Digital System Design

Digital Circuit VerificationHardware Descriptive Language

Verilog

Overview

More Structural Verilog Examples Dataflow Modeling Dataflow Verilog Examples Writing Test Bench

4-1 Multiplexer - Structural Verilog Description module multiplexer_4_to_1_st_v(S, I, Y); input [1:0] S; // S is a vector with components S[1] and S[0] input [3:0] I; // I is a 4-bit input; vectors: I[0]…I[3] output Y; wire [1:0] not_S; // connecting NOT-AND wire [0:3] D, N; // D connecting AND-AND not // N AND outputs

gn0(not_S[0], S[0]), // gn0(output, input) gn1(not_S[1], S[1]);

and g0(D[0], not_S[1], not_S[0]), g1(D[1], not_S[1], S[0]), g2(D[2], S[1], not_S[0]), g3(D[3], S[1], S[0]), g4(N[0], D[0], I[0]), g5(N[1], D[1], I[1]), g6(N[2], D[2], I[2]), g7(N[3], D[3], I[3]); or go(Y, N[0], N[1], N[2], N[3]);endmodule

D0g0

not_S[0]S[0]

g4N[0]

2-to-4 Line Decoder Structural Verilog

// 2-to-4 Line Decoder: Structural Verilog Description. // (See Figure 4-10 for logic diagram) module decoder_2_to_4_st_v(EN, A0, A1, D0, D1, D2, D3); input EN, A0, A1; output D0, D1, D2, D3; wire A0_n, A1_n, N0, N1, N2, N3; not go(A0_n, A0), g1(A1_n, A1); and g3(N0, A0_n, A1_n), g4(N1, A0, A1_n), g5(N2, A0_n, A1), g6(N3, A0, A1), g7(D0, N0, EN), g8(D1, N1, EN), g9(D2, N2, EN), g10(D3, N3, EN); endmodule

A1_n

N0

N1

g3

A0_n

g6 N3

Dataflow Verilog Modeling

A dataflow description is based on function rather thanstructure. A dataflow uses a number of operators that act on operandsto produce the desired function Boolean equations are usedin place of logic schematics.

4-1 Multiplexer - Dataflow Verilog

// 4-to-1 Line Multiplexer: Dataflow Verilog Description (Boolean)// (See Figure 4-14 for logic diagram)module multiplexer_4_to_1_df_v(S, I, Y); input [1:0] S; input [3:0] I; output Y;

assign Y = (~ S[1] & ~ S[0] & I[0])| (~ S[1] & S[0] & I[1]) | (S[1] & ~ S[0] & I[2]) | (S[1] & S[0] & I[3]);endmodule

2-to-4 Line Decoder Dataflow Verilog

// 2-to-4 Line Decoder with Enable: Dataflow Verilog Desc.// (See Figure 4-10 for logic diagram)module decoder_2_to_4_df_v(EN, A0, A1, D0, D1, D2, D3); input EN, A0, A1; output D0, D1, D2, D3; assign D0 =(EN & ~A1 & ~A0); assign D1 =(EN & ~A1 & A0); assign D2 =(EN & A1 & ~A0); assign D3 =(EN & A1 & A0); endmodule

Writing a Test Bench

A test bench in an HDL program for applying stimulus toan HDL design in order to test it and observe the responseduring simulation.

initial begin A = 0; B = 0; // @ t = 0 A and B are set to 0 #10 A = 1; // 10 time units later, A is changed to 1 #20 A = 0; B = 1; // @ t = 30 A is changed to 0 and B to 1 end

Test Bench Example

//HDL Example//------------------------------------------ //Gate-level description of a combinational circuit module analysis (A,B,C,F1,F2); input A,B,C; output F1,F2; wire T1,T2,T3,F2not,E1,E2,E3; or g1 (T1,A,B,C); and g2 (T2,A,B,C); and g3 (E1,A,B); and g4 (E2,A,C); and g5 (E3,B,C); or g6 (F2,E1,E2,E3); not g7 (F2not,F2); and g8 (T3,T1,F2not); or g9 (F1,T2,T3);endmodule

//Stimulus to analyze the circuitmodule test_circuit; reg [2:0]D; wire F1,F2; analysis circuit(D[2],D[1],D[0],F1,F2); initial begin D = 3'b000; repeat (7) #10 D = D + 1'b1; end initial $monitor ("ABC = %b F1 = %b F2 =%b ",D, F1, F2);endmodule

Test Bench Example (continued)