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PICList Thread
'[PIC]: Input Resistors'
2001\04\26@140114 by Raymond Choat

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part 1 2016 bytes content-type:text/plain; (decoded 7bit)

Yep I would of burned something up doing it that way. Attached  is my
corrected version. Let me see if I have this right. I connected the switches
between the two resistors (pullup and zap protection). So does this mean the
line from pic1 and the button both are held high and when I ground either
one the line to pic2 goes low which activates part of the program? Now the
line from pic1 has zap protection but the buttons do not. I have used
buttons before without zap protection without trouble. Do I need the zap
protection on the switches and if I do, will I just put 1k resistors  in
series with the buttons? Normally when I use the buttons (to test board) the
pic1 portB output would be high (not activating anything) or not connected
at all.
Wrong Way Ray
(Raymond Choat)

{Original Message removed}
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part 3 105 bytes
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2001\04\26@145207 by David W. Gulley

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Raymond Choat wrote:
> Yep I would of burned something up doing it that way. Attached  is my
> corrected version. Let me see if I have this right. I connected the switches
> between the two resistors (pullup and zap protection). So does this mean the
> line from pic1 and the button both are held high and when I ground either
> one the line to pic2 goes low which activates part of the program? Now the
> line from pic1 has zap protection but the buttons do not. I have used
> buttons before without zap protection without trouble. Do I need the zap
> protection on the switches and if I do, will I just put 1k resistors  in
> series with the buttons? Normally when I use the buttons (to test board) the
> pic1 portB output would be high (not activating anything) or not connected
> at all.

Whether you need ZAP protection on the switches depends on the type of
switch you are using. For example, if these are metal snap dome type
switches that are uncovered, then a finger charged with static can
discharge into the PIC2 inputs. If these are insulated switches, then
zap protection is (probably) not required. To be on the SAFE side (if
you are in doubt), add resistors (~1K) in the wires connecting going to
the PIC2.

If you add 1K resistors in series with the buttons, and the PIC1 is
driving high, then the voltage at the PIC2 is ~2.5V (which will NOT
provide a valid signal).

David W. Gulley
Destiny Designs

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2001\04\26@155036 by David Cary

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"David W. Gulley" <dgulleyspamKILLspamDESTINYDESIGNS.COM> on 2001-04-26 01:29:04 PM made
some good points, mentioning:
>Whether you need ZAP protection on the switches depends on the type of
>switch you are using. For example, if these are metal snap dome type
>switches that are uncovered, then a finger charged with static can
>discharge into the PIC2 inputs.

What about metal snap domes where the dome (the part you actually touch) is
grounded ?

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2001\04\26@164029 by David W. Gulley

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David Cary wrote:
> "David W. Gulley" <EraseMEdgulleyspam_OUTspamTakeThisOuTDESTINYDESIGNS.COM> on 2001-04-26 01:29:04 PM made
> some good points, mentioning:
> >Whether you need ZAP protection on the switches depends on the type of
> >switch you are using. For example, if these are metal snap dome type
> >switches that are uncovered, then a finger charged with static can
> >discharge into the PIC2 inputs.
>
> What about metal snap domes where the dome (the part you actually touch) is
> grounded ?

 What if the finger touches a trace before the dome?

As I said, it depends on the type...
 Depending on the application, you may be able to eliminate the ZAP
resistors:
    a) if the domes are grounded
    b) if there is no chance of static discharge
    c) if there is static discharge, you don't mind replacing parts.

Years ago, I remember one of my designs being testing by having a pair
of wires from a spark type charcoal lighter inserted through vents in
the system to see what would happen! If I had been warned of the test, I
would have provided all kinds of baffles to prevent the wires from
entering the device. My design passed, not based upon design, but on
luck!

 The "What Ifs" are no comparison for a well written specification.

Remember "It is impossible to design a fool-proof system, because fools
are so ingenious!"

David W. Gulley
Destiny Designs

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2001\04\26@173531 by James Paul

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Hi David,

Jim Paul here.  Remember me fron TI Microcontroller Applications Lab?
I worked for Bob Crosby.  I thought that was you when I seen Destiny
Designs.  Just thought I'd say hello, and see how things are going.
TTYL.

                                             Regards,

                                               Jim




On Thu, 26 April 2001, "David W. Gulley" wrote:

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