The Arduino Uno (Revision 3) is a microcontroller board based on the ATmega328. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16MHz ceramic resonator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
Also, it features the Atmega16U2 programmed as a USB-to-Serial converter.
The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically.
Each of the 14 digital pins on the Uno can be used as an input or output. They operate at 5V. Each pin can provide or receive a maximum of 40mA and has an internal pull-up resistor (disconnected by default) of 20-50KΩ.
In addition, some pins have specialized functions:
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Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are connected to the corresponding pins of the ATmega16U2 USB-to-TTL Serial chip.
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External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value, a rising or falling edge, or a change in value.
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PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output.
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SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication.
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LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value, the LED is on, when the pin is LOW, it's off.
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The Uno has 6 analog inputs, labeled A0 through A5, each of which provide 10 bits of resolution (i.e. 1024 different values). By default they measure from ground to 5 volts, though is it possible to change the upper end of their range using the AREF pin.
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TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the Wire library.
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AREF. Reference voltage for the analog inputs.
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Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button to shields which block the one on the board.
Features:
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Microcontroller: ATmega328
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Operating Voltage: 5V
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Input Voltage (recommended): 7-12V
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Input Voltage (limits): 6-20V
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Digital I/O Pins: 14 (of which 6 provide PWM output)
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Analog Input Pins: 6
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DC Current per I/O Pin: 40mA
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DC Current for 3.3V Pin: 50mA
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Flash Memory: 32KB (ATmega328) of which 0.5KB used by bootloader
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SRAM: 2KB (ATmega328)
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EEPROM: 1KB (ATmega328)
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Clock Speed: 16MHz