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

From: Sam Saprunoff (
Date: Thu Feb 03 2000 - 12:48:53 EST

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..."

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