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

Praveen G Shekokar (praveen@comit.com)
Fri, 19 Dec 1997 10:50:28 -0800

We are using our foundaries SSO rules to calculate number of VDD/VSS
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 e.g.
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.

thanks,

- Praveen Shekokar