Keyestudio 4DOF Manual page 100

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Microcontroller
Each Arduino board has its own microcontroller. You can regard it as the brain of your board.
The main IC (integrated circuit) on the Arduino is slightly different from the panel pair. Microcontrollers
are usually from ATMEL. Before you load a new program from the Arduino IDE, you must know what IC
is on your board. This information can be checked at the top of IC.
ICSP (In-Circuit Serial Programming) Header
In most case, ICSP is the AVR，an Arduino micro-header consisting of MOSI, MISO, SCK, RESET, VCC,
and GND. It is often called the SPI (serial peripheral interface) and can be considered an "extension" of
the output. In fact, put the output devices subordinate to the SPI bus host.
Power LED Indicator
Powering the Arduino, LED on means that your circuit board is correctly powered on. If LED is off,
connection is wrong.