From: Andrew Ingraham
Sent: 14 August 1998 20:45
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
years now. It's not surprising to find ourselves now
four logic supply voltages on one board. It can get
keeping it all straight.
The big problem is with the interfaces between different
domains. Some lower-voltage ICs are tolerant of inputs
over their supplies; others are not, and some of those
above their supply voltage. Pay close attention to the
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,
since such a part has no input overshoot clamp diode.
Bus-hold inputs at the voltage-domain interface might
interesting problems. A bus-hold input is a weak output
could back-drive a tri-stated low-voltage output pin
max Vout spec (but maybe not enough to damage it).
Also watch for lower-voltage ICs with true CMOS
as opposed to LVTTL levels. The latter are much more
3.3V supply ICs, but not universal.
The faster switching speeds of some of the newer ICs is
cause for a lot of concern. Reduced noise margins just
your job more interesting.
Some of the EIA's JEDEC documents at
be worth a look.
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