Circuit simulation with hardware description languages - Test Run

Designing field-programmable gate arrays is only half the job: The hardest part is the simulation, but Linux is the best place to tackle certain challenges.

- Marco Fioretti

In the previous issue of Linux Magazine, I introduced the digital integrated circuits called field-programmable gate arrays (FPGAs) [1], explained why they are important, and described the bare minimum needed to set up and try a state-of-the-art FPGA design environment on Linux. This article, which is divided into three parts, concludes that introduction.

First I’ll describe the main types of elementary digital circuits that can be built, combining the logic gates covered in the first tutorial and the basic features of hardware description languages (HDLs) that allow their implementation and simulation.

The central part combines some HDL code that's freely available online [2] [3] into one simple HDL module and the code necessary to simulate it. The final part shows a run of that simulation and concludes by explaining why Linux is the perfect platform for working with FPGAs.

Representing Digital Circuits with HDL

FPGAs (and every other digital integrated circuit) comprise millions of logic gates, with boolean input and output signals – that is, having one of two states, conventionally represented as 1 or 0.

Sequential components are combinations of logic gates that sample all their input ports, and (with constant, predefined delays) update their outputs on the positive or negative fronts of one or more signals called clocks (i.e., square waves of a fixed frequency that constitute common timing references for the chip).

The simplest sequential block is a memory element, also called a flipflop [4], that can load and preserve binary values: Its output changes only on the fronts of a control signal, almost always a clock, copying and then preserving whatever value the input pin has in those instants.