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ECE 444

ECE 444 Session 5 Dr. John G. Weber KL-241E 229-3182 John.Weber@notes.udayton.edu jweber-1@woh.rr.com http://academic.udayton.edu/JohnWeber Combinatorial Logic Blocks Multiplexers Used to steer data

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ECE 444

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  1. ECE 444 Session 5 Dr. John G. Weber KL-241E 229-3182 John.Weber@notes.udayton.edu jweber-1@woh.rr.com http://academic.udayton.edu/JohnWeber

  2. Combinatorial Logic Blocks • Multiplexers • Used to steer data • Suppose we want the output of a circuit, D, to be equal to the input A if a select signal is false and equal to the input B if the select signal is true • This circuit is a two-input, one-bit multiplexer which steers the data to the D input of a flip-flop

  3. Verilog for 2-input mux //mux_2in_1b.v //a two input, one-bit mux module mux_2in_1b(d, a, b, sel); input a, b, sel; output d; assign d = a && !sel || b && sel; endmodule

  4. Multiplexers • Extend this to a two-input, n-bit multiplexer

  5. Verilog for n-bit mux //mux_2in_nb.v //parameterized, n-bit mux module mux_2in_nb(D, A, B, sel); parameter width =4; input [width-1:0] A, B; input sel; output [width-1:0] D; reg [width-1:0] D; always @(sel or A or B) if (sel) D = B; else D = A; endmodule

  6. Behavioral Modeling Styles • Continuous Assignment Models • Used in the full-adder • Data Flow Models • Concurrent continuous assignment or asynchronous cyclic behavior • Algorithm Based Models • Abstract Model of behavior

  7. Continuous Assignment Model • Consider a Simple Comparator • compares two vectors and returns A.lt.B, A.gt. B, or A = B • Verilog • //compare_4.v • //4-bit comparator • module compare_4(A_lt_B, A_gt_B, A_eq_B, A, B); • input [3:0] A, B; • output A_lt_B, A_gt_B, A_eq_B; • assign A_lt_B = (A < B); • assign A_gt_B = (A > B); • assign A_eq_B = (A == B); • endmodule

  8. Parameterized Compare Module • Since the logic does not depend on the size of the vectors being compared, we can easily convert this to a parameterized module. • //compare_param_16.v • //16-bit parameterized comparator • module compare_param_16 (A_lt_B, A_gt_B, A_eq_B, A, B); • parameter width = 16 • input [width-1:0] A, B; • output A_lt_B, A_gt_B, A_eq_B; • assign A_lt_B = (A < B); • assign A_gt_B = (A > B); • assign A_eq_B = (A == B); • endmodule

  9. Data Flow Model • Model with cyclic behavior • Use always construct • //compare_4.v • //4-bit comparator • module compare_4(A_lt_B, A_gt_B, A_eq_B, A, B); • input [3:0] A, B; • output A_lt_B, A_gt_B, A_eq_B; • always@(A or B) • begin • A_lt_B = (A < B); • A_gt_B = (A > B); • A_eq_B = (A == B); • end • endmodule Note: Procedural assignment operator, =, is called a blocking assignment and statements are executed in order.

  10. Data Flow Model (Cont.) • Consider a 4-bit shift register (see fig 5-7) • Flip-flops labeled (left to right) D, C, B, A • Verilog Code • //shiftright_4.v • module shiftright_4(E, D, C, B, A, clk, rst); • output A; • input E, clk, rst; • reg A, B, C, D; • always @(posedge clk or posedge rst) • begin • if (rst) begin A = 0; B = 0; C = 0; D = 0; end • else begin • A = B; • B = C; • C = D; • D = E; • end • end • endmodule Blocking assignment

  11. Data Flow Model (Cont.) • To see effect of blocking assignment, reverse order of register assignment statements • //shiftright_4.v • module shiftright_4(E, D, C, B, A, clk, rst); • output A; • input E, clk, rst; • reg A, B, C, D; • always @(posedge clk or posedge rst) • begin • if (rst) begin A = 0; B = 0; C = 0; D = 0; end • else begin • D = E; //A = B; • C = D; //B = C; • B = C; //C = D; • A = B; //D = E; • end • end • endmodule Blocking assignment causes D to have contents of E, then C to receive contents of D, then B to receive contents of C and finally A to receive contents of B on the same clock edge. Degenerates into a single bit register.

