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'[PIC]: can input be connected directly to VDD'
2003\01\24@135928 by Tal

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Can an input of a PIC be connected directly to VDD to set a High level ?
What about other digital IC's such as 74HCXXX or FPGA's ?

One of my guys here insists that input should be tied to VDD through a
resistor.

Thanks,

Tal

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2003\01\24@141611 by Dwayne Reid

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At 10:58 AM 1/24/03 -0800, Tal wrote:
>Can an input of a PIC be connected directly to VDD to set a High level ?
>What about other digital IC's such as 74HCXXX or FPGA's ?
>
>One of my guys here insists that input should be tied to VDD through a
>resistor.

Your guy is correct.

Think about what happens if, somehow, that PIC pin gets turned into an
output.  Many things can cause this - an ESD event that corrupts the TRIS
register is only one such possibility.

In general, input only pins on CMOS devices can tie directly to GND or VDD
without intervening resistors.  Un-used pins on TTL devices should tie HI
via a 1K or so resistor.

The way I prefer to deal with un-used I/O PIC pins is to leave them
un-connected and set as outputs.

dwayne

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2003\01\24@143428 by Spehro Pefhany

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At 10:58 AM 1/24/03 -0800, you wrote:
>Can an input of a PIC be connected directly to VDD to set a High level ?
>What about other digital IC's such as 74HCXXX or FPGA's ?
>
>One of my guys here insists that input should be tied to VDD through a
>resistor.

Input-only can be tied to Vdd (except for old bipolar TTL parts).
Otherwise (bidirectional microcontroller pins) best to use a series
resistor.

Same with ground. You can safely tie an open drain output I/O to ground,
but other bi-di should be through an R.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2003\01\24@144222 by Josh Koffman

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It can, whether it should or not is another matter. Some might worry
that if the pin is accidentally set as an output and driven low, you
will be shorting out your power supply.

Josh
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Tal wrote:
>
> Can an input of a PIC be connected directly to VDD to set a High level ?
> What about other digital IC's such as 74HCXXX or FPGA's ?
>
> One of my guys here insists that input should be tied to VDD through a
> resistor.

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2003\01\24@155654 by Chris Loiacono

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I hope you didn't take a strong position on this...
Give him a pat on the back for insisting on good design practice. If he
knows the difference between a soft-pullup and a hard one, give him a raise
also...

CL
{Quote hidden}

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2003\01\24@164150 by Peter L. Peres

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On Fri, 24 Jan 2003, Tal wrote:

*>Can an input of a PIC be connected directly to VDD to set a High level ?
*>What about other digital IC's such as 74HCXXX or FPGA's ?
*>
*>One of my guys here insists that input should be tied to VDD through a
*>resistor.

It's ok for a PIC but not for older 74xx. 74[H]Cxx inputs can be connected
directly to Vdd. FPGAs are usually cmos, so yes, unless the manufacturer
says otherwise. You would use a pullup if you plan to drive the pin low at
some time or other (f.ex. MCLR on a PIC).

Peter

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2003\01\24@174557 by Olin Lathrop
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> Can an input of a PIC be connected directly to VDD to set a High level ?
> What about other digital IC's such as 74HCXXX or FPGA's ?

No problem as long as the connection is close to the IC's power pin.


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2003\01\26@034355 by James Newton, webhost

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source= http://www.piclist.com/postbot.asp?id=piclist\2003\01\24\135928a

This is FAQ

http://www.piclist.com/logic/xtrapins "What should I do with pins that I am
not using? "

This covers all sides of the issue and I think it does so fairly.

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2003\01\26@134259 by Tal

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Thanks James.

Tal

{Quote hidden}

http://www.piclist.com/logic/xtrapins "What should I do with pins that I
am not using? "

This covers all sides of the issue and I think it does so fairly.

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