4 bit comparator


This is the code for a 4 bit equality comparator. 



module eq4bit ( a, b, led2);
input wire [3:0] a, b;
output wire led2;
 
wire p1,p2,p3,p4;
 
comp1bit compare1 (a[0],b[0],p1);
comp1bit compare2 (a[1],b[1],p2);
comp1bit compare3 (a[2],b[2],p3);
comp1bit compare4 (a[3],b[3],p4);
 
assign led2 = p1 & p2 & p3 & p4;
 
endmodule
 
module comp1bit
(
input a, b,
output p
);
assign p = (a & b) | ( ~a & ~b); 
endmodule


Essentially, it gives output 1 when all 4 bits of a are equal to the 4 bits of b.
The video shows the result when it is implemmented on Spartixed board.



Following ucf file is used. 



NET "led2" LOC = P131;
 
NET "a[3]" LOC = P33;
NET "a[2]" LOC = P32;
NET "a[1]" LOC = P30;
NET "a[0]" LOC = P29;
 
NET "b[3]" LOC = P27;
NET "b[2]" LOC = P26;
NET "b[1]" LOC = P24;
NET "b[0]" LOC = P23;






In place of the module instantiation, we could also use a much simpler implementation as this one. 



module eq4bit ( a, b, led2);
input wire [3:0] a, b;
output wire led2;
 
assign led2 = (a == b) ? 1'b1 :
           1'b0; 	
endmodule
 




Suggested Exercises

1. Modify the code so that it gives output when a is less than b - use the 1 bit istantiation method ( is slightly difficult) and well as the shorter version .
2. Modify the code so it compares if a is greater than b.