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From: "Brill Pappin"
Subject: Re: help with logic voltage translation requested.
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Tue, 10 Sep 2002 07:34:24 GMT
NNTP-Posting-Date: Tue, 10 Sep 2002 03:34:24 EDT
The device is a hub that will take mostly 5V tolerant devices, but may have
3.3 sensitive devices connected... So, there is a switch for each "port" to
specify the voltage...
The problem comes because I have to interface the interrupt and reset from
the device to a 5V bus. This is fine when the device is 5V tolerant, but
won't do so well for 3.3 sensitive devices.
So, if I run the(a) 5V tolerant buffer on the 3.3 supply, all devices see
the 3.3 but can send logic at 5V or 3.3V... does that look like it would
Mostly I'm concerned about the 3.3 buffer not having the "nads" to bring the
INT high or low depending on the bus configuration (the system is highly
configurable, and I won't know what sort of crazy things are going to be put
on the bus)... though I suppose I could use two inverters, one on the 5V and
one on the 3.3V and connect them back to back... just seems a little like a
kludge to me ;)
- Brill Pappin
"Bob" wrote in message
> 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
> 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
> > 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
> > 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
> > - Brill Pappin
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