Logic Design for Array-Based Circuits
Copyright © 1996, 2001, 2002 Donnamaie E. White
Timing Analysis for Arrays
Last Edit July 22, 2001
The most accurate method of computing a circuit propagation delay requires that the circuit be completed through layout (routing). Back-Annotation software adds the actual metal delay to the electrical fan-out and wire-OR delays in the path. As for Front- and Intermediate-Annotation, Back-Annotation files provide the net rising and falling edge delays. These files can be used to compute accurate predictive path propagation delays and external set-up and hold times.
Back-Annotation delay files include metal load delays which account for the differences in rise and fall times due to differences in metal load units for the rising and falling edges of metal 1 and metal 2.
This software is undergoing further evolution as RC distributive networks are being examined by several vendors as a possible refinement. There is also research being undertaken to examine the topological effects of the path delays (nearness of a signal on one level to another active signal).
The Back-Annotation software uses a mainframe-based database and is not available on the individual workstations. The program provides files that can be used on the workstations and which include the actual metal delays of each net. The Back-Annotation delay files are used in the simulations in the same manner as the Front-Annotation files.
Back-annotation must be run and accepted as final before the generation of the actual arrays.
Vendors will guarantee that the array will match (will not be slower than) the results of the Back-Annotation.
Back Annotation Load Units Example - specific to the metal layer
Data used to be supplied such that computations had to be made, hence the conversion tables. These computations are now performed by software. Synopsys PrimeTimeTM and DesignTimeTM STA (Static Timing Analysis) imports back-annotation data (PDEF, edif, SDF and RC_parametric file) and performs the sack-annotated timing analysis.
Remedial Steps - Back-Annotation
If Intermediate-Annotation analysis has been performed, the probability of a failure during Back-Annotation analysis is significantly reduced. When a path is having timing problems after routing, the solutions are still those listed before, to reevaluate the problem and to determine an acceptable solution.
Minor problems can often be solved by a post-route edit. More serious problems require a more serious solution, including a return to the design phase. All timing problems must be resolved before committing the design to production.