题目：
Exams/2013 q2bfsm
Consider a finite state machine that is used to control some type of motor. The FSM has inputs x and y, which come from the motor, and produces outputs f and g, which control the motor. There is also a clock input called clk and a reset input called resetn.

The FSM has to work as follows. As long as the reset input is asserted, the FSM stays in a beginning state, called state A. When the reset signal is de-asserted, then after the next clock edge the FSM has to set the output f to 1 for one clock cycle. Then, the FSM has to monitor the x input. When x has produced the values 1, 0, 1 in three successive clock cycles, then g should be set to 1 on the following clock cycle. While maintaining g = 1 the FSM has to monitor the y input. If y has the value 1 within at most two clock cycles, then the FSM should maintain g = 1 permanently (that is, until reset). But if y does not become 1 within two clock cycles, then the FSM should set g = 0 permanently (until reset).

(The original exam question asked for a state diagram only. But here, implement the FSM.)

解题：

module top_module (input clk,input resetn,    // active-low synchronous resetinput x,input y,output f,output g
); parameter s0=0,s1=1,s2=2,s3=3,s4=4,s5=5,s6=6,s7=7,s8=8;reg [3:0]state,next_state;always@(posedge clk)beginif(!resetn)state=s0;elsestate=next_state;endalways@(*)begincase(state)s0:next_state=s8;s8:next_state=s1;s1:next_state=x?s2:s1;s2:next_state=x?s2:s3;s3:next_state=x?s4:s1;s4:next_state=y?s6:s5;s5:next_state=y?s6:s7;s6:next_state=s6;s7:next_state=s7;endcaseendassign f=(state==s8);assign g=(state==s4)|(state==s5)|(state==s6);endmodule

结果正确：

注意点：
专门用一个状态，来表示f置为1 只有一个时钟周期。