Chapter8 - Chapter 8 Cell Characterization S TANDARD CELLS...

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Unformatted text preview: Chapter 8 Cell Characterization S TANDARD CELLS must be characterized so that they can be understood and used by the various tools in the CAD flow. So far, if youve designed schematic (or The Encounter Library Characterizer (ELC) is based on (basically a rename of) an earlier tool named SignalStorm Library Characterizer (SLC) . If you are using an earlier version of the Cadence tools you might be using the latter tool. The overall flow in this chapter is the same, but some files and commands need to be renamed to reflect the change in the name of the tool. cmos sch ) and layout views of your cells, the tools understand the transistor netlist and the layout of the cells, but they dont understand the function of the cells in a way that can be used to synthesize behavioral descriptions into these cells. Other metrics, such as the input load, speed, and power of the cell, are also not available in a way that the tools can understand. The synthesis tools coming up in Chapter 9, for example, need to know the logic function of the cell, the load that the cell input will present to a signal connecting to it, the speed of the cell under different input slope and output loading conditions, the power that the cell will consume, and the area of the cell in order to do a good job of synthesizing a behavioral description to a collection of standard cells. Cell characterization is a process of simulating a standard cell with an analog simulator to extract this information in a way that the other tools can understand. This can be done through specific analog simulation (using Spectre ), whose output you look at to generate the characterization data, or by using a library characterization tool. In this case well use Encounter Library Characterizer ( ELC ) from Cadence. This tool actually uses Spectre to simulate the circuit being characterized, but it wraps a very nice interface around that so you dont have to specify in detail what should be measured. 8.1 Liberty file format In order for the tools to understand all this characterization data, it should be in a standard format that most tools agree on. The most standard format is called liberty format, which usually uses a .lib file extension. Liberty format is an ASCII file that describes a cells characterized data. This file is used by the synthesis tools described in Chapter 9 and by the place and route tools in Chapter 11. The general form of a Liberty file is shown in 218 CHAPTER 8: Cell Characterization / * General Syntax of a Technology Library * / library (nameoflibrary) { ... / * Library level simple and complex attributes * / ... / * Library level group statements * / ... / * Default attributes * / ... / * Scaling Factors for delay calculation * / / * Cell definitions * / cell (cell_name) { ... / * cell level simple attributes * / / * pin groups within the cell * / pin(pin_name) { ... / * pin level simple attributes * / / * timing group within the pin level * / timing(){ ... / * timing level simple attributes...
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Chapter8 - Chapter 8 Cell Characterization S TANDARD CELLS...

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