[SI-LIST] Re: SSO pushout
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: <linwee70@xxxxxxxxx>
- Date: Thu, 11 Apr 2002 09:34:01 -0400
> " SSO pushout is a result of multiple drivers
> switching simultaneously. It impacts signal integrity
> through adding extra delay to the propagating signal"
>
> I understand that we'll have more current in the
> return path due to simultaneous switching that may
> cause ground/plane bounce, but couldn't relate that to
> delay.
>
> Can someone explain this delay, as the text doesn't
> explain that ?
There are a few mechanisms that result in extra delay. (It isn't
really a delay in the propagating signal, i.e. wires, but rather in
the driven signal coming out of the IC outputs.)
One of them is simply this. When several outputs switch from high to
low, switching current flows in through the signal pins and N times
as much current goes out through the "ground" pins.
The ground system impedance turns this current into ground bounce,
where on-die "ground" momentarily lifts above board "ground".
For the outputs switching low, the pull-down transistor is on, so
this ground bounce gets added to their outputs. Imagine simply
adding a small positive pulse to the outputs, while they switch.
Their falling edges are lifted up slightly, which also has the
appearance of moving them a little to the right --> greater delay.
So it is really just crosstalk by way of the on-die ground bounce.
The same thing happens for rising edges, except it is the VDD or VCC
that bounces or sags.
I'm sure there are other mechanisms at work too. Note that SSO
can both increase and decrease delays, at least in principle.
Regards,
Andy
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