Billy Fung

VHDL testbenches

Always be testing

Billy Fung / 2015-10-06


One of the key elements in writing synthesizable VHDL is to test your VHDL before loading it onto the FPGA. For very simple and basic designs, it might not be worthwhile to test in a separate program, but like in software engineering, nothing is simple and basic in the real world. So tests are very useful in debugging your design before figuring out things are working as they should be. The typical workflow I do is I write the code in Quartus, or whatever editor you have that allows for tab complete since it is definitely more efficient to tab complete than to type out some of these very verbose descriptors (stdlogicvector is quite a number of keystrokes). Then ModelSim is used to run your test bench.

Before even starting, I’d like to point out that one of the more annoying features is that the subset of synthesizable code is not the same in ModelSim and Quartus, depending on which version of VHDL you are using. In my case, I’m using VHDL 2008, which supports make functions that make designs nicer, like the matching case statement. Just keep that in mind when your VHDL compiles and your tests all pass in ModelSim, and then fails when you go to synthesize in Quartus.

Moving along, the method of testing your VHDL design is quite straightforward. You create a new test bench file, and within it you instantiate the component you wish to test. The test bench entity will be empty, and the architecture will consist of the signals that will propagate the design, mainly clock and reset.

entity cpu_tb is
end entity;

architecture test of cpu_tb is
signal reset : std_logic; 
signal clk : std_logic;

beginDUT : controller port map(  reset => reset, clk  => clk);

The DUT here is a label meaning device under test, which is the component you are testing. This simple example only uses 2 signals within the controller entity, but if your entity has more signals, then when you port map the signals, you will put the signals there as well. In the case of a complex design with multiple entities, the syntax to test signals within an entity is:

<<signal DUT.entity.signal: std_logic_vector>> <= force ‘1’;

This command will force the signal to be whatever value you’d like it to be for testing purposes. This is a very handy tool for controlling the test bench if your signals require several cycles to change. I’ll call it a driving signal since you can use a signal to drive the test forward in certain situations. This kind of testing is mainly useful when you are testing components being added to a larger more complex design. By forcing a signal, you are able to see what changes your added component has.

The following shows how to simulate a clock in ModelSim:

process begin
clk <= '0’; wait for 5 ps;
clk <= '1’; wait for 5 ps;
end process;

So knowing the cycles, you can change the signals accordingly in each cycle.

To automate the detection of failures, the following is used:

report to_string(x) & “ ” & to_string(o) & “ -> ” & to_string(xnext);
assert xnext = “000010000” report “output should be 000010000” severity failure;

The report to_string(x) outputs the string values in the ModelSim console for easier reading. The assert checks the value of the signal, and if it is not equal, issues a failure result. This way you can run your test and the console will show you where all the failures are.

The ideal test bench is one in which your input data/files is loaded into the VHDL, and then run according with all the failures displayed. But sometimes this is not possible, and the next best thing is to view the signal waveforms in the waveform viewer in ModelSim. If you are testing components individually by driving them the signal to change, it is advisable to not do this in the final test, because if the signal you are driving is actually dependent on another component, it does not show that the component is working properly.