Quote:
Originally Posted by jhersh
the "count" is the number of samples in the sliding window average.
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Joe,
How is this implemented under the hood in FPGA? Is there a ring buffer in the FPGA which the FPGA populates with the 6.525us_resolution_timestamp for each rising edge it detects (assuming it's not in semi-period mode), and then when requested retrieves the elapsed time between the N+1 most recent samples in the ring (which the cRIO CPU then divides by N)?
Quote:
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Count will always be 1 if you specify not to average (NumberOfSamplesToAverage = 1).
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How large can "NumberOfSamplesToAverage" be? I searched the 2012 C++ and the 2013 Java WPILib code but couldn't find that search string.
EDIT:
@all:
I attached a small Excel app that computes the RPM jitter caused by the 6.525us timing resolution of the edge detections. There will be additional jitter due to manufacturing tolerances in the physical locations of the edges in the sensor, but I've not included those here.
Note how large the jitter is with a 360 PPR sensor at 5000 RPM with averaging set to 1. If you set the averaging to 120 (1/3 revolution) you can reduce the jitter dramatically, at the cost of some phase lag in the signal.
If you make a 1 PPR sensor with averaging set to 1 you'll be able to get an updated reading every 12ms at 5000 RPM with very low jitter.