Re: [oc] Output loading in digital circuit

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

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.


----- Original Message -----
From: "Steve Wilson" <stevew@intrinsix.com>
To: <cores@opencores.org>
Sent: Tuesday, April 02, 2002 4:19 AM
Subject: Re: [oc] Output loading in digital circuit


> Let me try my hand at this - having designed boards in a past life.
>
> One thing you might be seeing is the "test circuit" that was used to
actually measure (or spice if
> you like) the timing numbers represented in the device data sheet.   They
have to quote something,
> so usually represent what that something is relative too.
> This allows you to scale your numbers if necessary OR use it as an
absolute load number you don't
> want to exceed in your own design.  Example: The test circuit had 50pf in
it.  My design has 20 pf
> of load estimated on the board between the I/O's and board trace.  I'm
under their 50 pf number,
> so I should see the worst-case delay or better guaranteed.
>
> Does this help any?
>
> Steve
>
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