  12. Non-Blocking Assignment • Non-blocking assignment all assignment statements execute concurrently • Use non-blocking assignment operator <= • //shiftright_4.v • module shiftright_4(E, D, C, B, A, clk, rst); • output A; • input E, clk, rst; • reg A, B, C, D; • always @(posedge clk or posedge rst) • begin • if (rst) begin A = 0; B = 0; C = 0; D = 0; end • else begin • D <= E; //A <= B; • C <= D; //B <= C; • B <= C; //C <= D; • A <= B; //D <= E; • end • end • endmodule Either order is fine and results in the desired 4-bit shift register

  13. Algorithm Based-Model • Describes function of module using an algorithm • Looks more like software but beware • //compare_algo_4.v • //4-bit comparator using an algorithm • module compare_algo_4(A_lt_B, A_gt_B, A_eq_B, A, B); • input [3:0] A, B; • output A_lt_B, A_gt_B, A_eq_B; • reg A_lt_B, A_gt_B, A_eq_B; • always@(A or B) • begin • A_lt_B = 0 //initialize outputs • A_gt_B = 0 • A_eq_B = 0 • if (A ==B) A_eq_B = 1; • else if (A > B) A_gt_B = 1; • else A_lt_B = 1; • end • endmodule

  14. Sequential Circuits • Use language features discussed last time to design sequential circuits • Discuss each feature as we use it in a series of examples • Sequential Circuit Block Diagram • Memory used to track the state of the circuit • Combinatorial logic used to determine next state as a function of inputs and current state • Combinatorial logic also used to derive output functions

  15. Classical Tools for Sequential Circuit Design • State Diagram • Provides a diagram of the available states, the conditions for switching states and the output at each state • State Table • Provides a tabular (truth table like) view of the states and the logic required • Algorithmic-State Machine (ASM) Charts (Ch 5 in text) • Flow chart type of presentation for finite state machines • Register Transfer Language • Provides a sequential language description of the inputs, outputs and next state requirement

  16. Example Sequential Circuit Requirement • Problem • Design a circuit that monitors a serial input and detects a string of three or more contiguous “ones”. When three or more “ones are detected in a row, generate a “one” on the output line.

  17. State Diagram

  18. Present State Input Next State Output A B x A B y 0 0 0 0 0 0 0 0 1 0 1 0 0 1 0 0 0 0 0 1 1 1 0 0 1 0 0 0 0 0 1 0 1 1 1 0 1 1 0 0 0 1 1 1 1 1 1 1 State Table

  19. ASM Chart

  20. State Output Next State 0 y  0 (x,!x)/(1,0) 1 y  0 (x,!x)/(2,0) 2 y  0 (x,!x)/(3,0) 3 y  1 (x,!x)/(3,0) Register Transfer Language

  21. Verilog HDL //seq_detect.v //sample sequential machine to detect a sequence of three ones in a serial //bit stream. module seq_detect (x, y, clk, rst, state); input x, clk, rst; output y; output [1:0] state; reg y; reg [1:0] state; parameter S0 = 2'b00, S1 = 2'b01, S2 =2'b10, S3 = 2'b11;

  22. Verilog HDL(cont) always @ (posedge clk or negedge rst) if (~rst) begin state = S0; y=0; end else case (state) S0: begin y = 0; if(x) state = S1; else state = S0; end S1: if(x) state = S2; else state = S0; S2: if(x) state = S3; else state = S0; S3: begin y = 1; if(x) state = S3; else state = S0; endendcase endmodule

  23. Simulation

  24. Finite State Machine Types • Two types—Mealy and Moore • Which was the Example?

  25. Assignment 3 – Due Wednesday 9/15/2004 • Goal • Practice in Verilog continuous and procedural assignments • Practice in Verilog module design. • Problems • Include a design specification and a structure description for each problem • Develop a verilog module and simulation for each problem • Design a 4-bit encoder • Design a 4-input priority encoder • Design a 8-input majority function (output is one if five or more inputs are asserted • Problem 1—page 100 • Problem 2 – page 101

  26. Project 1– Due 2 October 2004 • Goal • Develop and demonstrate good design discipline and technique • Practice design of combinatorial logic circuits using Verilog HDL • Project • Develop 8-bit, two’s complement arithmetic ALU with data inputs A[7:0] and B[7:0] which performs the following operations asynchronously: • A + B • A – B • B – A • A OR B • A AND B • A XOR B • NOT A • NOT B • - A • - B For each function above, provide the following outputs • A > B • A < B • A = B • Timing Requirements: Maximum propagation delay for any function shall be less than 25 ns. • Provide a specification for each module, a structure write-up, a test plan, and a report describing your approach, results and conclusions

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