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Unformatted text preview: Chapter 7 Spectre Analog Simulator T HE SIMULATIONS described in Chapter 4 are either behavioral simulation , where the Verilog describes high-level behaviors in a software-like style, or switch-level simulation , where the circuit is expanded all the way to transistors that are mod- eled as perfect switches using built-in Verilog transistor-switch models. A more accurate simulation would model the transistors as analog devices and use as much detail as pos- The FORTRAN history of Spice explains why even today the input files for analog simulators are called spice decks. That term goes back to the punched card decks that were used for input to these FORTRAN programs. sible in that analog model to try to accurately reflect the real electrical behavior of the transistor network. Most analog simulators of this sort can trace their background to a sim- ulator called Spice that was originally developed at Berkeley in the early 1970s. Through the early 1980s Spice was still written in FORTRAN, and you can actually still obtain the Spice2G6 FORTRAN code from Berkeley if you look around carefully enough. Spice3, developed in 1985, was the first C-language version. Since escaping from Berkeley, there have been many commercial versions of Spice and Spice-like programs, including Hspice , Pspice , IS Spice , and Microcap . The analog simulator integrated into the Cadence framework is called Spectre . It is similar to Spice in terms of simulating the analog behavior of the transistors, and it even accepts spice decks as input in addition to its own Spectre format. It operates slightly differently internally and is claimed to be a little faster than Spice . From your point of view, though, they are essentially identical simulators. They take the same input decks and produce the same waveform outputs that show the analog behavior of the circuit network. Note that the simulator described in this chapter is Spectre and not SpectreS ; the latter This chapter uses Spectre from the MMSIM 6.2 release. is an older version with a socket interface. Depending on which version of the NCSU CDK you are using, SpectreS may be the default, and your local site administrator may have to modify the NCSU Analog Parts library to be used with Spectre . An important thing to keep in mind is that whereas the Verilog simulations used abstract logical 0 and 1 signals for inputs and outputs, analog simulations with Spectre will use analog voltages for inputs and outputs. Thus, you now need to concern yourself with 168 CHAPTER 7: Spectre Analog Simulator Tranistor circuit to test (DUT) Test Schematic Waveform Viewer DUT outputs Testbench Circuit DUT inputs Figure 7.1: The analog simulation environment for a circuit (DUT) things like what voltage the power supply is set to, what the voltages should be in order to be considered a logic 1 or logic 0, what the slope of changing inputs should be, and other analog electronic considerations....
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