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From: firstname.lastname@example.org (Tim Shoppa)
Subject: Re: Why use resistors to tie a logic input to ground?
Date: 17 Oct 2002 06:04:39 -0700
NNTP-Posting-Date: 17 Oct 2002 13:04:39 GMT
email@example.com (Manvinder Bhullar) wrote in message news:<firstname.lastname@example.org>...
> 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?
If you tie them directly to ground or Vcc, *and* you have substantial
ground bounce (most of a volt), then you can violate the "Absolute
Maximum Ratings" for input voltage and current. For typical TTL-like
logic these say that inputs cannot go below -0.5V or above Vcc+0.5V
unless you ensure the current is limited.
If you put a resistor in series, then you do not violate the "Absolute
Maximum Ratings" because you stay under the input diode current rating.
Typical ground bounce in a small, well bussed system will not be above
half a volt and you can hook the input directly to Vcc or Ground. Even
if you have severe ground bounce problems you can typically get away
with hooking genuine TTL inputs directly to Vcc or Ground because the
ground bounce is a transient effect - but you *are* violating the specs.
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