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您好，
EN_PLL2_XTAL bit 不是用来断开PLL2 参考输入的，在数据手册73页 8.6.3.4.9 EN_PLL2_XTAL有说明：If an external crystal is being used to implement a discrete VCXO, the internal feedback amplifier must be enabled with this bit in order to complete the oscillator circuit.
如果使用外部的晶振实现离散VCXO，则必须使用该位使能内部反馈放大器，以完成振荡器电路。
