一个关于状态机的问题

 

下面是状态机的一种写法：

这个状态机在满足 h_nx_state == S_H_SYNC 和h_pre_state == S_H_FRONT_PORCH 的状态下进行状态转换。

但是实际上实现出来的效果如下面的图一样，在复位过后，状态机所有的状态都可能同时为高，状态转换根本不正常。

 

always @( * )
    begin
            if( h_nx_state == S_H_SYNC &&  h_pre_state  == S_H_FRONT_PORCH  ) 
            begin
            case( v_pre_state )
            S_V_SYNC :
            begin 
                    if( v_cnt == V_SYNC -1 )
                            v_nx_state <= S_V_BACK_PORCH;
                    else
                            v_nx_state <= S_V_SYNC;
            end
            S_V_BACK_PORCH :
            begin
                    if( v_cnt == V_BACK_PORCH -1 )        
                            v_nx_state <= S_V_ACTIVE;
                    else
                            v_nx_state <= S_V_BACK_PORCH;      
            end
            S_V_ACTIVE    :
            begin
                   if( v_cnt == row -1 )     
                        v_nx_state <= S_V_FRONT_PORCH;
                    else
                            v_nx_state <= S_V_ACTIVE;        
                    
            end
            S_V_FRONT_PORCH :    
            begin
                if( v_cnt == V_FRONT_PORCH -1'b1 )                
                        v_nx_state <= S_V_SYNC;
                    else
                        v_nx_state <= S_V_FRONT_PORCH; 
            end        
            default:;
            endcase    
            end
    end

 

 

 

 

经过思考之后，笔者把程序修改成了下面这个样子。也就是状态机在任何时候都在判断状态，这样之后，状态就很确定。

always @( * )
    begin
            case( v_pre_state )
            S_V_SYNC :
            begin 
                    if( h_nx_state == S_H_SYNC &&  h_pre_state  == S_H_FRONT_PORCH  ) begin
                            if( v_cnt == V_SYNC -1 )
                                    v_nx_state = S_V_BACK_PORCH;
                            else
                                v_nx_state = S_V_SYNC;
                    end else begin
                            v_nx_state = S_V_SYNC;
                    end
            end
            S_V_BACK_PORCH :
            begin
                    if( h_nx_state == S_H_SYNC &&  h_pre_state  == S_H_FRONT_PORCH  ) begin
                            if( v_cnt == V_BACK_PORCH -1 )        
                                    v_nx_state = S_V_ACTIVE;
                            else
                                    v_nx_state = S_V_BACK_PORCH;    
                    end else begin
                            v_nx_state = S_V_BACK_PORCH;  
                    end  
            end
            S_V_ACTIVE    :
            begin
                    if( h_nx_state == S_H_SYNC &&  h_pre_state  == S_H_FRONT_PORCH  ) begin
                           if( v_cnt == row -1 )     
                                v_nx_state = S_V_FRONT_PORCH;
                            else
                                    v_nx_state = S_V_ACTIVE; 
                end else begin
                        v_nx_state = S_V_ACTIVE; 
                end       
                    
            end
            S_V_FRONT_PORCH :    
            begin
                    if( h_nx_state == S_H_SYNC &&  h_pre_state  == S_H_FRONT_PORCH  ) begin
                        if( v_cnt == V_FRONT_PORCH -1'b1 )                
                                v_nx_state = S_V_SYNC;
                            else
                                    v_nx_state = S_V_FRONT_PORCH; 
                    end else begin
                            v_nx_state = S_V_FRONT_PORCH;
                    end
            end        
            default:;
            endcase    
    end