ECE351 - System Verilog code  - Homework #1  - Solved

Step 1: Create a combinational RTL model for a 4-bit expandable carry lookahead adder (CLA). You do not need to create or use gate-level modules for full or half adders. 

 

A combinational RTL model uses only continuous assignment (assign) statements or always_comb procedural block with or without gate delays.

 

Use these gate delays in your model:

●     AND gate 2ns

●     OR gate 2ns

●     XOR gate 3ns

 

Step 2: Simulate your 4-bit CLA module with the testbench we provided. Save a transcript of your successful simulation results.

 

Step 3: Create an expandable 8-bit adder using the 4-bit CLA module you created and verified.  

 

Step 4: Simulate your 8-bit CLA adder.  Save a transcript of your successful simulation results.

 

Your design modules MUST be declared as follows to be instantiated correctly in the testbench:

 

module CLA4Bit(ain, bin, cin, sum, cout);    input [3:0]     ain, bin; 

     input          cin; 

     output logic [3:0] sum;       output logic cout; 

 module CLA8Bit(ain, bin, cin, sum, cout);   input [7:0]    ain, bin; 

     input          cin; 

     output logic [7:0] sum; 

     output logic cout; 

 

 

             

Refer to the schematic below for a 4-bit CLA.  FA is a full adder without the carryout logic: 

 

  

 

 

Refer below equations for the carry out logic: 

 

Cout = A⋅B + A⋅Cin + B⋅Cin =

Cout = A⋅B + Cin ⋅ (A+ B) =

Cout = G + Cin ⋅ P

 

where,

Gn = An.Bn and Pn = An + Bn

 

G (carry generate) is dependent upon current stage's ability to generate a carry.

 

P (carry propagate) is dependent upon current (and prior) stages’ ability to propagate a carry.

 

A carry is generated at a stage i if it is generated by stage i’s inputs (Ai, Bi) or by any prior stage and propagated by every succeeding stage.

 

Hence,

 

CO1 = G0 + CI0.P0

=> CO1 = G0 

 

CO2 = G1 + CI1.P1 

=> CO2 = G1 + CO1.P1 

=> CO2 = G1 + G0.P1 

 

CO3 = G2 + CI2.P2 

=> CO3 = G2 + CO2.P2 

=> CO3 = G2 +(G1 + G0.P1).P2 

=> CO3 = G2 +G1.P2 + G0.P1.P2 

 

And so on...

 

And the equation for Sum output will be as follows: 

 

S = A ^ B ^ CIN; 

 

(Hint: For the sum output, you need to figure out the logic for CIN using the schematic, it is not straight forward, unlike inputs A and B which can be used as is!)

 

             

How to use the test bench provided? 

 

Part 1: Testing your 4 bit CLA design module. 

 

1.    Create a top module as shown below and add this module to your CLA4Bit module and save this single file as CLA4Top.sv:

 

module CLA4Top;     parameter nBITS = 4;       logic [nBITS - 1 : 0] ain, bin, sum;       logic cin, cout; 

 

                                     // instantiate your 4 bit CLA design module here 

 

     // instantiate the test bench module as follows   test #(4) TB(.*); endmodule 

 

2.    Add CLA4Top.sv and test.sv to your project in QuestaSim.

3.    Simulate the CLA4Top.sv file

4.    Save the transcript.

 

Part 2: Testing your 8 bit CLA design module. 

 

1.    Create a top module as shown below and add this module to your CLA8Bit module and save this single file as CLA8Top.sv:

 

module CLA8Top;     parameter nBITS = 8;       logic [nBITS - 1 : 0] ain, bin, sum;       logic cin, cout; 

 

                                     // instantiate your 8 bit CLA design module here 

 

     // instantiate the test bench module as follows   test #(8) TB(.*); endmodule 

 

2.    Add CLA8Top.sv and test.sv to your project in QuestaSim.

 

3.    Simulate the CLA8Top.sv file

 

4.    Save the transcript.

 

You are encouraged to do further readings on the working of a CLA and ask questions if things are unclear.