How to build a self-checking testbench

William Kafig, Xilinx
EETimes (2/17/2012 7:15 AM EST)

A testbench, as it’s known in VHDL, or a test fixture in Verilog, is a construct that exists in a simulation environment such as ISim, ModelSim or NCsim. Simulation enables a unit under test (UUT) – typically, your synthesizable FPGA design – to connect to virtual (simulated) components such as memory, communication devices and/or CPUs, and be driven with a known set of stimuli. These stimuli cause the UUT to react and interact with the virtual components. You can view both the stimulus and the reaction as waveforms in the simulation environment.

Here’s quick example to illustrate how to implement a testbench using a simple 8-bit up/down with reset as the FPGA design (UUT). The testbench provides clock, up/down, enable and reset control signals. Figure 1 shows how to connect the UUT (central gray box) to a testbench.

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