[SI-LIST] Re: Back of the envelope termination resistor calculation
- From: Dan Bostan <dbostan@xxxxxxxxx>
- To: alexh1@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 8 Dec 2005 16:50:15 -0800 (PST)
I think the resistance is different for a transition
from "0" to "1" vs. a transition from "1" t "0".
/dan
--- Alex Horvath <alexh1@xxxxxxxxxxxxx> wrote:
> Hi,
>
> In the past when trying to determine if a series
> term resistor is required or what the value should
> be I would use the IBIS file to determine the
> approximate output resistance of the gate. Also it
> could be used to determine if incident wave
> switching is possible which can often be a make or
> break situation. I think I read about this technique
> in Halls book but I'm not sure. I presently have
> occasion to do that and although I know this is a
> crude approximation I wanted to make sure that I was
> doing it correctly.
>
> I'm referring to run of the mill LVTTL outputs
> here.
>
> From the IBIS I determined the following from the
> Pulldown curve (taking voltage and dividing by typ
> current) -
>
> 0.5V - 25 Ohms
> 1.0V - 29 Ohms
> 1.5V - 35 Ohms
> 2.0V - 45 Ohms
>
> The thing that disturbs me here is that the
> non-linearity is much higher than I would have
> expected so perhaps this method should be applied
> with extreme caution.
>
> I find it amazing that I still end up working with
> experienced engineers that have little concept of
> signal integrity or timing verification (hence the
> unavailability of an SI tool at my current
> employer). In my case my first job out of college
> was designing avionics. Obviously avionics must be
> completely reliable and we had a rule that all
> interchip paths had to have a 20% timing margin so
> the importance of "correct by design" was instilled
> in me early in my career.
>
> Of course in those days edge rates were relatively
> slow so most paths could be treated as lumped and if
> not the ridiculously high drive current of the
> jellybean logic of that time allowed us to use
> parallel terminations. Thus the 20% margin rule was
> easy to apply from the device data sheets in most
> cases.
>
> Thanks
>
>
>
>
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