[求助] Isim状态机不跳转

只看该作者 · 2014-1-25 09:14

请教大家一个问题，
我写了一个状态机，代码如下：
      always @(posedge p0rxc or negedge rst_n)
begin
      if(!rst_n)
         current_state <= idle;
      else
         current_state <= next_state;
end

always @(current_state)
begin
      case(current_state)
         idle             : if(read_fifo_empty)
                                    next_state <= idle;
                                 else if(start_read_fifo)
                                    next_state <= first_read;
                                 else
                                    next_state <= idle;
         first_read          : if(read_fifo_cnt == 16'd7)
                                    next_state <= judge_data_type;
                                 else
                                    next_state <= first_read;

         judge_data_type       : if(video_data_type)
                                    next_state <= judge_video_range;
                                 else
                                    next_state <= judge_cardNo;
         judge_video_range : next_state <= local_forward;
         judge_cardNo       : next_state <= local_forward;
         local_forward       : if(forward_video||forward_notvideo||forward_frame)
                                    next_state <= forward;
                                 else
                                    next_state <= local;
         local             : if(read_fifo_empty)
                                    next_state <= idle;
                                 else
                                    next_state <= local;
         forward             : if(read_fifo_empty)
                                    next_state <= idle;
                                 else
                                    next_state <= forward;
         default             :       next_state <= idle;
      endcase
end
这个状态机下载到fpga是能正常工作的。
问题是在Isim仿真中，当read_fifo_empty=0而start_read_fifo=1时，curent_state为什么不跳转到first_read。如图1.

图1 状态不转变
然后我将状态机修改如下，再仿真可以看到状态机跳转了。如图2.
always @(posedge p0rxc or negedge rst_n)
begin
      case(current_state)
         idle             : if(read_fifo_empty)
                                    current_state <= idle;
                                 else if(start_read_fifo)
                                    current_state <= first_read;
                                 else
                                    current_state <= idle;
         first_read          : if(read_fifo_cnt == 16'd7)
                                    current_state <= judge_data_type;
                                 else
                                    current_state <= first_read;

         judge_data_type       : if(video_data_type)
                                    current_state <= judge_video_range;
                                 else
                                    current_state <= judge_cardNo;
         judge_video_range : current_state <= local_forward;
         judge_cardNo       : current_state <= local_forward;
         local_forward       : if(forward_video||forward_notvideo||forward_frame)
                                    current_state <= forward;
                                 else
                                    current_state <= local;
         local             : if(read_fifo_empty)
                                    current_state <= idle;
                                 else
                                    current_state <= local;
         forward             : if(read_fifo_empty)
                                    current_state <= idle;
                                 else
                                    current_state <= forward;
         default             :       current_state <= idle;
      endcase
end

图2 状态转变

请问为什么前面那个状态机仿真不正确呢？谢谢！