CSE 201 Computer Logic Design * * * * * * * Verilog Modeling Basics CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Two Primary Ways of Modeling Logic Structural Behavioral DO NOT use “always” Use “always” block Outputs declared as “wire” Outputs declared as “reg” “Schematics using text” “Instantiate” other modules and interconnect them with wires and busses… Combinational Logic Sequential Logic … or, alternatively, use the “assign” statement along with the verilog operators for “and” (&), “or” (|), “not” (~) and xor (^). always @ (*) always @ (posedge clk, posedge reset) case ( {all inputs} ) if ( reset == 1’b1 ) //check for reset first // assign output(s) for reset condition // each row assigns output(s) // based on input combinations else //got a clock // assign output(s) based on decisions For example, assign F1 = (~W & X & ~Y) | (~X & ~Z); CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Fundamentals Verilog Module Definitions (structural) Module name (must begin with a letter or underscore) Port list (input and output names) module addsub4 ( M, A, B, Cin, Y, CB ); input [3:0] A, B; input M, Cin; output wire [3:0] Y; output wire CB; input/output declarations size specifier (default size is 1-bit scalar) type specifier (for outputs only, default type is wire) // structural logic of 4-bit add/sub // using 4 instances of 1-bit addsub addsub a0 ( M, A[0], B[0], Cin, Y[0], cb0), a1 ( M, A[1], B[1], cb0, Y[1], cb1), a2 ( M, A[2], B[2], cb1, Y[2], cb2), a3 ( M, A[3], B[3], cb2, Y[3], CB); single line comments endmodule CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Fundamentals Verilog Module Definitions (behavioral) Module name (must begin with a letter or underscore) Port list (input and output names) module addsub ( M, A, B, Cin, Y, CB ); input [3:0] A, B; input M, Cin; output reg [3:0] Y; output reg CB; input/output declarations size specifier (default size is 1-bit scalar) type specifier (for outputs only) event operator sensitivity list always @ ( M, A, B, Cin ) begin // behavioral logic of add/sub if ( M == 1’b0 ) {CB,Y} = A + B + Cin; else {CB,Y} = A - B - Cin; single line comment Note: Verilog is an “event driven” language, not a “sequential” language. concatenation operator As such, the most powerful operator in the verilog language is the “event” operator (@). end // end of always endmodule In practical use, the second most powerful operator is the concatenation operator ( { , } ). CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Structural Verilog Module Example (2-to-4 decoder) En Y3 Y2 Y1 Y0 decode_2to4 One could create four 3-variable K-maps to find the S.O.P expressions for the four outputs, but, by observation, each output only has one minterm in the truth table where it is “1.” En I1 I0 Y3 Y2 Y1 Y0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 0 1 1 0 1 0 0 1 0 1 1 0 0 1 0 0 1 1 1 1 0 0 0 Thus, Y3 = En & I1 & I0 Y2 = En & I1 & I0’ Y1 = En & I1’ & I0 Y2 = En & I1’ & I0’ CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Structural Verilog Module Example (2-to-4 decoder) Thus, Y3 = En & I1 & I0 Y2 = En & I1 & I0’ Y1 = En & I1’ & I0 Y2 = En & I1’ & I0’ Structural using “gates” module decoder_2to4 (En, I1, I0, Y3, Y2, Y1, Y0);   input En, I1, I0; output wire Y3, Y2, Y1, Y0; wire i1_n, i0_n; // interconnection wires for not gate outputs // Structural model of decoder using 4 3-input “and” gates not n1 (i1_n, I1), n0 (i0_n, I0); and ag3 (Y3, En, I1, I0), ag2 (Y2, En, I1, i0_n), ag1 (Y1, En, i1_n, I0), ag0 (Y0, En, i1_n, i0_n); endmodule CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Structural Verilog Module Example (2-to-4 decoder) Thus, Y3 = En & I1 & I0 Y2 = En & I1 & I0’ Y1 = En & I1’ & I0 Y2 = En & I1’ & I0’ Structural using “assign” module decoder_2to4 (En, I1, I0, Y3, Y2, Y1, Y0);   input En, I1, I0; output wire Y3, Y2, Y1, Y0; // Structural model of decoder using 4 3-input “assign” statement assign Y3 = (En & I1 & I0), Y2 = (En & I1 & ~I0), Y1 = (En & ~I1 & I0), Y0 = (En & ~I1 & ~I0); endmodule CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Behavioral Verilog Module Example (2-to-4 decoder) En Y3 Y2 Y1 Y0 decode_2to4 Having the truth table for ANY logic function, we can create a “behavioral” module using the verilog “case statement.” In essence, the “case statement” has exactly the same input/output relationships seen in a table; yet the table data must be placed in the case “template.” // behavioral logic of 2-to-4 decoder case ( {En, I1, I0} ) 3’b000: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b001: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b010: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b011: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b100: {Y3,Y2,Y1,Y0} = 4’b0001; 3’b101: {Y3,Y2,Y1,Y0} = 4’b0010; 3’b110: {Y3,Y2,Y1,Y0} = 4’b0100; 3’b111: {Y3,Y2,Y1,Y0} = 4’b1000; endcase En I1 I0 Y3 Y2 Y1 Y0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 0 1 1 0 1 0 0 1 0 1 1 0 0 1 0 0 1 1 1 1 0 0 0 Inputs Outputs CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison

Behavioral Verilog Module Example (2-to-4 decoder) The “case statement” must be placed within the verilog “always” block, the only procedural block used in behavioral modeling. The complete module is created by “packaging” the procedural block within the applicable verilog module declarations ... module decoder_2to4 ( En, I1, I0, Y3, Y2, Y1, Y0); input En, I1, I0; output reg Y3, Y2, Y1, Y0; endmodule // Although verilog allows for “free // formatting,” you are encouraged to // develop good coding style. case ( {En, I1, I0} ) 3’b000: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b001: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b010: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b011: {Y3,Y2,Y1,Y0} = 4’b0000; 3’b100: {Y3,Y2,Y1,Y0} = 4’b0001; 3’b101: {Y3,Y2,Y1,Y0} = 4’b0010; 3’b110: {Y3,Y2,Y1,Y0} = 4’b0100; 3’b111: {Y3,Y2,Y1,Y0} = 4’b1000; endcase // behavioral logic of 2-to-4 decoder always @ (En, I1, I0) begin end // end of always block // “Programming styles commonly deal // with the visual appearance of source // code, with the goal of requiring less // human cognitive effort to extract // information about the program.”1 1 ref. http://en.wikipedia.org/wiki/Programming_style CSULB -- CECS 201– Verilog Basics © 2013 -- R.W. Allison