[time-nuts] Trimble Resolution SMT GG weirdness

[Daniel Ginsburg]

On 23.04.2013, at 3:02, Jim Lux wrote:

> 
> Assuming the software is reading some hardware counter when it gets the pulse, there could be variable latency in the routine.  8000 ticks is a lot of instructions though..

The hw timer latches the counter value to a separate register when the leading edge of PPS arrives, and raises an interrupt. The ISR simply reads the latched value and subtracts the previous value to get the duration. So interrupt latency shouldn't be an issue.


> 
> If you read, say, 100 intervals, what does the distribution look like?
> is it always the same number (crystal is too high in frequency), or does it vary (software might be an issue)


These are 100 intervals, you can see them going steadily up and then down, and up again. A sawtooth of gigantic proportions.
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