FSM的優化寫法 
    采用所謂"三段式"的狀態機寫法清晰而且可以有效的避免毛刺,列一例來體會.
  
module FSM ( nrst,clk,

i1,i2,

o1,o2,

err

);

input nrst,clk;

input i1,i2;

output o1,o2,err;

reg o1,o2,err;

reg [2:0] NS,CS;

parameter [2:0] //one hot with zero idle"獨熱碼"

IDLE = 3'b000,

S1 = 3'b001,

S2 = 3'b010,

ERROR = 3'b100;

always @ (posedge clk or negedge nrst)

if (!nrst)

CS <= IDLE;

else

CS <=NS;

always @ (nrst or CS or i1 or i2)

begin

NS = 3'bx;

case (CS)

IDLE: begin

if (~i1) NS = IDLE;

if (i1 && i2) NS = S1;

if (i1 && ~i2) NS = ERROR;

end

S1: begin

if (~i2) NS = S1;

if (i2 && i1) NS = S2;

if (i2 && (~i1)) NS = ERROR;

end

S2: begin

if (i2) NS = S2;

if (~i2 && i1) NS = IDLE;

if (~i2 && (~i1)) NS = ERROR;

end

ERROR: begin

if (i1) NS = ERROR;

if (~i1) NS = IDLE;

end

endcase

end

always @ (posedge clk or negedge nrst)

if (!nrst)

{o1,o2,err} <= 3'b000;

else

begin

{o1,o2,err} <= 3'b000;

case (NS)

IDLE: {o1,o2,err}<=3'b000;

S1: {o1,o2,err}<=3'b100;

S2: {o1,o2,err}<=3'b010;

ERROR: {o1,o2,err}<=3'b111;

endcase

end

endmodule//采用時序輸出的方式可以減少毛刺