The data sheet of Si4720 says integrated crystal oscillator can be activated at booting, and relevant sample circuit can be found in the data sheet.
As you see, one of the terminal of crystal as shown inset of figure is not written. One side is RCLK, which is clear, but the other is where!!!!!
The answer is,,,,
It's GPO3. In the sample circuit, GPO3 of Si4720 is connected to GND but cut it and connect one of the additional 32.768 kHz crystal circuit and activate it on booting, it starts oscillate. I don't know why but inside Si4720 data sheet, this fact is not described.. why?