Re: [uCsimm] Are the uCsimm I/O pins 5V Tolerant?

From: Kevin Dawson (kevind@esi.com.au)
Date: Fri Feb 04 2000 - 16:51:08 EST


> David Williams wrote:
>
> > On page 2-3 of the 68EZ328 user manual it says
> >
> > [ EXTAL CMOS input levels, others TTL-ok but power consumption increased ]
> >
> > I think this is the info you require. So 5V is OK, but will cause system
> > to consume more power when going into low power modes.

G'day all.

I don't think that's a logical conclusion.
"CMOS levels" means you go to within a few hundred millivolts of the supply
and has to do with the hard switching of the FETs.
There's nothing that requires that supply to be 5 volts.
After all - this is a "nominal" 3.3 volt part...

If there are diodes on the inputs (has Motorola given an answer to that?)
and you drive it with 5V, then you'll also pull the CPU supply up.
At least until your 3.3V supply becomes a sink :-)

I'd be driving the outside world with HCT or ACT if I wanted 5V swings.
Listening to it depends on what voltage I'm being fed,
whether there is diode protection and how much speed is needed.
It can be as simple as a resistor,
or more complex like a 5V-input-tolerant gate or TTL output gate.

Kevin

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