ECO Flow in Physical Design

 The tapeout is the final stage of the physical design process which definitely gives a big mental relax to the entire team involved in the project. The process of sending a clean layout file in form of gds/oasis to the foundry for fabrication after passing all the checks set by the foundry is termed as tapeout. But before the tapeout there might be many sleepless nights which physical design engineers / Signoff engineers spend and close the design. There are many signoffs like physical signoff, timing signoff and IR signoff which we need to get a closer state after which our layout is ready to send the foundry. And all these final achievements are done in the ECO (Engineering Change Order) phase. In this article, we will discuss the various aspects of the ECO cycle and how it works

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ECO Phase:

ECO phase is the phase of design where we close all the signoff checks which remain open in the PnR stage. Generally in PnR we make timing, DRC and IR closable but the final closing is done in ECO phase. In ECO phase, we close the PnR implementation activities and solve all the open issue through the ECO only. But before entering the ECO stage we need to achieve good timing and DRC numbers and we must have confidence that all the open issues are closable in the ECO phase. In ECO phase we focus on closing each open issue, we generate ECO file and implement them on PnR tool incrementally. 

What is ECO cycle and how it works? 

In the ECO cycle, we perform various analysis one by one for every check which we need to close but not closed till PnR stage. There are specialized signoff tools that help us to analyze the issue and also suggest the changes we need to do in order to close the issue. The suggested change is captured in an eco file. 

Once we generate the ECO file for the fixes, we implement that on the PnR database on which we have performed the analysis. After the implementation of ECO file, we save the updated database which we carry forward for the next ECO generation and implementation. We repeat this ECO cycle for every open issue and close one by one all issues. There are chances that we might need multiple ECO cycle to close a single issue. 

ECO Cycles

So basically there are the following steps in each ECO cycle.

1. Analysis of an issue on the latest database
2. ECO generation for fixing the issue
3. ECO Implementation on the analyzed database
4. Save the database after ECO implementation for the next ECO cycle

Signoff Tools

Signoff tools are very specialized tools to perform the analysis for a particular issue thoroughly and also have the capability to generate the ECO file for the fixes.  We have various types of signoff tools as per the issue like timing signoff tool, Physical Signoff tool and IR signoff tools. Some of the popular signoff tools are as bellow.

Timing Signoff: 

1. PrimeTime or PT (of Synopsys)
2. Tempus (of Cadence Design Systems)
3. Tweaker (of Dorado, now a part of Synopsys)

Physical verification signoff tools:

1. Calibre (of Mentorgaphics now a part of Siemens)
2. IC Verification or ICV (of Synopsys)

IR Signoff Tools:

1. Redhawk (of Ansys)
2. Voltus (of Cadence Design Systems) 

ECO file

ECO file contains a series of changes required in the form of PnR tool command for fixing the issue. Based on the analysis, sometimes we generate the eco file from the signoff tool itself or sometimes we create our own eco file. 

For example, to fix the setup timing, we need to upsize the combinational cells or need to convert them to lower vt cells. In the case of a few hundred violating paths, these conversions of cells might be in thousands or more. So signoff tool will generate commands for each cell that need to be changed and write in a file that we call eco file. Later we source these file in PnR tool followed by refine placement and eco route. 

ECO Implementation

Once we have the solution of any issue in the form of ECO file, we need to load the database which was used to generate the ECO file and source the eco file. ECO implementation is generally done in the batch mode of the tool. We need to delete the fillers before sourcing the eco file. Once the eco file is sourced, all the required changes is done. Now there might be a change in the size of cells or addition/deletion of some cells so we need to do refine placement to followed by the eco route. These two steps will take care of any overlap of cells and routing of cells. The updated database need to be saved for next stage of ECO or the final database in case of all issues are fixed.

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