RE: [SI-LIST] : SSO : How to identify SSO groups?

Greg Edlund (
Mon, 22 Dec 1997 09:21:29 -0500


I would have a look at the current vs. time profile for 1) the driver
you're interested in with 2) the package you will be using and 3) the
actual system load.

Unless your vendor has done all of this work already, you'll want to get
the SPICE model for the driver (the real thing, extracted from layout,
and not somebody's sanitized swag) and a FULLY-COUPLED package model.
Depending on how much you know about your vendor's capabilities, how
well the models have been characterized against test hardware, and how
sensitive your design is to the relative phase of SSO groups, you may
also want to build yourself a test vehicle to check these things out in
the lab. You almost always find a few surprises when you do!

Greg Edlund, Principal Engineer
Server Product Development
Digital Equipment Corp.
129 Parker St. PKO3-1/20C
Maynard, MA 01754
(508) 493-4157 voice
(508) 493-0941 FAX

Sent: Friday, December 19, 1997 1:50 PM
To: si-list@silab.Eng.Sun.COM
Subject: [SI-LIST] : SSO : How to identify SSO groups?

We are using our foundaries SSO rules to calculate number of
pins required for our ASIC. Before we can use the SSO rules we
need to
identify signals that form a SSO group. The conventional
technique I know
of is to group signals of similar functionality into a SSO group
Address/Data bus.

In practice we have noticed that not all the signals of such a
group have
identical delays. Further they may not have identical output
loads. Variable
delays implies that not all the outputs switch at the same. E.g.

Let output1 switch at time T and output2 at time T+t1. If t1 is
small then the
switching current for output1, say I1, will overlap with
switching current
for output2, say I2, leading to overall increase in the
switching current.
But if t1 is more than the time required for I1 to reduce to 0
then the
overall switching current is going to be limited to max of I1
and I2.

I want to know whether there are any guide lines for value of t1
that one
can use to decide whether a signal is part of a SSO group.

Value of t1 is also required in designing address/data stepping.


- Praveen Shekokar