Mega is basically used when there is more requirement of pins as there are more components in the project. Arduino UNO has less number of pins compared to MEGA.

54 digital input/output pins
15 DO pins can be used as PWM outputs
16 analog inputs

Main Overview
5V 8-bit
16 MHz
USB Host
AVR
The Arduino MEGA ADK is a microcontroller board based on the ATmega2560. It has a USB host interface to connect with Android based phones, based on the MAX3421e IC.
It has 54 digital input/output pins (of which 15 can be used as PWM outputs), 16 analog inputs,UARTs (hardware serial ports), a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button.
The MEGA ADK is based on the Mega 2560.
Getting Started

In the Getting Started section, you can find all the information you need to configure your board, use the Arduino Software (IDE), and start to tinker with coding and electronics.Google’s ADK documentation.

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Technical Specs

Microcontroller ATmega2560
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
Digital I/O Pins 54 (of which 15 provide PWM output)
Analog Input Pins 16
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 256 KB of which 8 KB used by bootloader
SRAM 8 KB
EEPROM 4 KB
Clock Speed 16 MHz
USB Host Chip MAX3421E
Length 101.52 mm
Width 53.3 mm
Weight 36 g

Explanation of Pins:

 

 

The power pins are as follows:

  • VIN. The input voltage to the Arduino board when it’s using an external power source (as opposed to 5 volts from the USB connection or other regulated power sources). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
  • 5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 – 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don’t advise it.
  • 3V3. A 3.3 volt supply generated by the onboard regulator. The maximum current draw is 50 mA.
  • GND. Ground pins.
  • IOREF. This pin on the Arduino board provides the voltage reference with which the microcontroller operates. A properly configured shield can read the IOREF pin voltage and select the appropriate power source or enable voltage translators on the outputs for working with the 5V or 3.3V.
Memory

The MEGA ADK has 256 KB of flash memory for storing code (of which 8 KB is used for the bootloader), 8 KB of SRAM and 4 KB of EEPROM (which can be read and written with the EEPROM library).

Input and Output

Each of the 50 digital pins on the MEGA ADK can be used as an input or output, using pinMode()digitalWrite(), and digitalRead() functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:

      • Serial: 0 (RX) and 1 (TX); Serial 1: 19 (RX) and 18 (TX); Serial 2: 17 (RX) and 16 (TX); Serial 3: 15 (RX) and 14 (TX). Used to receive (RX) and transmit (TX) TTL serial data. Pins 0 and 1 are also connected to the corresponding pins of the ATmega8U2 USB-to-TTL Serial chip.
      • External Interrupts: 2 (interrupt 0), 3 (interrupt 1), 18 (interrupt 5), 19 (interrupt 4), 20 (interrupt 3), and 21 (interrupt 2). These pins can be configured to trigger an interrupt on a low value, a rising or falling edge, or a change in value. See the attachInterrupt() function for details.
      • PWM: 2 to 13 and 44 to 46. Provide 8-bit PWM output with the analogWrite() function.
      • SPI: 50 (MISO), 51 (MOSI), 52 (SCK), 53 (SS). These pins support SPI communication using the SPI library. The SPI pins are also broken out on the ICSP header, which is physically compatible with the Uno, Duemilanove and Diecimila.
      • USB Host: MAX3421E.

     

  • The MAX3421E communicate with Arduino with the SPI bus. So it uses the following pins:
  • Digital: 7 (RST), 50 (MISO), 51 (MOSI), 52 (SCK).
    NB: Please do not use Digital pin 7 as input or output because is used in the communication with MAX3421E
  • Non broken out on headers: PJ3 (GP_MAX), PJ6 (INT_MAX), PH7 (SS).
  • 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.
  • TWI: 20 (SDA) and 21 (SCL). Support TWI communication using the Wire library. Note that these pins are not in the same location as the TWI pins on the Duemilanove or Diecimila.

The MEGA ADK has 16 analog inputs, each of which provides 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 and analogReference() function.There are a couple of other pins on the board:

  • AREF. The reference voltage for the analog inputs. Used with analogReference().
  • 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.

Source : https://www.arduino.cc/en/Main/ArduinoBoardMegaADK?from=Main.ArduinoBoardADK