D Flip-Flop (DFF)

Code located at: Verilog D Flip-Flop

This example demonstrates the design and verification of a simple D flip-flop (Wikipedia link).

Design

The DFF module has the following pins:

Name	Type	Description
clk	input	the clock; rising edge of the clock captures the value
reset	input	asynchronous reset; when reset is high, the DFF output q is 0
d	input	the main input
q	output	the d value captured at the last rising clock edge
qb	output	inverted version of q

Testbench

The testbench is a simple directed test which toggles the DFF inputs and displays the outputs to the console.

The reg signals are used to drive inputs, and wire signals are used to observe outputs:

reg clk;
reg reset;
reg d;
wire q;
wire qb;

The DFF design is instantiated:

dff DFF(.clk(clk), .reset(reset),
        .d(d), .q(q), .qb(qb));

The initial block contains the actual test. First, reset is driven to 1 to reset the flop, while d is driven with an X:

clk = 0;
reset = 1;
d = 1'bx;

From the console display, we see that the flop has been properly reset with q == 0

Reset flop.
d:x, q:0, qb:1

Next, reset is released, while input d is driven to 1:

d = 1;
reset = 0;

The output q remains at 0 because the design did not see a rising edge of clk and did not capture the d input:

Release reset.
d:1, q:0, qb:1

Finally, we drive clk to 1 to create a rising edge:

clk = 1;

Now we see q output change to match the d input:

Toggle clk.
d:1, q:1, qb:0

Note: Before calling the $display task, we always tell simulation to proceed for 1 time unit #1 to allow the output signals to propagate.