问题如下：

F2812可以采用有源晶振吗？如采用有源晶振，怎么旁路内部OSC？

解答如下：

不可以直接接一般市面上的有源晶振，因为大部分都是5v输出电平，而TI文档中提到Oscillator Input – input of the internal oscillator. This pin is also
used to feed an external clock. The 28x can be operated with
an external clock source, provided that the proper voltage
levels be driven on the X1/XCLKIN pin. It should be noted that
the X1/XCLKIN pin is referenced to the 1.8-V core digital
power supply (VDD), rather than the 3.3-V I/O supply (VDDIO).
A clamping diode may be used to clamp a buffered clock
signal to ensure that the logic-high level does not exceed VDD
(1.8 V or 1.9 V) or a 1.8-V oscillator may be used.

不能高于1.8V的电平！