Test bench program for full adder

There are three inputs and two outputs to the full adder circuit. The construct of Verilog behavioral modeling consists of three main parts:. The behavioral style mainly has two prominent statements. One is an initial statement which is executed only once and the other is always statement which gets executed once the sensitivity list gets enabled.

The syntax for always statement is:. The most commonly used are begin … end. This is the most general way of coding in behavioral style. What we do over here is; select the sensitivity list first, the change in which your output depends in almost every case, the input ports comprise the sensitivity list. Then comes the logical expression which will be assigned to the output registers S and Cout. Remember that the left-hand side entities must always be a reg register since registers are data storing elements.

The case statement in Verilog is much similar to switch- case statement in C language. It is one of the procedural statements in Verilog.

The set of statements that match the first true condition is executed. For the full adder, we just try to write the statements according to the truth table; each row considering to be a case. There we use sequence generators for input and probe the output to a capturing device. And here we do the same thing virtually. In some books, authors also refer to them as an entity under test. We write a testbench to inject inputs stimulus to the Device Under Test and reads its output.

After that, we plot the results, where the waveform of all input and outputs are plotted showing their values at all instants of time.

Talking in terms of an HDL program, the HDL code for every functional block, whether it is an adder, multiplexer, or memory, they are all called DUTs during verification. A testbench is specific for a DUT. It contains a blank entity and its architecture. It really does not mean anything. It is there because as said earlier, a testbench is also a VHDL program. So it must have an entity.

Also, the entity describes the input and output of the circuit that we are testing. Inside the architecture of testbench, we declare a component which is actually our DUT.

So we use signals for internal calculations and in the end, assign the signal value to the port. The next step is to generate a stimulus, or you may say sequences for inputs.

We have two ways to generate an in-program stimulus. To test it, we will need to apply sixteen 2 4 input combinations. In the repetitive pattern method of generation, we dedicate one statement to generate only one bit. The above lines of code only represent one input combination. We have to repeat this fifteen times more for each set of inputs. That will make the testbench much longer. There surely has to be a better way right?

Here, we use vector notation to apply a group of bits to a port a group of pins. And bit assigned to the respective pin according to their position. We will be writing one example of each type for the same DUT so that you can compare them and understand them better. We will be creating a testbench for a full adder. You can find the code of full adder below for your reference. As the name suggests, it is the simplest form of a testbench that uses the dataflow modeling style.

We start writing the testbench by including the library and using its necessary packages. Here's a good reference , one of the first that came up when I googled how to write a testbench. You should google first, give it an honest shot, then come back here with more specific questions.

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