Re: [SI-LIST] : Input switching threshold & CPCI

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Tue Jan 04 2000 - 09:44:51 PST


Abe Riazi wrote:
>
> D. C. Sessions Wrote:
> >
> >PCI does the old (idiotic) 0.8-2.0 input thresholds that were first
> >documented on cave walls. In contrast, anyone trying to do serious
> >signaling at nontrivial speeds uses very tight thresholds, usually
> >scaled to the driver supply and usually with differential receivers.
> >
> Hi D. C.,
>
> This is a good point. For 3.3 V signaling, the input voltage threshold
> values (based on PCI specs) consist of:
>
> Minimum Vih (Input High Voltage) = 0.5 Vcc
> Maximum Vil (Input Low Voltage) = 0.3 Vcc

Hmmm... 0.9v to 1.8v -- slightly tightened TTL again.
The big evil of course is the 0.4 Vcc centerpoint, which
Intel reflects in their drive specs making for rotten
line matching. They made the same mistake with AGP
(Vref of 0.39 to 0.41 Vcc, 2/3 PCI drive) even though
there wasn't any need for backward compatibility.

At least it's scaled. I'm tired of arguing with customers
who insist on running timing analyses against Vih(max) at
slow corner Vcc min.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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