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VHDL Testbench Tutorial

2VHDL Testbench Tutorial

IntroductionThe purpose of this advanced testbench tutorial is to acquaint you with methods of automatic generationof WAVES-based testbenches. The basic testbench tutorial, which you are assumed to have alreadygone through, deals with the single process testbenches. A single process testbench can only drive inputports of the UUT entity while the WAVES-based testbench simultaneously drives the inputs andcompares the output response with a previously saved pattern. Test vectors used to stimulate the UUTentity are furnished in an external file (*.VEC) based on a text format specified by the WAVES standard.Additional procedures compare output signals of the UUT entity with the pattern vectors. Discrepanciesare reported in a log file. The Test Bench Wizard generates the test vector file (*.VEC) from a waveformfile (*.AWF) created with Waveform Editor. It also verifies if port names of the UUT entity agree with thesignal names saved in the waveform file.Once the user has generated a testbench and prepared specification of test vectors, the testbench can beused many times to perform automatic verification of successive revisions of a VHDL design. Moreover,the same testbench can be used with different test vector files, provided that they define timings of thesame signals. The wizard is not the only tool that generates test vector files in the WAVES format. Active-HDL provides the Export Waveform command that can be used to convert waveform files (*.AWF) toWAVES format test vector files(*.VEC).

Creating New DesignIn the tutorial, you will create a simple design. To save time, you will reuse VHDL code from theLoadable_counter sample design, shipped with Active-HDL.1. Start Active-HDL. If the Getting Started dialog opens, click Cancel.2. Choose New | Design from the File menu. The New Design Wizard will open.3. In the first wizard dialog, select the Add existing resource files option, and then click Next. You willuse an existing VHDL source file from the Loadable_counter sample design.

4. In the next wizard dialog, click the Add files button to open the Add Files to Design dialog box.

35. In the Add Files to Design dialog box, select the counter.vhd source file from the src folder of theLoadable_counter sample design folder.

6. Select the Make local copy check box, and then click Add to copy the counter.vhd file to your designdirectory. Then click Next to display the last wizard dialog.7. Specify desired synthesis and implementation tools for the current design (if there are any installed).Choose the default family, block diagram configuration and default HDL language. Click the Next button.8. Enter L_counter as the design name, and then click Next.

49. The next wizard dialog shows a summarized design specification so that you can view it before thedesign will be created.Select the Compile source files after creation check box, and then click Finish.The design will be created and the counter.vhd file will be compiled.Choose the counter as a top-level.

After this operation, the Design Browser window should look as shown below:

Generating Test BenchUsually, the user performs the functional simulation and defines test vectors required to verify operationof the design before generating a testbench. To save time, in this tutorial you will use the waveform filesaved in the sample Loadable_counter design to generate the testbench, and then perform the functionalsimulation using the testbench macro.1. Select the Add files to design option from the Design menu.

52. In the Add Files to Design dialog box, select All files from the Files of type list box, then select theFunctional.awf waveform file from the src folder of the Loadable_counter design, and then click Add.3. In the Design Browser click the icon to the left of the counter.vhd icon.4. Click the counter(counter_behav) design entity with the right mouse button, and then choose GenerateTest Bench from the shortcut menu to start the Test Bench Wizard.

5. The first Test Bench Wizard dialog will appear in which you have to choose the desired testbench type.

6. Select the WAVES Based option, and then click Next.7. Select the Test vectors from file check box, to make the wizard use the previously saved waveformfile to generate the testbench stimuli. Next, click the Browse button to display available waveform files.Select the Functional.awf waveform file in the Open dialog, and then click Open.

6The Signals found in file box will display input ports found in the specified waveform file, and the UUTports box will display the input ports of the COUNTER entity. Note, that the COUNTER entity is the UUTentity. Click Next.8. The next wizard dialog allows you to specify the names of the testbench entity, architecture and VHDLsource file. The wizard automatically suggests default names. Click Next to accept them and go to thenext wizard dialog.

