HDLbits: Fsm ps2

本题目理解起来有点难，要观察题目中给的三个时序图，通过时序图可以发现，状态有四个：byte1、byte2、byte3，还有一个“？”状态。其中，byte1的下一个状态一定是byte2，byte2的下一个状态一定是byte3，但是byte3的下一个状态可能是“？”，需要通过in[3]来确定。于是先写出下面的代码（运行错误）：

module top_module(input clk,input [7:0] in,input reset,    // Synchronous resetoutput done); //parameter byte1=4'b0001, byte2=4'b0010, byte3=4'b0100, byte_fault=4'b1000;reg [3:0] state,next_state;// State transition logic (combinational)always@(*)begincase(state)byte1: next_state = byte2;byte2: next_state = byte3;byte3: next_state = in[3]?byte1:byte_fault;byte_fault: next_state = in[3]?byte1:byte_fault; default: next_state = byte1;endcaseend// State flip-flops (sequential)always@(posedge clk)beginif(reset)state <= byte1;elsestate <= next_state;endalways@(posedge clk)beginif(state == byte3)done <= 1;elsedone <= 0;end// Output logic//assign done = (state == byte3);endmodule

通过对比大神写的代码，发现两个问题：1、reset时，state不是byte1而是byte_fault，这一点题目没说明白。2、done判断条件为什么不能用我写的always块而是assign？

module top_module(input clk,input [7:0] in,input reset,    // Synchronous resetoutput done); //parameter byte1=4'b0001, byte2=4'b0010, byte3=4'b0100, byte_fault=4'b1000;reg [3:0] state,next_state;// State transition logic (combinational)always@(*)begincase(state)byte1: next_state = byte2;byte2: next_state = byte3;byte3: next_state = in[3]?byte1:byte_fault;byte_fault: next_state = in[3]?byte1:byte_fault; default: next_state = byte1;endcaseend// State flip-flops (sequential)always@(posedge clk)beginif(reset)state <= byte_fault;elsestate <= next_state;end// Output logicassign done = (state == byte3);endmodule