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

From: Tom Walsh (tom@mytoys.com)
Date: Thu Feb 03 2000 - 17:23:36 EST


Sam Saprunoff wrote:

> Hi Tom, I thought I would keep you informed as to the answer to my
> question... So far the response from Motorola has been conspicuously
> quiet! However, upon reviewing the 68EZ328 user's manual, it does
> imply that the inputs can accept 5V inputs. The reason I say this is
> on page 18-1... Supply Voltage (Vcc) -0.3 to 7.0V Input
> Voltage (Vin) -0.3 to 7.0V Normally, Vin is referenced to Vcc
> and not an absolute value. If I look at other data sheets for other
> devices, such as the 74LVX parts (5V tolerant parts) as you mentioned,
> they define Vcc and Vin the same way (absolute). If I look at 3.3 V
> only parts, I see that Vin is referenced to Vcc... So, it would seem
> that the 68EZ328 is a 5V tolerant device... I sure wish Motorola would
> place a definitive statement in their user's manual /data sheet so as
> to remove any doubt! Anyway, I have sent an additional e-mail to a FAE
> that has access to Motorola tech people. I am hoping to hear from him
> over the next few days to see what he has found out. As soon as I
> find out, I will post the info to the list. Cheers, Sam

Right, those are the Absolute Rating limits, those are the values at
which you will destroy the gate input / output. I find it interesting
that Motorola makes no reference to the a ViH or VoH limit in there
specs, they only mention the 'typical' ViH & VoH values. Normally, the
Absolute Ratings are given so that if you are determine the amount of
voltage spikes & undershoot that signals driving a reactive load will
withstand, these transients are a relatively short pulse time, but
experience has shown to keep them within the device absolute ratings.

I am designing an ethernet card for my EZ328SIMM card, and the SMC docs
for the LAN 91C96 device clearly state that when using a VDD supply of
3.3volts that the I/O pins are '+5V tolerant'.

I remain suspicious of using the DragonBall EZ processor at 5 volt
levels unless I see it in writing. I have designed embedded stuff for
years and have learned that you can hide a multitude of sins within the
documentation. I have learned to be cautious, rather than have 200-300
units failing in the field...

Regards,

TomW

>
>
> ----- Original Message -----
> From:Tom Walsh
> To: ucsimm@uClinux.com
> Sent: Wednesday, January 26, 2000 6:49 PM
> Subject: Re: [uCsimm] Are the uCsimm I/O pins 5V Tolerant?
> Tom Walsh wrote:
>
> I miskeyed, use the 74LVX family
>
> TomW
>
>
> > Sam Saprunoff wrote:
> >
> > > Hello everyone,
> > >
> > > I was curious... Are the uCsimm I/O pins 5V Tolerant? I
> > > have been unable to
> > > verify this from searching through the uCsimm manual (Vdd
> > > -0.5 to +4.0,
> > > Vin -0.5 to +7.0), nor the Motorola MC68EZ328 users
> > > manual (Vdd/Vin -0.3 to
> > > +7.0).
> > >
> > > Thanks in advance!
> > >
> >
> > No, they are not. You must use a logic family such as the
> > 74LV which also provides for voltage level shifting.
> > Nothing in the 68EZ328 User Manual says anything either
> > way regarding the voltages on the i/o (and other) pins, as
> > it usually happens, attaching a voltage higher than that
> > of the the specified supply of A processor, generally
> > results in damage to the pin through long term use.
> >
> > I would certainly welcome being wrong on this issue, but
> > as yet, I have read no definitive documentation that
> > specifically addresses the issue of TTL family
> > interfacing. Rule of thumb? If they don't spec it, don't
> > believe it!
> >
> > Regards,
> >
> >
> > TomW
> >
> >
> >
> > --
> > Tom Walsh - WN3L - Embedded Systems Consultant - tom over_at mytoys(.)com
> > 'www.mytoys.com', 'www.openhardware.net', 'www.cyberiansoftware.com'
> > "Windows? No thanks, I have work to do..."
> >
> >
>

--
Tom Walsh - WN3L - Embedded Systems Consultant - tom over_at mytoys(.)com
'www.mytoys.com', 'www.openhardware.net', 'www.cyberiansoftware.com'
"Windows? No thanks, I have work to do..."

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