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根据题意设计了四个状态，写出代码如下：
module top_module(input clk,input in,input reset, // Synchronous resetoutput done
); parameter IDLE = 3'b000, START = 3'b001, DATA = 3'b010, STOP = 3'b100, bit_counter_end = 4'd7;reg [2:0] state,next_state;reg [3:0] bit_counter;always@(*)begincase(state)START: next_state = DATA;DATA: next_state = (bit_counter >= bit_counter_end)? (in?STOP:IDLE):DATA;STOP: next_state = in?IDLE:START;IDLE: next_state = in?IDLE:START;default: next_state = IDLE;endcaseendalways@(posedge clk)beginif(reset)state <= IDLE;elsestate <= next_state;endalways@(posedge clk)beginif(reset)bit_counter <= 4'd0; else if(state == DATA)bit_counter <= bit_counter + 4'd1; elsebit_counter <= 4'd0;endassign done = (state == STOP); endmodule
时序图如下，有误：
参考网上的答案，加入了一个ERROR状态表示例题时序图"？"的时候，下面代码没问题了
注意bit_counter计数的那块如果用state==DATA判断，那么上面长度判断就得用7，如果用next_state==DATA判断，上面长度判断就得用8
module top_module(input clk,input in,input reset, // Synchronous resetoutput done
); parameter IDLE = 4'b0000, START = 4'b0001, DATA = 4'b0010, STOP = 4'b0100, ERROR = 4'b1000, bit_counter_end = 4'd7;reg [3:0] state,next_state;reg [3:0] bit_counter;always@(*)begincase(state)START: next_state = DATA;DATA: next_state = (bit_counter >= bit_counter_end)? (in?STOP:ERROR):DATA;STOP: next_state = in?IDLE:START;ERROR: next_state = in?IDLE:ERROR;IDLE: next_state = in?IDLE:START;default: next_state = IDLE;endcaseendalways@(posedge clk)beginif(reset)state <= IDLE;elsestate <= next_state;endalways@(posedge clk)beginif(reset)bit_counter <= 0; else if(state == DATA) bit_counter <= bit_counter + 1;elsebit_counter <= 0;endassign done = (state == STOP); endmodule
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