At 05:17 AM 9/22/2009, you wrote:
>Hi guys,
>    Any chance of a quick sanity-check?
>
>    I'm playing around with a design that needs to drive a bunch of
>open-collector outputs to a 5V-powered peripheral, and receive data
>along a bunch of 0V/5V open-collector lines back from said peripheral.
>
>    The drivers specified in the datasheet for the device are the 7438
>"or similar" (so 7407/74LS07 would work too) for the O/C drivers, and
>74LS14 plus a 1k pull-up to +5V for the receivers.
>
>    The thing is, the rest of my hardware operates at a Vcc of 3.3V, not
>5V. I was originally going to use some level translators to convert the
>3.3V to 5V then use LS07/LS14 chips to drive the I/Os, but then I found
>out about the TI 74LVC series:
>
>74LVC07A: http://focus.ti.com/docs/prod/folders/print/sn74lvc07a.html
>74LVC14A: http://focus.ti.com/docs/prod/folders/print/sn74lvc14a.html
>
>    The datasheet for these specifies a Vcc of 0V to 3.6V (for the LVC14)
>and 0V to 5V for the LVC07. Input voltage is specified as 0V to 5.5V
>regardless of Vcc voltage -- even the LVC14 can apparently take a 5V
>input, despite its Vcc being limited to 3.3V or less.
>
>    Somehow this doesn't seem right -- these are CMOS parts (LVC = Low
>Voltage CMOS), so surely there are parasitic diodes to Vcc and Vss,
>which would prevent any input from being driven above Vcc without the
>chip latching up?
>
>    Am I missing some critical point here?
>
>Thanks,

Diodes to Vdd (or "Vcc" as it's called on 74xx devices) are not
parasitic, rather they are deliberately formed structures. They (the IC
designers) can leave them out or replace them with something else, 
which is what
happens on inputs of devices which are tolerant of input voltages much higher
than the positive supply. There is something like a Zener diode in there
to provide some protection for the input gate oxide layer. A similar thing
happens on open-drain outputs.

(Finally) some PIC chips now have this feature- eg.PIC24HJXXXGPX06/X08/X10
with more than half the I/O pins being 5V-tolerant (basically, the I/O pins
that cannot be used for analog functions such as ANx or OSCx are tolerant
of inputs as high as 5.5V).

OTOH, diodes to the substrate are not optional in the typical* CMOS process--
that's what provides isolation.

* There are exceptions to the typical.. eg.
http://en.wikipedia.org/wiki/Silicon_on_insulator



>Phil.
>piclist@philpem.me.uk
>http://www.philpem.me.uk/
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