Quoting Tony <[email protected]>:
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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