9. The last dialog displays the paths of the files that will be generated by the wizard.They are: the testbench source files a simulation macro file which automatically compiles the testbench files, inserts the UUT ports

into the Waveform Editor window and runs simulation of the testbench an additional source file containing a configuration declaration for timing simulation (generated

optionally)10. Select the Generate check box to generate the optional configuration file, and then click Finish.11. The wizard will generate the testbench files and attach them to the design in a special file folder,WAVES. Note, that Design Browser uses a special icon for testbench source files to distinguish themfrom regular sources. The Counter_TB.vhd file will open in the HDL Editor window. View the source code

7to see how the testbench has been constructed by the wizard.12. In addition to the files created by the wizard, the WAVES folder also contains external auxiliary filesthat have been attached to the design. The external files are represented in Design Browser by theicon.13. Click the icon to the left of the WAVES folder on the Files tab of the Design Browser. The WAVESfolder contents is as shown in the figure below:

Running TestBench MacroIn the previous paragraph you have generated the WAVES-based testbench for your design. Now, youcan run the testbench macro to view the results of functional simulation of the design. Note, that thegenerated waveform will contain not only the actual result timings of your design but also the expectedtimings ( the output Q port is represented by the ACTUAL_Q signal, whose value is compared with theexpected Q port value, represented by the EXPECT_Q signal). The waveform will also contain theERR_STATUS additional signal, whose value of 'L' indicates that no discrepancies have been detectedbetween expected and actual timings.The testbench macro will perform the following operations: compiles the UUT design files compiles the WAVES testbench necessary files initializes simulation adds ports of the design to the Waveform Editor window runs simulation, comparing the actual results with the expected result values

All you have to do to run the test of the design is execute the macro and view the simulation results. If theERR_STATUS signal value equals 'L' then the results are the same as expected. If, at any time value thediscrepancy has been detected, then the ERR_STATUS signal value will equal 'H'. The details of thedetected discrepancies are logged to the counter_report.log file.1. Select the Counter_TB_runtest.do macro on the Files tab of the Design Browser window.2. Click the right mouse button and choose Execute from the shortcut menu.3. Wait until the shown below message box appears, then click OK.

8You should obtain waveforms as shown in the figure below.

4. End simulation by choosing End Simulation from the Simulation menu.

Timing SimulationThe final stage of the design development process is the verification of the design behavior after itsimplementation. The Place & Route implementation tools produce structural VHDL code. Such code isusually supplied with SDF file with timing information. So obtained source file can be verified with thesame testbench that has been used for functional simulation. In this tutorial, you will not have to runimplementation software by yourself. Instead, you will use implementation files from the original sampledesign Loadable_counter, suitably modifying the testbench macro for the needs of timing simulation.1. Select Add Files to Design from the Design menu. In the Add Files to Design dialog select the srcfolder located in the Loadable_counter folder, and then select the following two files: counter_tim.vhd - the backannotated post place-and-route VHDL structural code counter_tim.sdf - SDF (standard delay format) timing file

Make sure that the Make local copy check box is selected, then click Add.2. Switch to the Files tab of Design Browser.Now you will have to edit the counter_TB_tim_cfg.vhd configuration and the counter_TB_runtest.domacro so that they can be used for timing simulation.3. Double-click the counter_TB_tim_cfg.vhd file on the Files tab. The file will open in the HDL Editor.Uncomment the line:

use entity work.ENTITY_NAME (ARCH_NAME);Replace the ENTITY_NAME and ARCH_NAME with the entity and architecture names from thebackannotated VHDL file. The line should read as follows:

use entity work.COUNTER (STRUCTURE);

Save the timing configuration file by clicking the toolbar button.4. Double-click the counter_TB_runtest.do macro on the Files tab. The file will open in the HDL Editor. Dothe editing as follows:a) In the #Compiling UUT entity design files section, replace the line

comp $DSN\src\counter.vhdwith the line:

comp "$DSN\src\counter_tim.vhd"

9b) In the #Compiling timing configuration section, uncomment the lineacom -work L_counter "$DSN\src\WAVES\counter_TB_tim_cfg.vhd"

c) In the #Run simulation section replace the lineasim TESTBENCH_FOR_counter

with the line:asim TIMING_FOR_counter -sdftyp /UUT=$DSN\SRC\counter_tim.sdf

5. Save the counter_TB_runtest.do file. Now, the macro is ready for execution. The macro will compile theimplementation source files, initialize simulation and run the testbench.6. Select the macro label on the Files tab of the Design Browser window. Click the right mouse button,and then choose Execute from the shortcut menu.Wait until the shown below message box appears, then click OK.

