We recently studied the Composite Current Source (CCS) model and Effective Current Source (ECSM) models in detail.  After analyzing the similarities and differences between the formats, we developed an open-source translation tool that converts Liberty CCS libraries to ECSM.  We feel that this tool can be very useful to the EDA industry so we made it available for free by posting it on the Liberty Forum:

http://synopsys.infopop.net/1/OpenTopic?a=cfrm&s=7741942603&f=340199990

Background on our Company:

Electronic Tools Company (E-Tools) develops and distributes high quality translators that simplify tasks of integrating, migrating, and archiving design data from various EDA platforms. E-Tools is a strong supporter of standards like the Liberty format, EDIF and XML. Over the last decade, we provided the best EDIF and XML tools to the EDA industry on Mentor Graphics, Viewlogic and Cadence Design Systems platforms. In 2000, E-Tools developed the first version of the Liberty open-source parser for use by the entire electronics industry. Over the past 6 years, we have consistently updated the Liberty parser, driven by extensions of the Liberty format and via enhancements from numerous Liberty parser users. On July 26, 2006, E-Tools won the Tenzing Norgay Interoperability Award in recognition for the work done on Liberty parser.

CCS and ECSM:

We have been following the recent advances in library modeling technology and we agree with many ESNUG readers that current-based modeling is the way to go for 90-nm & below process nodes.  The problem for many companies is that they have two formats to choose from.  If they have a multi-vendor tool flow, there's a good chance they need to support both CCS and ECSM.  Since ECSM is a set of user-defined attributes in Liberty, it does not interfere with the standard Liberty CCS syntax.  In other words both CCS and ECSM can co-exist in the same .lib.  However, library file size becomes an issue:  Current-based models contain a lot more data than Non-Linear-Delay Models (NLDM.)  Our analysis has shown that both CCS and ECSM libraries are roughly the same in size but are each about 8X the size of NLDM.  Of course library size depends on many factors including the number of index points, current/voltage segmentation tolerance and so on.  But 8X is a good rule-of-thumb. If your timing NLDM .lib is 30Mb, you can expect your CCS and ECSM timing libraries to be 240Mb each. Then total library size of NLDM+CCS+ECSM = 510Mb.  Multiply this by many corners and data-size becomes overwhelming. 

What we have discovered, however, is that the CCS data is a super-set of the ECSM data.  The good news from this is that designers can use CCS models as their main libraries and they can re-create ECSM from CCS if needed in a matter of minutes.  Best of all, in-theory, these indirectly created ECSM models should be as accurate as the natively generated ones. Note that the converse is not true.  Since CCS timing models contain direct current waveforms (as opposed to voltage waveforms) and have a 2-Capacitor receiver model (as opposed to one), they cannot be accurately re-created from ECSM models.  In other words if you convert ECSM to CCS, the resulting models will be less accurate than the natively generated CCS models.

How our CCS2ECSM translator works:

To translate from CCS to ECSM, we perform 5 steps:

1- Parse the .lib containing CCS:  This is simple since we developed the parser in the first place.

2- Create the driver voltage waveform: The CCS current source model consists of current samples as a function of time. This current is the measured current that flows into the known load capacitor during characterization. Since we know the value of the capacitor and we have the current samples as a function of time, it is a simple matter to numerically integrate to get the corresponding voltage waveform. Since the CCS segmentation algorithm controls error in this resurrected voltage waveform during characterization you can be assured that the integration will be accurate. In other words we can recreate the voltage waveform. Note from this voltage it is straightforward to extract an equivalent NLDM or ECSM model.

3-Create the ECSM driver voltage waveform: ECSM consists of samples of the driver voltage waveform as a function of time. It is straightforward to sample the voltage waveform above to create the ECSM driver waveform. Typical ECSM libraries have samples at fixed voltage values, so if you want 10 sample points, you sample at: 10%Vdd, 20%Vdd etc.

 4- Create the ECSM receiver capacitors: CCS uses a two segment capacitor and there is subtlety in the way it which it models the dependency on input slew. The two segment model means that the input slew is considered as a two segment slew.

The ECSM receiver capacitor model uses one segment and treats the input slew as uniform. We have implemented a number of translation schemes. But the two approaches that we recommend are (i) take the average of the C1 and C2 as the ECSM receiver cap, or (ii) only take the C1 value. The latter model will match the delay and have some pessimism in the slew calculation.

5- Take data from 2 & 3 and properly format into ECSM models.

How to get CCS2ECSM:

Our CCS2ECSM translation tool is available for free.  It's an open-source tool that we will upload as an attachment to a posting on Liberty Forum.  Here's a link to the liberty forum: 

http://synopsys.infopop.net/1/OpenTopic?a=cfrm&s=7741942603&f=3401999903

Since this tool is open-source, it can be downloaded, modified and improved by anyone interested.  In fact, we welcome industry experts who want to improve on this tool and re-post it on Liberty Forum.  E-Tools has the expertise to provide bug fixes or other necessary support for this tool, but given our company size and very limited resources, we cannot commit to do it free of charge in the long term.  If a company requires any specialized support, they should feel free to contact us: antoinebigirimana@yahoo.com or antoine@e-tools.com .

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Electronic Tools Company

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Telephone: +1 (707) 996-3320
e-mail: sales@e-tools.com