speedometer

marcla

Hello!

How do i measure the time betvin two pulses.
it's goning to be used in a speedometer and the sensor gives one pulse per each rotation.
The speead never goes faster then 50kmh (30mph).



/Martin
Sweden

Mike Green

You wait for the beginning of the first pulse with WAITPNE or WAITPEQ, then save the value of CNT in a variable.

You then wait for the beginning of the second pulse, then subtract the saved value from CNT to get the time difference in system clock time (12.5ns for an 80MHz system clock).

Look in the manual for descriptions of WAITPNE, WAITPEQ, and CNT.

Martin Hebel

You can setup the counters to measure the time a pulse lasts. The BS2_Functions library has functions for PULSIN or FREQIN to measure the length of a pulse or the frequency of a signal in case that helps too. For the down and dirty method of setting things up yourself, check out the counters chapter of the PEK downloads.

You can also use the WAITPNE and WAITPEQ to instructions to wait for the transitions:
Wait for pin to go high
record cnt value
wait for pin to go low
record new cnt value
subtract the 2 values to determine speed.

This relies on Spin though, and can suffer from some delay in spin whereas the counters don't.

-Martin

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Counters:


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Post Edited (Bob Lawrence (VE1RLL)) : 2/21/2008 9:24:54 PM GMT

deSilva

Martin Hebel said...
...Wait for pin to go high
record cnt value
wait for pin to go low
record new cnt value
subtract the 2 values to determine speed.

This relies on Spin though, and can suffer from some delay in spin whereas the counters don't

The inaccuracy caused by SPIN is generally overestimated. When using such a SYMMETRIC design as Martin gives here, all SPIN delays will compensate and the result will be AS PRECISE AS WITH COUNTERS!!

dfletch

I've been meaning to update it, but you could try the frequency meter which takes samples, averages, and then dumps the result to a term. It's very primitive, but I think it does what you're asking for:

obex.parallax.com/objects/256/

There's a slightly newer version of this in the VGA learning driver (search this forum for it if you want that). I'll try to get a new rev up soon, but my evenings have been filled trying to get a graphics object working for that driver

Cheers,

--fletch