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From: Keith R. Williams
Subject: Re: Why use resistors to tie a logic input to ground?
Date: Thu, 17 Oct 2002 11:00:12 -0400
References: <email@example.com> <firstname.lastname@example.org>
X-Newsreader: MicroPlanet Gravity v2.60
In article <email@example.com>, Petrov_
> firstname.lastname@example.org (Manvinder Bhullar) wrote in message news:<email@example.com>...
> > Hi All,
> > I'm looking at this circuit from Anchor Chips that shows unused inputs
> > of a 74LCX14 tied to ground with 10K resistors.
> > Can someone please advise what the rationale is behind this? I've
> > always tied unused inputs directly to ground. What role do the
> > resistors play?
> > Thanks
> > Manvinder S. Bhullar
> My guess is that the resistors are there to permit easy design
> changes during debug. If you need one of the unused inputs, you
> merely have to remove the resistor and solder a wire to the pad.
> the pin were connected directly to the ground plane, you have to lift
> the leg of the chip and solder a wire to it directly.
Which is why I never tie inputs directly to planes. I always run a
back surface wire to a via tied to the plane. If I need the input I
can simply cut the trace and solder to the chip pad/via. Lifting the
leg of a BGA isn't easy. ;-)
Resistors aren't a bad idea, but take space and cost a bit.
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