Test Benches
To simulate your design, you need both the design under test (DUT) or unit under test (UUT) and the stimulus provided by the test bench. A test bench is HDL code that allows you to provide a documented, repeatable set of stimuli that is portable across different simulators. A test bench can be as simple as a file with clock and input data or a more complicated file that includes error checking, file input and output, and conditional testing.
Note Verilog designers sometimes refer to a Verilog test fixture. "Test bench" and "test fixture" are used synonymously throughout this documentation.
You can create the test bench using either of the following methods:
  •  Text editor
    This is the recommended method for verifying complex designs. It allows you to use all the features available in the HDL language and gives you flexibility in verifying the design. Although this method may be more challenging in that you must create the code, the advantage is that it may produce more precise and accurate results than using the Test Bench Waveform Editor.
    To assist in creating the test bench, you can create a template that lays out the initial framework, including the instantiation of the UUT and the initializing stimulus for your design. Create this template as described in Creating a Source File, selecting VHDL Test Bench or Verilog Test Fixture as your source type.
    Note   You can also use the Language Templates to create the VHDL or Verilog code for both the test bench and the design. See Working with Language Templates for details.
  •  Xilinx® Test Bench Waveform Editor
    This is the recommended method for verifying less complicated simulation tasks and is recommended if you are new to HDL simulation. It allows you to graphically enter the test bench to drive the stimulus to your design. For detailed information, see the ISim Help. You can also create a self-checking test bench. For details, see Generating a Self-Checking Test Bench.
    Note The Test Bench Waveform Editor flow is being considered for deprecation in this release. If you use this tool and do not want to see it deprecated, please send feedback through the mechanism in the software. To learn more, see the ISim Help.
Test Bench Strategies
Because the test bench becomes a part of the hierarchy in your code, the following is recommended:
  •  Make the test bench the top level of the code.
    The test bench instantiates the unit under test (UUT), and stimulus is applied from the top-level test bench to the lower-level design or portion of the design being tested.
  •  Use the instance name UUT for the instantiated unit under test.
    This is the default instance name that Project Navigator expects.
You can use the same test bench for both functional and timing simulation. Following are general recommendations that apply to both types of simulation:
  •  Initialize all input ports at simulation time zero, but do not drive expected stimulus until after 100 nanoseconds (ns) simulation time.
    During timing simulation, a global set/reset signal is automatically pulsed for the first 100 ns of simulation. To keep the test bench consistent for both timing and functional simulation, it is recommended that you hold off input stimulus until the global set/reset has completed.
    Note You can still run the clocks during the first 100 ns of simulation.
  •  Do not provide data to the input ports at the same time as the clock.
    For non-timing simulation, this can cause some signals to be applied before the clock and some after the clock. Apply data only after the clock is applied to the input ports. This makes it easier to keep track of which clock edge data changes are being applied.
  •  If output checking is performed in the test bench, apply data just before the next clock cycle.
    For timing simulation, it could take up to an entire clock cycle for the data to propagate through the logic and settle to a known value. Checking data too early in the clock cycle may yield incorrect results.
See Also

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