RE: [SI-LIST] : Interfacing 3.3V device with 2.5V device.

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri May 18 2001 - 05:19:00 PDT


> Even if the manufacturer of the 2.5V claims that the device is 3.3 V
> tolerant, is it really advisable to subject it to 3.3V signals
> continuously
> ? Will it have an adverse effects on the long term reliability of the
> device
> ?
 
If the manufacturer claims it is 3.3V tolerant, it should mean that it
really can tolerate (at least) 3.3V indefinitely. However, don't go by the
advertising claims alone. Read the detailed specs. Ask the vendor about
reliability.

Make sure the absolute maximum Vin, as well as the recommended Vin range, is
much greater than 3.3V. I have seen a part that claimed the inputs were 5V
tolerant, yet the absolute maximum was exactly 5.0V so it wouldn't even
tolerate the +5V supply being just slightly above nominal! Nor would it
tolerate any signal overshoot.

There are a few other issues. One of the biggest is overshoot tolerance.
Tolerating 3.3V (or 3.6V) continuously is one thing. Tolerating 0V/3.3V
signals with typical overshoots on the order of a volt (-1.0V to +4.6V), is
another. Work with the vendor to understand their real overshoot tolerance.

Do the inputs have clamp diodes to 2.5V (which could begin conducting at
around 2.3+0.6=2.9V)? Clamps could be both good and bad. Good because they
absorb overshoot energy and reduce ringing if nets are long or uncontrolled.
Bad because they draw current from your 3.3V signal sources and load them
down.

Check the input current spec, over a range that includes at least Vin=3.6V.

Make sure you consider what happens when either supply is margined in either
direction. Don't assume that they track, so you might have 2.3V with 3.6V,
or 2.7V with 3.0V.

If these are bidirectional transceivers, or if you plan to connect the
tri-stated 2.5V outputs to your 3.3V signals, watch for differences between
the input pin specs and output or I/O pin specs. It is harder to make an
output or I/O buffer tolerant of signals that go past its own Vdd supply.

Regards,
Andy

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