对编译特性(* ASYNC_REG = “TRUE” *)的理解

（*ASYNC_REG = "TRUE"*）命令用于声明寄存器能够接收相对于时钟源的异步数据，或者说寄存器是一个同步链路上正在同步的寄存器。这条命令可以放在任何寄存器上，除了设置它的值为TRUE外还可以设置为FALSE.

例子：（*ASYNC_REG = "TRUE"*） reg [0:0]  async_rst = 0;

这样可以强制指定async_rst为异步时钟输入，防止综合器对打拍寄存器进行优化，类似DONT_TOUCH。 详情见UG901-Vivado Design Suite User Guide.pdf

module reset_sync
  (input clk,
   input reset_in,
   output  reset_out);

   (* ASYNC_REG = "TRUE" *) reg reset_int = 'b1;
   (* ASYNC_REG = "TRUE" *) reg reset_out_tmp = 'b1;

   always @(posedge clk or posedge reset_in)
     if(reset_in)
       {reset_out_tmp,reset_int} <= 'b11;
     else
       {reset_out_tmp,reset_int} <= {reset_int,'b0};

   assign reset_out = reset_out_tmp;

endmodule // reset_sync

module cdc#(
    parameter   DW  = ,
    parameter   SL  =
    )
    (
    input               sys_clk ,
    input               sys_rst ,
    input   [DW-:]   A_Din   ,
    output  [DW-:]   S_Dout
    );

(*ASYNC_REG="true"*)reg     [DW-:]   ccsrl_A_Din[:SL-];
                    reg     [DW-:]   ccr_A_Din   ={DW{'b0}} ;

always@(posedge sys_clk)
begin
    if(sys_rst)
    begin
        ccsrl_A_Din[SL-]   <=  'b0    ;
    end
    else
    begin
        ccsrl_A_Din[SL-]   <=  A_Din   ;
    end
end

genvar i;
generate
   for(i=;i<SL-;i=i+)
     begin
        always@(posedge sys_clk)
           begin
               if(sys_rst)
                  begin
                      ccsrl_A_Din[i]      <=  'b0    ;
                  end
               else
                  begin
                      ccsrl_A_Din[i]      <=  ccsrl_A_Din[i+]   ;
                  end
           end
     end
endgenerate

always@(posedge sys_clk)
begin
    if(sys_rst)
       begin
           ccr_A_Din   <=  {DW{'b0}} ;
       end
    else
       begin
           ccr_A_Din   <=  ccsrl_A_Din[]  ;
       end
end

assign          S_Dout  =ccr_A_Din;

endmodule

相关参考：

DONT_TOUCH Verilog Examples

Verilog Wire Example
(* dont_touch = “yes” *) wire sig1;
assign sig1 = in1 & in2;
assign out1 = sig1 & in2;

Verilog Module Example
(* DONT_TOUCH = “yes” *)
module example_dt_ver
(clk,
In1,
In2,
out1);

Verilog Instance Example
(* DONT_TOUCH = “yes” *) example_dt_ver U0
(.clk(clk),
.in1(a),
.in2(b),
.out1(c));