Re: [oc] Output loading in digital circuit

["Kenneth Hung" <hungwaiming@hotmail.com>]

Hi John

Do u know why old TTL logic has hundreds of ohms in R ?
I think I really want to know what information decides the R value
and C value.

Say in typical CMOS 3.3V spec, you can see R=500ohm in parallel
with C=50pF as an example. So I really want to know how those
values come from ? From what theory or information to decide such
values ? It's not from God, rite ? haha..

Kenneth

----- Original Message -----
From: "John Sheahan" <jrsheahan@optushome.com.au>
To: <cores@opencores.org>
Sent: Tuesday, April 02, 2002 3:39 PM
Subject: Re: [oc] Output loading in digital circuit


> On Tue, Apr 02, 2002 at 11:48:07AM +0800, Kenneth Hung wrote:
> > Hi Steve
> >
> > Ur answer might be correct in some sense in pF.
> > But do u know why there's an R=500 or R=1kohm in parallel
> > with the C ? I can't see no relation why there's a R and why is such
> > value ? For a typical transmission line, even the termination resistor
> > is 50ohm.
> >
>
> Rl in the hundreds of ohms used to be specified for old TTL logic,
> back when it actually consumed static input current.
>
> You need to to check DC limits to ensure you reach Vil and Vih (plus
> noise margin)
>
> I have not seen Rl spec'ed for a while.  Can you point
> me at an example?
>
> john
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