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Verilog

Verilog. One-bit Full Adder. module full_adder (A,B,Cin,Sum, Cout); input A,B,Cin; output Sum, Cout; assign Sum = (A & B & Cin) | (~A & ~B & Cin) | (~A & B & ~Cin) | (A & ~B & ~Cin); assign Cout = (A & Cin) | (A & B) | (B & Cin); endmodule. Four-bit Adder.

JasminFlorian
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Verilog

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  1. Verilog

  2. One-bit Full Adder module full_adder (A,B,Cin,Sum, Cout); input A,B,Cin; output Sum, Cout; assign Sum = (A & B & Cin) | (~A & ~B & Cin) | (~A & B & ~Cin) | (A & ~B & ~Cin); assign Cout = (A & Cin) | (A & B) | (B & Cin); endmodule

  3. Four-bit Adder module four_bit_adder (A,B,Cin,Sum, Cout); input [3:0] A; input [3:0] B; input Cin; output [3:0] Sum; output Cout; wire C0, C1, C2; full_adder FA1(A[0], B[0], Cin, Sum[0], C0); full_adder FA2(A[1], B[1], C0, Sum[1], C1); full_adder FA3(A[2], B[2], C1, Sum[2], C2); full_adder FA4(A[3], B[3], C2, Sum[3], Cout); endmodule

  4. MIPS ALU module MIPSALU (ALUctl, A, B, ALUOut, Zero); input [3:0] ALUctl; input [31:0] A,B; output reg [31:0] ALUOut; output Zero; assign Zero = (ALUOut==0); //Zero is true if ALUOut is 0; goes anywhere always @(ALUctl, A, B) //reevaluate if these change case (ALUctl) 0: ALUOut <= A & B; 1: ALUOut <= A | B; 2: ALUOut <= A + B; 6: ALUOut <= A - B; 7: ALUOut <= A < B ? 1:0; 12: ALUOut <= ~(A | B); // result is nor default: ALUOut <= 0; //default to 0, should not happen; endcase endmodule

  5. Register • Register is built with gates, but has memory. • The only type of flip-flop required in this class – the D flip-flop • Has at least two inputs (both 1-bit): D and clk • Has at least one output (1-bit): Q • At the rising edge of clk (when clk is changing from 0 to 1), Q <= D. • Q does not change value at ANY OTHER TIME except at the rising edge of clk

  6. D flip-flop module Dff (D, clk, Q); input D, clk; output reg Q; always @(posedge clk) begin Q = D; end endmodule

  7. D flip-flop can hold value • Note that the output of the D flip-flop (Q) does not follow the change of the input (D). It holds the value until the next time it is allowed to change – the rising edge of the clock. • This is why you can write to a register and expect that the next time you want to use it the value is still there. Your MIPS code won’t work if the values in the registers can change at random time.

  8. Delay • Real circuits have delays caused by charging and discharging. • So, once the input to a gate changes, the output will change after a delay, usually in the order of nano seconds. An and gate: A B output

  9. Delay • A more realistic D flip-flop: module Dff1 (D, clk, Q, Qbar); input D, clk; output reg Q, Qbar; initial begin Q = 0; Qbar = 1; end always @(posedge clk) begin #1 Q = D; #1 Qbar = ~Q; end endmodule

  10. What happens if… What happens if I connect a Dff like this? wire Q2, Qbar2; Dff1 D2 (Qbar2, clk, Q2, Qbar2);

  11. The verilog code used in the class • http://www.cs.fsu.edu/~zzhang/CDA3100_Spring_2009_files/week11_1.v

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