[time-nuts] Ublox neo-7M GPS

Dan Drown dan-timenuts at drown.org
Thu Aug 21 21:44:14 UTC 2014


Quoting Tony <tnuts at toneh.demon.co.uk>:
> I don't have any means to do that at the moment - when I get time  
> I'll try programming my STM32F4 discovery board (168MHz ARM Cortex  
> M4) to take some measurements. However as I haven't used the timers  
> on that yet it'll take a bit of time to get it right!

Be careful interpreting those results.  I have a stm32f4 discovery and  
its timer  occasionally wanders very far.  The measurements I have are  
using the internal PLL as the timer's source, which is driven by the  
external crystal.  I assume it wouldn't be an issue if you used an  
external clock source for the timer, but I haven't tested that.

I captured timer measurments of PPS signals from a Navspark GPS over  
20 hours with a timer capture channel on TIM2.  TIM2 was setup with a  
10.5Mhz rate (84Mhz / 8).  Mostly the intervals between PPS were  
within 1ppm (10hz).

Looking at a random 4 second interval where they were not within 1ppm:
10500782HZ (normal)
10499731HZ (-100ppm)
10501831HZ (+99ppm)
10500781HZ (normal)

This happened 7 times within 20 hours, with the same pattern.  This  
also happens when I switch out the Navspark GPS for a different  
navigation-grade GPS.

In case you're interested in the stm32 code, it's based on the STM libraries:
https://github.com/ddrown/usb-gps/blob/master/src/pps.c
https://github.com/ddrown/usb-gps/blob/master/src/mytimer.c



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