VHDL Programming

CPU: RTL Simulation 331

Testbench

Stimulus Driver

DUT

OK Errors

Good Results

Figure 14-2

Testbench Block

Diagram.

Kinds of Testbenches

There is a myriad of ways to write a testbench, but some of the most com-

mon are described in this section. The following are the most common

testbench types:

Stimulus only—Contains only the stimulus driver and DUT; does

not contain any results verification.

Full testbench—Contains stimulus driver, known good results, and

results comparison.

Simulator specific—Testbench is written in a simulator-specific

format.

Hybrid testbench—Combines techniques from more than one

testbench style.

Fast testbench—Testbench written to get ultimate speed from

simulation.

To show the different types of testbenches, a common example is used.

To make it simple to understand the stimulus and response, a counter

example is used. The following description is the package, entity, and ar-

chitecture for an 8-bit counter: