Verilog code for a comparator

In this project, a simple 2-bit comparator is designed and implemented in Verilog HDL. Truth table, K-Map and minimized equations for the comparator are presented. The Verilog code of the comparator is simulated by ModelSim and the simulation waveform is presented.


Verilog code for a comparator


The specification of the 2-bit comparator is as follows:

  • Input: 2-bit A and B for comparison
  • Output:
    • A_greater_B: high if A > B else low
    • A_equal_B: high if A = B else low
    • A_less_B: high if A<B else low
The truth table for the comparator:


A1
A0
B1
B0
A_greater_B
A_equal_B
A_less_B
0
0
0
0
0
1
0
0
0
0
1
0
0
1
0
0
1
0
0
0
1
0
0
1
1
0
0
1
0
1
0
0
1
0
0
0
1
0
1
0
1
0
0
1
1
0
0
0
1
0
1
1
1
0
0
1
1
0
0
0
1
0
0
1
0
0
1
1
0
0
1
0
1
0
0
1
0
1
0
1
1
0
0
1
1
1
0
0
1
0
0
1
1
0
1
1
0
0
1
1
1
0
1
0
0
1
1
1
1
0
1
0

K-Map tables and the corresponding equations for the comparator:



A_greater_B
A1A0
00

01

11

10
B1B0
00
0
1
1
1
01
0
0
1
1
11
0
0
0
0
10
0
0
1
0

A_greater_B = B0 B1 A0 + B1 A1 + A1 A0 B0
= A0 B0 (B1 + A1) + + B1 A1



A_equal_B
A1A0
00


01


11


10
B1B0
00
1
0
0
0
01
0
1
0
0
11
0
0
1
0
10
0
0
0
1

A_equal_B = (B1 A1+A1 B1) (B0 A0+A0 B0)



A_less_B
A1A0
00


01


11


10
B1B0
00
0
0
0
0
01
1
0
0
0
11
1
1
0
1
10
1
1
0
0

A_less_B = A0 A1 B0 + A1 B1 + B1 B0 A0

After obtaining the minimized equations of the outputs of the comparator, it is easy to write Verilog code for the comparator.

Below is the Verilog code for the comparator:

 // FPGA projects using Verilog/ VHDL  
 // fpga4student.com : FPGA projects, Verilog projects, VHDL projects
 // Verilog code for 2-bit comparator   
 module comparator(input [1:0] A,B, output A_less_B, A_equal_B, A_greater_B);  
 wire tmp1,tmp2,tmp3,tmp4,tmp5, tmp6, tmp7, tmp8;  
 // A = B output   
 xnor u1(tmp1,A[1],B[1]);  
 xnor u2(tmp2,A[0],B[0]);  
 and u3(A_equal_B,tmp1,tmp2);  
 // A less than B output   
 assign tmp3 = (~A[0])& (~A[1])& B[0];  
 assign tmp4 = (~A[1])& B[1];  
 assign tmp5 = (~A[0])& B[1]& B[0];  
 assign A_less_B = tmp3 | tmp4 | tmp5;  
 // A greater than B output   
 assign tmp6 = (~B[0])& (~B[1])& A[0];  
 assign tmp7 = (~B[1])& A[1];  
 assign tmp8 = (~B[0])& A[1]& A[0];  
 assign A_greater_B = tmp6 | tmp7 | tmp8;  
 endmodule   
 `timescale     10 ps/ 10 ps  
 // FPGA projects using Verilog/ VHDL  
 // fpga4student.com  
 // Verilog testbench code for 2-bit comparator   
 module tb_comparator;  
 reg [1:0] A, B;  
 wire A_less_B, A_equal_B, A_greater_B;  
 integer i;  
 // device under test  
 comparator dut(A,B,A_less_B, A_equal_B, A_greater_B);  
 initial begin  
      for (i=0;i<4;i=i+1)  
      begin   
           A = i;  
           B = i + 1;  
           #20;  
      end   
      for (i=0;i<4;i=i+1)  
      begin   
           A = i;  
           B = i;  
           #20;  
      end   
      for (i=0;i<4;i=i+1)  
      begin   
           A = i+1;  
           B = i;  
           #20;  
      end   
 end   
 endmodule   

Finally, simulate the Verilog code and testbench in ModelSim and verify the operation of the comparator through simulation waveform. 

Verilog code for 2-bit comparator

By observing the waveform, Verilog code for the comparator are working correctly. 


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