Full layers functional ECO can add or delete gates freely. The ECO operations are done in a GofCall script which is compatible with Perl, and it uses exported APIs to access, modify and save the netlist database. GOF reads in two netlist files, Implementation Netlist which is under ECO and Reference Netlist which is re-synthesized from modified RTL with the same constraints as the pre-layout netlist. In the GofCall script, the API 'fix_design' is used to fix the top level module and its sub-modules in global mode. GOF uses the built-in Logic Equivalent Check Engine to figure out the non-equivalent points. And optimized minimum size gate patches are applied to fix the non-equivalent modules.
As shown in the Figure 1, two logic cones are extracted from the Implementation and Reference Netlist for the same compare point. The implementation point mismatches the reference point initially. GOF compares the two points and generated a patch from Reference logic cone and applies to Implementation Netlist. After the patching, the two points become equivalent.
Figure 1: Logic Cone Optimization
GOF does logic cone analysis and optimization for each failing point found in top down logic equivalence check. The failing point is in format of output port or sequential element’s input pin, such as flop’s D input. The final patch has the minimum number of gates to make the implementation logic cone equal to the reference logic cone.
The flow chart is shown in the Figure 2.
Figure 2: Automatic functional ECO flow
Steps for an automatic functional ECO:
The GofCall script has exact the same syntax of Perl script. It runs the exported APIs that access the netlist database and modify the netlist.
The following is an example script for an automatic functional ECO:
The GofCall Script can be run by '-run' option.
Check Run and debug GofCall script section in User Manual for more detail
The example is in the released package.
To run the example:
The log file shows there are total 35 gates in the initial logic cone. After optimization, the patch has only 1 complex cell and 8 inverts