verilog 综合时候出现xst 899 高手帮帮忙啊！不胜感激！
module ss(input sys_clk,
input rst_in_n,
input rx_clk,
input rx_en,
input rx_data,
output reg rxd_en=1'b0,
output reg rxd_valid=1'b0,
output reg rxd=8'b0
);
reg state;
reg cnt=1'b0;
parameter idle=4'b0001;
parameter state1=4'b0010;
parameter state2=4'b0100;
parameter state3=4'b1000;
always @(posedge sys_clk or posedge rx_clk)
case(state)
idle:
if(!rst_in_n)
begin
rxd_en<=1'b0;
rxd_valid<=1'b0;
rxd<=8'b0;
end
else state<=state1;
state1:
if(rx_en)
begin
rxd_en<=1'b1;
rxd_valid<=1'b1;
rxd<=rx_data;
state<=state2;
end
else
state<=idle;
state2:
if(rx_en)
begin
rxd<=rx_data;
cnt<=~cnt;
state<=state3;
end
else
state<=idle;
state3:
if(!cnt)
begin
rxd_en<=1'b0;
rxd_valid<=1'b1;
state<=state1;
end
default:
state<=idle;
endcase
endmodule
综合错误提示：ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd_en> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd_valid> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <rxd> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <state> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
ERROR:Xst:899 - "ss.v" line 41: The logic for <cnt> does not match a known FF or Latch template. The description style you are using to describe a register or latch is not supported in the current software release.
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