小弟接触FPGA不久，应老师要求设计一个关于IIC的接口，用状态机实现，结果出现如下错误：


ERROR:Xst:528 - Multi-source in Unit <iic_level> on signal <nstate<12>>
Sources are:
   Output signal of LD instance <nstate/12/0>
   Signal <nstate<12>> in Unit <iic_level> is assigned to GND

ERROR:Xst:528 - Multi-source in Unit <iic_level> on signal <nstate<11>>
Sources are:
   Output signal of LD instance <nstate/11/0>
   Signal <nstate<11>> in Unit <iic_level> is assigned to GND

ERROR:Xst:528 - Multi-source in Unit <iic_level> on signal <nstate<10>>
Sources are:
   Output signal of LD instance <nstate/10/0>
   Signal <nstate<10>> in Unit <iic_level> is assigned to GND

在网上搜了不少，也没有找到原因，源程序中也只在一个always中对  nstate  赋值  
求大神帮忙看看  在此谢过