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Signal Integrity Macromodel

Macromodels In the Mix

By Timothy Coyle

The hybrid model: SPICE + IBIS = Macromodel

There are many different definitions for a “macromodel” in the Signal Integrity field that are used for system level simulations. I am going to define a macromodel as a hybrid simulation model used to represent the analog behavior of an IO buffer. There are many different variations of a macromodel from a pure C implementation to a Matlab model and more since there is not a standard specification that any one company uses.

However, the IBIS Specification has tried to address this over the years by adding support for different types of IBIS models that can include SPICE models and VHDL-AMS/Verilog-AMS. The IBIS 5.0 Specification has also added support for an algorithmic modeling interface that uses separate executable DLL files in conjunction with an IBIS model to represent the equalization of high speed SerDes devices. These are the types of macromodels that will be discussed throughout the rest of the book.

Advantages

The advantage of using a macromodel is that it often gives the best trade-off between flexibility vs. simulation run time and still does not reveal any design IP. For example, if you want to model certain types of equalization with an IBIS model you have to use the external circuit links to a SPICE or AMS type model. (Remember, while this is still technically an IBIS model often times the implementation involves more than just an IBIS model file so we are going to call this a macromodel.) Using macromodels allows you to extend the base IBIS specification to meet technological advances before the IBIS Specification or an EDA tool vendor has added full support.

Disadvantages

The disadvantage of using a macromodel is that it is often tool dependent which is supposed to be one of the main benefits of using an IBIS model in the first place. While various macromodel implementations can conform to the IBIS Specification often times the adoption of these types of models across EDA tools is not consistent enough to be viewed as reliable. There is considerable work being done to improve this and there are work arounds but it is not as smooth as using standard IBIS models.

Macromodels have their place in Signal Integrity simulation but they require considerable care when being implemented and used to maximize the benefits and not trade one issue for another.




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