Arduino Uno is an ATmega328 microcontroller based circuit board. This IC (integrated circuit) has 14 digital inputs / outputs (6 outputs for PWM), 6 analog inputs, 16 MHz ceramic crystal resonator, USB connection, adapter socket, ICSP header pin, and reset button. This is what is needed to support a microcontroller that is easily connected with a USB power cable or AC adapter power supply cable to DC or battery.
Uno is different from all microcontroller boards in the beginning which did not use a special FTDI USB-to-serial driver chip. As a replacement for the application of USB-to-serial ATmega16U2 version R2 (previous version ATmega8U2). The Arduino Uno Rev.2 version features a resistor to 8U2 to the ground line which is more easily supplied to DFU mode.
The language “UNO” comes from Italian which means ONE, marked by the first launch of Arduino 1.0, Uno on version 1.0 as a reference to the next Arduino, the latest version of Uno is equipped with USB.
Arduino Uno can be supplied directly to the USB or an additional power supply that the power option automatically functions without a switch.
An external (non-USB) cable such as using an AC to DC adapter or a battery with a 2.1mm positive polarity plug connector in the middle to the power jack on the board. If using a battery, it can be pinned on the GND and Vin pins in the Power connector.
VIN – Input the voltage board when you use an external power supply source (5 Volt USB adapter or other 7-12 volt adapter), you can connect it with this VIN pin or directly to the 5V power jack. DC power jack (7-12V), USB connector cable (5V) or other power supply (7-12V). Connecting directly the external power supply (7-12V) to pin 5V or pin 3.3V can damage this Arduino circuit, don’t blame me ?!
3V3 – 3.3 volt voltage pin general power supply directly to the board. The maximum allowable current is 50 mA.
GND – Ground Pin.
IOREF – This pin provides a reference reference for the microcontroller to operate properly. Choose the right power source or enable voltage translator at the output to work with 5V or 3.3V.