Tristate Buffer Output if Input is High-Z?
Peter C. Wallace
pcw at mesanet.com
Thu Sep 29 09:30:18 CDT 2005
On Wed, 28 Sep 2005, Jeff Walther wrote:
>> Date: Tue, 27 Sep 2005 07:54:24 -0700
>> From: Eric J Korpela <korpela at ssl.berkeley.edu>
>> Subject: Re: Tristate Buffer Output if Input is High-Z?
>> On 9/26/05, Jeff Walther <trag at io.com> wrote:
>>> If a tristate buffer is enabled by its control line, but the input to
>>> the buffer is at high-Z is there a typical output?
>> If there is it probably varies between parts. If you need a defined output
>> for tristated inputs you should pull the inputs up or down.
> Thanks to everyone for all the replies.
> It sounds like the tristate buffers don't do exactly what I would like,
> although I think I can make it work with more logic in front of the control
> line for the buffer.
> However, doing a bit more research, it looks like what I really need is a
> Transmission Gate. The important difference is that a transmission gate
> will pass the input regardless of its state, so L passes L, H passes H and Z
> passes as Z. The disadvantage is that if you have a noisy signal, the TG
> doesn't clean it up at all the way a TB will.
> The catch is that I cannot find one listed anywhere as a part that one can
> actually buy. Are transmission gates purchasable parts? Or are they just
> something they discussed in my VLSI textbook?
Google for something called a quick-switch, It is basically a single
NMOS transistor transmission gate. Pericom ( and others) make them.
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