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Conventional static timing analysis (STA) has been a stock analysis algorithm for the design of digital circuits over the last 30 years. However, in recent years the increased variation in semiconductor devices and interconnect has introduced a number of issues that cannot be handled by traditional (deterministic) STA. This has led to considerable research into statistical static timing analysis, which replaces the normal deterministic timing of gates and interconnects with probability distributions, and gives a distribution of possible circuit outcomes rather than a single outcome. Why STA is popular Old fashioned deterministic STA is popular for good reasons: *It requires no vectors, so it does not miss paths. *The run time is linear in circuit size (for the basic algorithm). *The result is conservative. *It typically uses some fairly simple libraries (typically delay and output slope as a function of input slope and output load). *It is easy to extend to incremental operation for use in optimization. == 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Statistical static timing analysis」の詳細全文を読む スポンサード リンク
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