7. View the results of timing simulation in the Waveform Editor window and compare them with theresults of the functional simulation.As the frequency of 125 MHz has been chosen for the CLK input signal, which is too high for the realsynchronous counter implementation, the output ACTUAL_Q signal value does not change as expected.Note, that the ERR_STATUS signal value is 1, indicating discrepancies between EXPECT_Q andACTUAL_Q signals. View the log file to see how discrepancies are represented in it.

To do so, switch to the Resource tab of Design Browser, click the icon to the left of the Logs folder, andthen double-click the src\WAVES\counter_report.log label.

Adjusting Test Vector FileIn this paragraph you will edit waveforms so that the timing simulation could be successfully performed.The first thing you have to do is scaling the waveforms to obtain the 12.5 MHz frequency of the input CLKsignal, which should be acceptable by the design implementation. You will use another Active-HDLfeature, the Stretch command, that is available from the shortcut menu after selecting the waveform areato be scaled.1. Select End Simulation from the Simulation menu.2. Open the original Functional.awf waveform file, that has been used for testbench generation.3. Select the whole waveform area:

a) In Waveform Editor, switch to the Edit mode by clicking the toolbar button.b) Rest the mouse pointer over the origin of the waveform area to be selected. Press and hold the mouse

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button. The mouse pointer will adopt a new shape: . If the mouse pointer tends to switch to the event-dragging mode, hold the Ctrl key.c) Move the mouse so as to select the desired area. The selection should contain all waveforms. Thetooltip will show the time range of the current selection.d) Release the mouse button.4. Click the right mouse button, then select the Stretch command from the shortcut menu. This will openthe dialog shown below:

5. In the Scale box enter 1000%, and then click OK. This will change the CLK frequency to 12.5 MHz.

6. Save the waveform file, clicking the button.7. Choose Waveform from the File/Export menu. The following dialog will open:

8. Select the Waves vectors (*.VEC) from the Save as type box, select the Functional.vec file from the\\src\WAVES folder, and then click Save. Click Yes when asked if you want to replace an existing file.Select the L_counter design library on the Files tab of the Design Browser, click the right mouse button,and then choose Delete simulation data from the shortcut menu.Note, that the asim TIMING_FOR_counter -sdftyp /UUT=$DSN\SRC\counter_tim.sdf command instructsthe simulator to apply the time delays from the counter_tim.sdf file. This will result in certain delays of theoutput signal relative to the falling edge of the clock signal. The WAVES-based testbench declares thewindow frames, in which it will compare the actual signal values with the expected signal values duringsimulation. For the purpose of the functional simulation the default WND_BEGIN_DEFAULT andWND_END_DEFAULT window parameters have been used. The appropriate declarations can be foundin the counter_TB_declaration.vhd file:

constant WND_BEGIN_DEFAULT: EVENT_TIME := 1 ps;constant WND_END_DEFAULT: EVENT_TIME := 0 ps;

Those values can not be used for the purpose of timing simulation. You will have to edit thecounter_TB_declaration.vhd file to supply the appropriate values of the window frames. As shown in thefigure below, the WND_BEGIN_DEFAULT parameter for the design operating with 12.5 MHz clock signalshould be greater than 26 ns (which is the delay of the Q output relative to the CLK falling edge). Thefigure shows timings for the WND_BEGIN_DEFAULT equal to 30 ns, and the WND_END_DEFAULTequal to 40 ns.

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9. Double-click the counter_TB_declaration.vhd file on the Files tab of Design Browser to open the file forediting. In the editing window, enter WND_BEGIN_DEFAULT in the Find box located on the left of the

Find Next toolbar button . Next, click the Find Next button, and then replace the values in theconstant declarations as shown below:

constant WND_BEGIN_DEFAULT: EVENT_TIME := 40 ns;constant WND_END_DEFAULT: EVENT_TIME := 50 ns;

Click the button.10. Double-click the counter_tb_runtest.do macro on the Files tab of Design Browser to replace the line:run 400 ns withrun 4000 nsThis is required to simulate the whole stretched waveforms.

Save the macro, clicking the button.11. Select the counter_tb_runtest.do macro on the Files tab of the Design Browser window. Click theright mouse button, and then choose Execute from the shortcut menu.Wait until the shown below message box appears, then click OK.

The results should be as those shown in the picture below.

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The Console window still shows many VitalGlitch warnings. Choose End Simulation from theSimulation menu. If you want to disable the VitalGlitch warnings edit the counter_tb_runtest.do macro,adding the switch -noglitch to the asim command line.

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