Finite State Machine (Mealy Machine)
Prerequisites: Study the functionality of Mealy Machine.
Learning Objective: To develop the source code for Melay machine by using VERILOG and obtain the simulation and synthesis.
Meay Machine
Software and Hardware: Xilinx ISE 9.2i and FPGA Spartan3E.
Theory: The output of a Moore finite state machine depends only on the machine's current state; transitions are not directly dependent upon input.
 The use of a Mealy FSM leads often to a reduction of the number of states.
Present State  Next State  Output  

X = 0  X = 1  X = 0  X = 1  
S0  S0  S1  0  0 
S1  S0  S3  1  0 
S2  S0  S2  1  0 
S3  S0  S2  1  0 
Block Diagram:
Verilog Code:
module FSM_Mealy( y, x, clock, reset); input x, clock, reset; output y; reg y; reg [1:0] Prstate, Next_state; parameter s0 = 2'b00, s1 = 2'b01, s2 = 2'b10, s3 = 2'b11; always @(posedge clock) begin if (reset) Prstate = s0; else Prstate = Next_state; end always @(Prstate or x) begin case (Prstate) s0: if (x) begin Next_state = s1; y = 1'b0; end else begin Next_state = s0; y = 1'b0; end s1: if (x) begin Next_state = s3; y = 1'b0; end else begin Next_state = s0; y = 1'b1; end s2: if (~x) begin Next_state = s0; y = 1'b1; end else begin Next_state = s2; y = 1'b0; end s3: if (x) begin Next_state = s2; y = 1'b0; end else begin Next_state = s0; y = 1'b1; end endcase end endmodule
TEST BENCH:
module FSM_Mealy_tb_v; reg x; reg clock; reg reset; wire y; FSM_Mealy uut( .y(y), .x(x), .clock(clock), .reset(reset) ); initial begin x = 0; clock = 0; reset = 1; #10 x = 0; reset = 0; #10 x = 1; #10 x = 0; #10 x = 1; #10 x = 1; #10 x = 0; #100; end always #5 clock = ~clock; endmodule
Simulation Results:
Result: Thus the program for Mealy FSM has been verified and also simulation and synthesis reports have been verified.
Learning outcome: After completion of this experiment, students are able to design Mealy machine using Verilog code.
Viva Questions:
 What is the function of Mealy machine?
 What is the difference between blocking and non blocking statement?
 What is the difference between ‘always’ and ’initial’ statements?
 What is the difference between ‘case x’ and ‘case z’?

UpdatedNov 21, 2021

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