chip pinout help

From: Jim Battle <>
Date: Fri Aug 23 01:05:00 2002

At 01:33 PM 8/22/02 -0700, you wrote:
> >From: "Eric Smith" <>
> >
> >> Hmm, but then how is translation done on the inputs to the ROM? It is
> >> a TTL device feeding the address, so the 0-5v swing (0.5v to 4+v)
> >> swing would appear to be an input swing of +15 to +20.
> >
> >No translation required. PMOS thresholds are within a few volts of the
> >positive rail.
> >
> >
> >
> >
> The only issue with PMOS is on the outputs. They
>may not pull down as hard as a TTL input pulls up.
>This is generally fixed with a pull down resistor
>and/or a diode to limit the negative travel. Some
>TTL can tolerate some negative on the input, most
>CMOS can't ( I think the 4049 can though ).

Ah, master, you are precisely right. The circuit pulls down with 6.8K to
the -V. Still, I'm surprised that 74xx inputs will take the output (I'm
assuming that the PMOS outputs ran rail to rail). There is a clamp diode
on the input of 74xx devices, but with a >10V difference, I'd expect either
the driver or receiving clamp diode to complain.

This is all just for my education -- the board does exactly that and I've
learned that empirical proof takes priority over my speculations.

Jim Battle ==
Received on Fri Aug 23 2002 - 01:05:00 BST

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