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'Multiplexing switches'
1996\06\29@104221 by Shannon Watson

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       I'm working on a PIC project and have need to multiplex a bank of
switches. The PIC is a 16c84 and the switches are are simple maintained
slide switches. I am using AN529 for guidance. In this app they talk about
discharging the pins before bringing a new column low. My question is this -
what exactly is meant by "discharging pins" ?
What happens if I don"t follow this procedure - am I running the risk of
toasting the PIC? I would appreciate any light you could shed on the subject
for me.



Thanks,

Shannon

1996\06\29@144200 by fastfwd

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Shannon Watson <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> wrote:

> I'm working on a PIC project and have need to multiplex a bank of
> switches. .... I am using AN529 for guidance. In this app they talk
> about discharging the pins before bringing a new column low. My
> question is this - what exactly is meant by "discharging pins" ?
> What happens if I don"t follow this procedure - am I running the
> risk of toasting the PIC?

Shannon:

When you select a column of switches, capacitance in the circuit may
hold a small charge on the switches.  "Discharging the pins" just
means that you need to wait a few microseconds (between selecting
the column and reading the rows of switches) for that residual charge
to dissipate.

If you fail to do this, you won't damage the PIC... All that will
happen, worst-case, is that your switches will be read inaccurately.

-Andy

Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499

1996\06\29@144826 by Mark K Sullivan

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>        I'm working on a PIC project and have need to multiplex a bank of
>switches. The PIC is a 16c84 and the switches are are simple maintained
>slide switches. I am using AN529 for guidance. In this app they talk about
>discharging the pins before bringing a new column low. My question is this -
>what exactly is meant by "discharging pins" ?
>What happens if I don"t follow this procedure - am I running the risk of
>toasting the PIC? I would appreciate any light you could shed on the subject
>for me.

He means discharging and voltage stored on the capactiance of the I/O circuitry.
You can get by with a time delay between setting up the column and sampling the
row.  I suppose in an EXTREME case you might risk latch-up but don't worry about
it.  The only real risk is that you will read the wrong state for a switch.

BTW - Since you are reading an array of *maintained* switches, you will need
diodes to avoid phantom switch closures when multiple switches are closed.

- Mark Sullivan -

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