Re: Simplest way to propagate a digital pulse?
- From: whit3rd <whit3rd@xxxxxxxxx>
- Date: Tue, 24 Mar 2009 13:05:00 -0700 (PDT)
On Mar 23, 1:23 pm, Max Moor <maxmoor@xxxxxxxxxxxxxxxxxx> wrote:
I want to propagate a digital pulse down a line of maybe 100 modules,
each separated by three inches or so. I want to apply a pulse to the first
module which will turn on and do its thing, then as it turns off, pass the
pulse to the next module, etc., etc.
Currently, each module has a D flip-flop with the propagating pulse on
each module's D input (Q out from the previous module), and a separate,
common clock line going to all the modules. As you can imagine, I'd really
like to not have to drive a 25 foot clock line with a 100 device fan out.
You presumably also have a 'reset' line somewhere, so that this
bunch of modules will do the same thing all over again someday.
As far as long clock lines are concerned, a differential driver and
receiver can
do it fine (like AM26LS31 and AM26LS32). You can also
daisy-chain (each module receives the clock, buffers it, and
sends it to the next in ripple fashion).
If you really implement the 'wait till module N is done before
module N+1 starts' functionality, every single module is
a point-of-failure for all the rest. That can be made to work,
but this is a kind of control counter, and its state has
100 elements, so the problem is one of one hundred state
bits. If a central controller turns the modules on, the
problem is log_2(100), which is a seven bit state; this
is usually preferable because it is easier to troubleshoot.
.
- References:
- Simplest way to propagate a digital pulse?
- From: Max Moor
- Simplest way to propagate a digital pulse?
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