Limits of Conventional STA
STA, while very successful, has a number of limitations:
- Cannot easily handle within-die correlation, especially if spatial correlation is included.
- Needs many corners to handle all possible cases.
- If there are significant random variations, then in order to be conservative at all times, it is too pessimistic to result in competitive products.
- Changes to address various correlation problems, such as CPPR (Common Path Pessimism Removal) make the basic algorithm slower than linear time, or non-incremental, or both.
SSTA attacks these limitations more or less directly. First, SSTA uses sensitivities to find correlations among delays. Then it uses these correlations when computing how to add statistical distributions of delays.
Interestingly, there is no technical reason why determistic STA could not be enhanced to handle correlation and sensitivities, by keeping a vector of sensitivities with each value as SSTA does. Historically, this seemed like a big burden to add to STA, whereas it was clear it was needed for SSTA, so no-one complained. See some of the criticism of SSTA below where this alternative is proposed.
Read more about this topic: Statistical Static Timing Analysis
Famous quotes containing the words limits of, limits and/or conventional:
“In a virtuous action, I properly am; in a virtuous act, I add to the world; I plant into deserts conquered from Chaos and Nothing, and see the darkness receding on the limits of the horizon.”
—Ralph Waldo Emerson (18031882)
“Predatory capitalism created a complex industrial system and an advanced technology; it permitted a considerable extension of democratic practice and fostered certain liberal values, but within limits that are now being pressed and must be overcome. It is not a fit system for the mid- twentieth century.”
—Noam Chomsky (b. 1928)
“We must be generously willing to leave for a time the narrow boundaries in which our individual lives are passed ... In this fresh, breezy atmosphere ... we will be surprised to find that many of our familiar old conventional truths look very queer indeed in some of the sudden side lights thrown upon them.”
—Bertha Honore Potter Palmer (18491918)