我用了一个DLL的例化模块，如下：

dll_freq4x freq_4x (
    .CLKIN(CLKIN),
    .RESET(0),
    .CLK2X(CLK2X),
    .CLK4X(CLK4X),
    .LOCKED(LOCKED)
    );

将他直接插入我的顶层模块，主要是使用他的CLK4X时钟；单独运行时每个模块没错，但顶层模块综合时也每错，但翻译时错误如下：（add I/O buffer改了还是一样）
Checking timing specifications ...
Checking expanded design ...
ERROR:NgdBuild:455 - logical net 'N108' has multiple drivers. The possible
   drivers causing this are:
     pin G on block XST_GND with type GND,
     pin PAD on block N108 with type PAD
ERROR:NgdBuild:466 - input pad net 'N108' has illegal connection. Possible pins
   causing this are:
     pin G on block XST_GND with type GND,
     pin DI on block phase_contr1_Madd__n0010_inst_cy_10 with type MUXCY,
     pin CI on block phase_contr1_Madd__n0010_inst_cy_9 with type MUXCY,
     pin DI on block phase_contr1_Madd__n0010_inst_cy_12 with type MUXCY,
     pin DI on block phase_contr1_Madd__n0010_inst_cy_11 with type MUXCY,
     pin CI on block phase_contr1_Madd__n0006_inst_cy_0 with type MUXCY

NGDBUILD Design Results Summary:
  Number of errors:     2
  Number of warnings:   0

Total memory usage is 39768 kilobytes

One or more errors were found during NGDBUILD.  No NGD file will be written.

Writing NGDBUILD log file "sin_top.bld"...
ERROR: NGDBUILD failed
Process "Translate" did not complete.

请问是什么问题啊，该如何解决呢？谢谢