Philip Pemberton escreveu:
> 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 =3D 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,

I have never read the data-sheets for these devices, but in reality
there are lots of CMOS families that are 5V tolerant. They use different
methods of ESD protection, like zener diodes to ground instead of diodes
to VCC and ground.

I would say you can trust the data-sheets. There you should find what
ESD protection method the devices use and other useful information also.

Best regards,

Isaac

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