提问TD6.2.1 DR1M90 PLL 提示警告：WARNING: PLL clkc is driving an IO without location

TD6.2.1 DR1M90 PLL 提示警告：WARNING: PLL clkc is driving an IO without location ，代码只添加了一个PLL，但是实现后会报出如下警告，

这是哪里得问题呢？

module system_top(

input wire clk_in, //50M外部晶振

);

wire clk_100M;

wire pll_lock;

MAINPLL mainpll_u(

.refclk (clk_in ),

.reset (1’b0 ),

.clk0_out(clk_100M ),

.lock (pll_lock )

);

endmodule