Re: [uCsimm] SIMM hardware question

From: Jean-Marie Bussat (JMBussat@lbl.gov)
Date: Tue Feb 20 2001 - 11:38:27 EST


> I am working on a project in which I need to process inputs from a couple
> relays (i.e. the relay is either open or closed by an external source). Is
> it acceptable to simply use the relay to feed +3.3V from the uCgardener to
> one of the PDx pins, or does it need 5V, or is something else needed?

Hi,

Everything depends on how you would like to use those inputs. If
you just plan to check levels from time to time (poll), a straight
connection of the relay contact between +3.3V and the PDx input
will work. You may just need a pullup (pulldown) resistor if your
relays have single-way contacts (otherwise, you may connect the
normally open contact to +3.3V and the normally closed to ground
or the opposite depending of which logic you plan to use (negative
or positive).

But, if you plan to use them in an interrupt driven system (i.e. you
will look for an edge/change of the contact state), you will need an
"anti-ringging" circuit since when a relay contact closes or opens,
it doesn't not do that in a very "clean" way. You may have successive
closing/opening of the contact before it reach its mechanical stable
state. This may generate spurious interrupts that may confuse your
program.
"anti-ringging" circuits can be as simple as a R/C network or it
can be made using a monostable. It also exists some integrated
circuits that are doing this job (I dont remember any reference...).

I hope this helps. Feel free to ask for further details.

Jean-Marie

-- 
  _______________________________________
 |                                       | 
 | Jean-Marie Bussat                     |
 | Lawrence Berkeley National Laboratory |
 | 1 Cyclotron Road - MS 50A-6134        |
 | BERKELEY, CA 94720 - USA              |
 | Email: JMBussat@lbl.gov               |
 | Phone: (510)-486-5687                 |
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