IBUFG和IBUFGDS的输入端仅仅与芯片的专用全局时钟输入管脚有物理连接，与普通IO和其它内部CLB等没有物理连接。

所以，IBUFG输入的不能直接接另外信号。

GTH transceiver primitives are called GTHE3_COMMON and GTHE3_CHANNEL in
UltraScale FPGAs, and GTHE4_COMMON and GTHE4_CHANNEL in UltraScale+ FPGAs.

Four GTHE3/4_CHANNEL primitives clustered together with one GTHE3/4_COMMON
primitive are called a Quad or Q.

The GTHE3/4_COMMON primitive contains two LC-tank PLLs (QPLL0 and QPLL1). The
GTHE3/4_COMMON only needs to be instantiated when an LC-tank PLL is used in the
application.

refclk进来后要用IBUFDS差转单，然后根据情况使用QPLL或者CPLL。