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

From: John Ananny (
Date: Thu Feb 03 2000 - 16:15:31 EST

Hi all,

I've been lurking for a few months on this list, but would like to contribute my $0.02 to this 5V-tolerance discussion. 

I don't have the data sheets in front of me, but I'm 98% confident that the EZ's inputs are clamped to the rail with an input protection diode.  That means that you'll have two conflicting drives on the pin.  The diode wants to keep the input pin no higher than one forward diode drop above the EZ's supply rail, but whatever output is driving the input wants to keep the input pin at 5V.

This is bad.

At the very best, you get a valid logic high and nothing overheats at steady-state.  At the worst, a wirebond somewhere gives because of heating due to overcurrent, either at the input protection diode or in the output drive of the previous stage.  In any case, you squander power because of the overcurrent condition.

IMHO, absolute maximum specs aren't good design targets.

A simple option for prototyping is to use 5V LS buffers.  The output high of a 5V LS device is only actively driven to approximately 3.5V, so have your 5V HC device drive a 5V LS device, and have the 5V LS device drive the 3.3V HC inputs to the EZ.  In the other direction, it's not quite so clean, as you need a pullup on the output of the LS buffer to get it all the way up to a valid input high for the 5V HC.  This does give you an opportunity to make a response time/power efficiency tradeoff in your design.  For a production device, adding LS buffers likely wouldn't be cost-effective, but it does the trick for a quick prototype.

Horowitz and Hill (2nd ed.) section 9.03 has further details on interfacing logic families and levels.

Best regards,

John Ananny

At 10:48 AM 2/3/00 -0700, you 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.
----- Original Message -----
From: Tom Walsh
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

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

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!


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

John Ananny                    
Interval Research Corporation                (650) 842-6039 This message resent by the list server

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