9403 edge detections?

[TimDavis]

What is the maximum number of edge transitions per channel we can detect?  In the spec, the minimum signal time is 10nS stable transition and 60nS hold time, but presumably we are also limited by the FPGA performance (40MHz?).  The follow up question is how many total transitions across all input channels?

The context is the number of high speed sensors, and in particular quadrature encoders, we can connect to a single module.

Thanks,

Tim

[sciencewhiz]

See http://forums.usfirst.org/showthread.php?t=11589

[DAQjr]

Due to slight optimizations in the FPGA since that post, the current maximum is 39,987 counts per second.  This is derived from the sampling rate of 6.525us and requiring 4 edges per "count", where count assumes 1x decoding.  This also assumes that the quarature signal's edges all have perfect 90 degree phase relationships.

-Joe

[George_P_Burdell]

TimDavis-

The forum post sciencewhiz suggested you look at is accurate.  The max rate for the 9403 is 6.525 microsec (or about 153kHz).  As an fyi, if you think about this in round numbers, it takes 4 edges for a quadrature encoder.  This brings this number down to about 38,000 counts per second.  If you had an encoder with 1024 pulses per rev (a pretty good encoder), you can read it spinning at 38,000/1024 = 37 revolutions per second.  That’s pretty fast.  What is your application, and are you seeing an update rate that is not quick enough?

-Mike S

[TimDavis]

Thanks!

Our application is encoding shaft speed from one of the motors.  The sampling rate specified is more than adequate, the objective was to confirm the performance before we committed design time.

For my own edification and future reference, where in the 9403 or First datasheet can I find the sampling rate spec?  I seem to find only the signal rise and hold times.  The analog module is documented at 500kS/s aggregate, which was helpful when we were choosing to analog sensors.

Tim

[599mentor]

What is the maximum error for measuring the time between pulses?