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Subject: Re: help with logic voltage translation requested.
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Tue, 10 Sep 2002 05:44:18 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 22:44:18 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Typically, 5V systems operate with TTL logic levels, and 3.3V systems
sometimes run on LVTTL logic levels.
The logic thresholds for TTL and LVTTL are identical. The only difference is
that some TTL ICs' output stages will drive at levels greater than 3.3V
(well above the 2.4V voh(min) spec).
If the inputs on the 3.3V system chips are (what is called) "5V tolerant"
then you can directly interface the 5V TTL outputs into the 3.3V LVTTL
You can always run 3.3V LVTTL outputs into 5V TTL inputs (remember, the
logic thresholds of the two systems are identical).
If lack of "5V tolerance" turns out to be your problem then you can get
buffers that are tolerant.
Hope this helps.
"Brill Pappin" wrote in message
> I have a situation where I've got a variable logic voltage (3.3V or 5V)
> need to input and/or output a logic state to a 5V bus. The bus input are
> interrupt and a reset... so one is feeding onto the 5V bus, the other feed
> from the 5V bus.
> What I was thinking of doing was to use a CMOS buffer running at 5V. it
> should be able to distinguish between a high and low on the 3.3V side of
> buffer, and output H|L on the 5V side or vice versa.
> Anyone have any comments on this, or a better way to do it?
> any ICs I should look at?
> This application only needs two buffers, so I was thinking that I'd use to
> single buffers to save space and power. However I'm in the design stage
> though someone might have a better idea before I get all the way to proto.
> - Brill Pappin
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