorting that out is one 
of the "interesting" design problems.  Just because the data sheet says you 
have 2 foos and 3 bars that doesn't mean you can use them all at the same 
time.  You have to make sure you can find a combination of pins that works.

One possible use for almost all pins is GPIO.  Usually they are grouped 
together 32 to a register, so you can say things like GPIO[3] goes to LED[2] 
or PPS.  All the leftover pins that aren't used for things like serial I/O 
ports or uSD can be used as junk I/O aka GPIO for things like PPS or driving 
LEDs.

So "GPIO pin 18" probably refers to bit 18 of the GPIO register while "pin 4" 
probably refers to the connector.


> I tried building contrib/ppscheck on the Pi 3 and running it on /dev/ttyAMA0
> and saw no line transitions. 

The modem control signals on the serial port most likely aren't assigned to 
any external pins.


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