Ambit BuildGates Synthesis User Guide

9

Optimizing After Place and Route

After designs are synthesized to meet timing and design rule constraints, place and route is performed. During place and route, accurate capacitance and resistance information is extracted for nets and precise delays can be calculated for timing arcs. Deviations in wire RC from wireload table values can significantly change post-layout timing characteristics. As a result, further timing and design rule optimization is often required.

The purpose of this chapter is to familiarize you with the commands used to optimize after place and route, consequently, only a subset of the optimize commands are provided in the way of example. The examples in this chapter are presented in command line input format and may not include all possible options; the syntax is for example only. For complete descriptions of all commands and their options, please refer to the Envisia and Ambit Synthesis Command Reference.

Timing Analysis

The timing environment and constraints may need a slight adjustment after backannotation. Timing adjustments are a result of having more accurate clock tree timing information in the design after place and route. Handling clock information varies for different flows. Clock trees inserted at place and route yield accurate clock timing information previously not available. To use the accurate clock timing information during timing analysis and resynthesis, use the following command:

set_clock_propagation_mode propagated

Setting the clock propagation inserts actual timing for all gate and wire delays along clock signal routes, instead of using the zero delay assumption of the ideal clock. Because the clock timing is now accurate and skew variances can be sensed for different clock signal arrival points, the amount of clock uncertainty programmed with the set_clock_uncertainty command can be reduced or eliminated.

When analyzing backannotated timing information, it is useful to add the load, fanout, and slew fields to the report_timing format. To do this, use the following command:

set_global report_timing_format