[time-nuts] looking for low-power system for gps ntp timekeeping

Hal Murray hmurray at megapathdsl.net
Fri Jul 5 16:53:52 EDT 2013


tshoppa at gmail.com said:
> I still don't trust USB for precise PPS timing but folks could convince me
> different. Doubt GPIO14 would get there either but I could be educated there
> too. 

USB is probably good enough for ms level timing.


I haven't looked carefully at the details of the chips used in any of the 
boards discussed recently.

Several years ago, I worked with ARM SOCs.  These were the ones designed to 
run out of on-chip RAM and flash rather than external memory.  The limitation 
was pins.  The chips had more IO devices than there were pins for.  Each pin 
had 2 or 3 possible uses.  If you were considering using a chip, you had to 
do more than just check the summary info sheet to make sure in had enough 
timers, UARTs and whatever you needed.  You also had to make sure that you 
could find a pinout assignment that wouldn't conflict.

With that in mind, there are two possible ways to get PPS timing into one of 
those chips.

1) You could get an interrupt on a GPIO pin and save the time, just like the 
traditional PPS on a modem control signal.

2) You could find an unused GPIO pin that is connected to a previously unused 
counter/timer and set things up to latch the count when the level changes.  
...


-- 
These are my opinions.  I hate spam.





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