Verilog HDL: Gate Level Modeling

Gate Level Modeling:

à For small number of gate designs;

à Feedbackless structure;

à Used in top level to integrate the design.

Gate –Level Modeling (Structural Modeling):

à mainly includes instantiation of built-in primitives.

à and,nand,or,nor etc

à Design with top-down methodology.

à Implementation with bottom-up methodology.

module half_adder(s,c,a,b)

input a,b;

output s,c;

assign s=a^b;

assign c=a&b;

endmodule

module full_adder(sum,carry,x,y,z)

input x,y,z;

output sum,carry;

half_adder ha1(.s(s1), .c(c1), .a(x), .b(y));

//half_adder ha1(s1,c1,a,b);//above method is good

half_adder ha2(.s(sum), .c(c2), .a(s1), .b(z));

or o1(.c(carry), .a(c1), .b(c2));

endmodule



module adder4bit(sout,cout,ain,bin,cin)

input [3:0] ain,bin;

input cin;

output [3:0] sout,cout;

full_adder FA1 (.sum(s[0]),.carry(c[0]),.x(a[0]),.y(b[0]),.z(cin));

full_adder FA2 (.sum(s[1]),.carry(c[1]),.x(a[1]),.y(b[1]),.z(c[0]));

full_adder FA3 (.sum(s[2]),.carry(c[2]),.x(a[2]),.y(b[2]),.z(c[1]));

full_adder FA4 (.sum(s[3]),.carry(c[3]),.x(a[3]),.y(b[3]),.z(c[2]));

endmodule

gate level modeling of half Adder:

module halfadder (s,c,a,b)

input a,b;

output s,c;

and a1(c,a,b);     //List of ports

xor x1(s,a,b);      //primitive instatiation; x1 being instatiation label

endmodule;

Gate Level Modelling of Full Adder:

module full_adder(output s, cout, input a,b,cin)

wire s1,cout1,cout2,cout3;

xor x1(s1,a,b);

xor x2(s1,cin);

and a1(cout1,a,b);

and a2(cout2,b,c);

and a3(cout3,c,a);

or o1(cout,cout1,cout2,cout3);

endmodule

Data Flow Modeling:

 For small boolean equations; combinational circuits.

module multiplier(port_list)

                input IN1, IN2;

                output output1;                                                                             

assign output1=IN1*IN2

endmodule

Half Adder:

module half_adder(s,c,a,b)

input a,b;

output s,c;

assign s=a^b;                     //^ àxor

assign c = a&b;                  //& à and

endmodule

Truth Table For Half Adder:

a              b             s              c

------------------------------------

0              0              0              0

0              1              1              0

1              0              1              0

1              1              0              1

‘assign’ à called as continous assign statement; assignment can only to ‘wire’ not for ‘reg’ data type.

The above type of modeling is called “data flow” modelling.

Full –Adder –Data Flow Modelling:

‘assign’ à called as continous assign statement; assignment can only to ‘wire’ not for ‘reg’ data type.

The above type of modeling is called “data flow” modelling.

Full –Adder –Data Flow Modelling:

module full_adder(output s, cout, input a, b, cin);

wire s1;

assign s1=a^b;

assign s=s1^cin;

// can also be coded as s=((a^b)^c));

assign cout=(a&b) | (b&c) | (c&a) ;

endmodule

Truth Table For Full Adder:

cin          b             a              s              c

------------------------------------------------

0              0              0              0              0

0              0              1              1              0

0              1              0              1              0

0              1              1              0              1

1              0              0              1              0

1              0              1              0              1

1              1              0              0              1

1              1              1              0              1

Multiplexer:

 

module mux(z,sel,a,b)

input a,b,sel;

output z;

wire selbar=~sel;

assign z=(a & selbar) | (b & sel);

endmodule

2-Bit Comparator:

module (output g,l,e, input a1,a0,b1,b0)

assign g=({a1,a0}>{b1,b0});

assign l=({a1,a0}<{b1,b0});

assign e=({a1,a0}=={b1,b0});

endmodule