From: "Tom Faloon"
Subject: Re: Why use resistors to tie a logic input to ground?
Date: Thu, 17 Oct 2002 11:57:45 +0100
NNTP-Posting-Date: Thu, 17 Oct 2002 10:59:42 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
There is no 'electrical' reason why you cannot tie unused inputs of this
device directly to ground. I can't tell you why inputs are tied low with
resistors on your board, but I can tell you that I often do this, and why I
I often tie inputs of unused logic elements to ground via a resistor,
especially if the board is a prototype, or a low volume product. My reason
is that it makes the input easily accessible if you ever want to use it. (If
you have to do a cut and strap mod on the PCB, just throw away the resistor,
and you have a pad to solder to.)
If you don't have that resistor, and the PCB layout person autoroutes the
board, the unused input will be tied to the nearest ground. This may be a
heavy ground plane, or on an inner layer, or to a via located underneath the
chip! Try cutting the ground track! Often the only solution is to lift the
pin! That is not always acceptable to the customer.
It may seem easier to connect unused pins to ground through a standard
track, and make sure it is easily accessible should it ever have to be cut.
But try telling that to a draftsman. Nine times out of ten it won't happen.
This practice may not be acceptable on a high volume product where cost is
important. If you do implement it, use a resistor value which is used
elsewhere on the circuit, to reduce component count. Anything from 0 Ohms to
several hundred K is fine for most MOS logic families. Purchasing, accounts,
stores, and the person who loads the pick and place machine will all be
pleased. (Well they might be pleased if they knew. Like the rest of us, they
usually see only the things that bother them.)