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
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