Description
Objective
To understand how to build and use registers in Verilog.
1 Preparation
1. Read this document in its entirety.
2. Design a 4-bit register using D flip-flops.
3. Design the circuit in Exercise 3.19 from the main textbook. See details below.
2 D Flip-Flop
Inside your ECE2700 directory, create a new folder called Lab4. Start up Xilinx ISE and create a new
project called Reg inside the Lab4 directory.
Create a new file called dflipflop.v. Inside, design a D flip-flop. The inputs are the clock signal Clk,
hold signal Hold, and data input D. The output is Q. The hold signal is used to ensure no change is
made to the stored value even if the clock signal is high and is a custom functionality requested by, say,
your client. The D flip-flop also has two asynchronous inputs: Rst, which resets the D flip-flop to 0,
and Set, which set the D flip-flop to 1. Note that reset has priority over set and hold, while set has
priority over hold. Use the following declaration: dflipflop d1(Clk, Hold, Rst, Set, D, Q);
Test your design using dflipfloptest.v, which is available on Canvas. Make sure you understand what
the test cases are supposed to do. Feel free to modify the test cases. Show your working design to the
TA.
3 4-bit Register
Inside a new file called reg4bit.v, design a 4-bit register using instances of the D flip-flop you designed
earlier. You are required to use vectors in this exercise.
Test your design using simulation. It may be easier to follow the test cases in dflipfloptest.v and
make changes accordingly. Show your results to the TA.
4 Using Registers in a Combinational Circuit
Complete Exercise 3.19 from the main textbook. You can assume the input is 4-bit long instead of 8-bit
long. You are required to use the 4-bit registers you designed earlier. The 4-bit 4×1 mux is provided
in Canvas (mux4bit4x1.v). Note that you should design your circuit in such a way that if the control
inputs are 00, the oldest value is the output (and if the control inputs are 11, the newest value is the
output). Use vectors.
Test your design. It may be helpful to declare the output of each register as an output as well so that
you can see it in the waveform and debug the result.
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5 TA Checkoff
• (10 points) Prelab design.
• (20 points) D flip-flop.
• (30 points) 4-bit register.
• (40 points) Register use in combinational circuit.
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