Keyestudio 4DOF Manual page 98

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USB Connection
Arduino board can be powered via USB connector. Or you can program the board via the USB port.
DC Power Jack
Arduino board can be supplied with power from the DC power jack
Voltage Regulator
To control the voltage provided to the Arduino board, as well as to stabilize the DC voltage used by the
processor and other components.
Crystal Oscillator
How does Arduino calculate time? by using a crystal oscillator.
The number printed on the top of the Arduino crystal is 16.000H9H. It tells us that the frequency is
16,000,000 Hertz or 16MHz.