No matter how you run your power analysis - with Encounter Power System (EPS) or from within Encounter Digital Implementation (EDI) System - you're probably familiar with the result directory. It will look something like VDD_125C_avg_1 and have lots of files inside. The first ones you probably look at are the "results" text file and the ir_limit.gif (at least those are the first ones I look at). While these will give you the immediate information you're looking for regarding the analysis (IR-drop, EM, etc.), several releases ago there appeared a Reports directory which gathered a lot of other information to make your life easier!
Even if your IR-drop looks good, you may still have a few unconnected instances, or be missing a powergrid view, or may have had a problem with the power consumption run that the rail analysis uses. If you know what files to check out in this Reports directory, you can make sure that you're not missing anything. I'll use "VDD" in the filename examples, but of course it could be "VSS," "VDDCore," or whatever your rails are named. Here's my breakdown of what to check out:
- VDD.disconnected_inst.asc - This file lists the instances in the design that are not connected to the rail being analyzed. This file MUST be reviewed. There may be cells in this file that you can ignore, such as I/O cells if the LEF files do not model the pwr/gnd busses correctly. In that case, you just need to double-check that these cells are indeed getting power in the design, and make sure that the LEF pin connectivity is the reason they end up here. Once you've done that, you can easily manipulate the file to get rid of the cells you know are false violations and make sure there is nothing else.
- VDD.disconnected_pgv.asc - This file will list the cell types whose powergrid views are not connected to the pwr/gnd net you analyzed. This file MUST be reviewed. Some of the cells listed here may be ok - there could be a macro connected to a different power net than the one analyzed, or the same kind of LEF pin issues that are ok to ignore in the disconnected_inst file. But like the disconnected_inst file, make sure you take a look at each item and understand why it's here.
- VDD.missing_pgv.asc - This file lists the cells in your design that do not have a powergrid view. This file MUST be reviewed. Looking at this file enabled me to catch a mistake: my design had started using a few new RAM cells, but I had not regenerated the RAM powergrid views. Once I saw the results of this file, I realized I had to generate powergrid views for the new cells and then rerun the power and rail analysis.
- VDD.pwr_annotation.asc - This file lists the instances that do not have power consumption information for the rail you analyzed. This file MUST be reviewed. If something went very wrong with the power analysis before the rail analysis, you could catch it here. But there also may be cells listed here that you can ignore: an example would be a PLL instance, that attaches to VSS and VDDA, but you were analyzing VDD. Since this instance does not connect to VDD, the power analysis that was run before rail analysis does not have any power consumption for VDD for this cell.
- VDD.pgv_table.asc - This file lists each cell type and what kind of powergrid view was used (port, detailed, etc.), and more importantly, if the powergrid view was NOT used. Any cell types that say "disconnected" here will be in the disconnected_pgv file.
- VDD.unconnected_sections.asc - This file lists floating metal segments that are labeled as the pwr/gnd net you analyzed, but are not connected to the grid. Leaving these in the design may not be an issue, but it's a good idea to clean them up so that they don't mask any real issues.
- VDD.layerbased_ir.asc - This file is a breakdown of the IR-drop by layer. If you do have an IR-drop issue, you may be able to quickly narrow it down to one layer by looking at this file. If your IR-drop passes, it's still interesting information to have.
This is not an exhaustive list of all the files in the Reports directory; just the ones that I have found to be most useful. Please let me know in the comments if there are other files here you can't live without and how you use them. I'm interested in how other designers are making use of this information.
- Kari Summers
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