RE: [SI-LIST] : 3.3V Design

Tim Morley (tmorley@bluews.com)
Mon, 17 Aug 1998 09:47:13 +0100

Look out for 3.3V (or lower) devices that are tolerant of higher
voltages on input pins but NOT on bi-directional pins.
It easy to see "5V tolerant inputs" on a data sheet and to mistakenly
assume that there is no problem connecting the device to a 5V part.

Tim

-----Original Message-----
From: Andrew Ingraham
[mailto:Andrew.Ingraham@digital.com]
Sent: 14 August 1998 20:45
To: 'si-list'
Subject: RE: [SI-LIST] : 3.3V Design

>What about 1.8V / 2.5V devices ? Is special layout
needed? What about
>connectivity between other devices on the system?

We have been biting the bullet with mixed supply designs
for some
years now. It's not surprising to find ourselves now
with perhaps
four logic supply voltages on one board. It can get
messy, just
keeping it all straight.

The big problem is with the interfaces between different
voltage
domains. Some lower-voltage ICs are tolerant of inputs
that go
over their supplies; others are not, and some of those
will clamp
above their supply voltage. Pay close attention to the
data sheets.
A 3.3V IC with a max Vin spec of 5.5V doesn't
necessarily make it
tolerant of 5V inputs; overshoot can far exceed 5.5V,
especially
since such a part has no input overshoot clamp diode.

Bus-hold inputs at the voltage-domain interface might
cause
interesting problems. A bus-hold input is a weak output
and
could back-drive a tri-stated low-voltage output pin
above its
max Vout spec (but maybe not enough to damage it).

Also watch for lower-voltage ICs with true CMOS
switching levels
as opposed to LVTTL levels. The latter are much more
common with
3.3V supply ICs, but not universal.

The faster switching speeds of some of the newer ICs is
indeed
cause for a lot of concern. Reduced noise margins just
make
your job more interesting.

Some of the EIA's JEDEC documents at
http://www.jedec.org may
be worth a look.

Regards,
Andy Ingraham

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