Synplify综合问题

Amay

我用Synplify 7.0综合一VHDL时，当用Lattice片子时，综合一切正常，为什么在选用Altera的时候会出现：
  Net cao1 appears to be a clock source which was not identified .assuming default from frequency. 的警告
cao1是供下级使用的分频信号。