At 05:53 PM 5/27/02 +1000, Roman Black wrote:
> >
> > The problem with using just a cheap shift register is the limit on rise and
> > fall times on the clock inputs - most require rise and fall times to be
> > faster than 500 ns.  As I mentioned in my earlier post, some higher end
> > shift registers do have schmitt trigger inputs but those tend to cost much
> > more than a cheap SR (4021) and 2 sections of a hex inverter schmitt
> > trigger (4584 or 40106).
>
>
>I should have made it clearer, the clock input has
>NO RC filter so it does not suffer from rise time
>problems. Only the data input and reset inputs have
>the RC filters. :o)

Yeah, except that Russell said that it was coming from an opto type
sensor.  Those have fairly slow rise and fall times unless they are the
more expensive types with a schmitt trigger driven output stage.  The
prudent thing to do is add the schmitt trigger inverter to the inputs that
need them: clk and latch inputs.

I just had a thought, though.  I've been thinking along the lines of a
hc595 which does have edge triggered output latch.  But we've also been
talking about the 4094 and I *think* that has a level rather than edge
triggered output latch.  I'll have to check.  If that is the case, the
schmitt trigger could be eliminated from that input.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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