异步复位会影响寄存器的recovery时间，引起设计的稳定性问题。尤其对于状态机的无意识的复位导致进入不确定的状态。

       下面介绍一种更为可靠的异步复位、同步释放的双缓冲电路。该电路由两个同一时钟沿触发的层叠寄存器组成，该时钟必须和目标寄存器是一个时钟域。

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//两级层叠复位产生，低电平复位

reg rst_nr1,rst_nr2;

always @ (posedge clk or negedge rst_n)

       if(!rst_n) rst_nr1 <= 1'b0;

       else rst_nr1 <= 1'b1;

always @ (posedge clk or negedge rst_n)

       if(!rst_n) rst_nr2 <= 1’b0;

       else rst_nr2 <= rst_nr1;

//---------------------------------------   

//异步复位，同步释放

always @ (posedge clk or negedge rst_nr2)

       if(!rst_nr2) b <= 1'b0;

       else b <= a1;

always @ (posedge clk or negedge rst_nr2)

       if(!rst_nr2) c <= 1'b0;

       else c <= a2;

//---------------------------------------

//两级层叠复位产生，高电平复位

reg rst_r1,rst_r2;

always @ (posedge clk or posedge rst)

       if(rst) rst_r1 <= 1'b1;

       else rst_r1 <= 1'b0;

always @ (posedge clk or posedge rst)

       if(rst) rst_r2 <= 1’b0;

       else rst_r2 <= rst_r1;

//---------------------------------------   

//异步复位，同步释放

always @ (posedge clk or posedge rst_r2)

       if(rst_r2) b <= 1'b0;

       else b <= a1;

always @ (posedge clk or posedge rst_r2)

       if(rst_r2) c <= 1'b0;

       else c <= a2;