深圳大学第七章VLSI设计导论.ppt

* * ‘Useful Skew CTO’ is a new feature in 2005.09. It is invoked with the TCL command tcl “useful_skew_opt”. It performs an ‘incremental’ useful skew optimization, after global or local skew CTS/CTO, with the sole purpose of improving or eliminating setup time violations, while maintaining a balanced skew elsewhere. Useful skew CTO has several options to allow buffer insertion, buffer sizing and level adjustment. The recommended options are –buffer_sizing (first preference) and –level_adjustment (second preference). Multiple runs with the same option should be executed until no further improvement is seen. The –buffer_insertion option is not recommended after global or local CTS/CTO because it can significantly increase buffer count, insertion delay, run time, and can disturb the existing clock tree. The latter option can be used after a ‘useful skew CTS’ run (astCTS with the “Useful” skew type clock common option selected). You would do this if you are concerned with meeting timing only (as opposed to minimizing skew as well as meeting timing in the flow shown above). Note: You should not execute a Post-Route CTO (astPostRouteCTO) during the routing phase of the design if useful skew CTO was executed. Post-Route CTO focuses on balancing skew, which will undo the useful skew optimizations. Prior to 2005.09 there is no ‘incremental’ useful skew capability. You have to choose between minimizing skew (using ‘Global’ or ‘Local’ skew CTS) or meeting setup timing (using ‘Useful’ skew CTS) – you can not do both. If you run useful skew CTS after a global or local skew CTS run, nothing happens. You must first remove the ‘synthesis flag’ (astMarkClockTree? select ‘Reset Net Synthesiszed Flag’). Useful skew CTS re-synthesizes the entire clock tree from scratch. Since it uses an entirely different algorithm than global/local skew CTS, the results can be quite different – may be better for timing but worse for skew. * * The preferred routing direction guides Astro to route a