The little chip in the middle of the PCB is the actual MPR121 sensor that does all the capacitive sensing and filtering. We add all the extra components you need to get started, and 'break out' all the other pins you may want to connect to onto the PCB. For more details you can check out the schematics in the Downloads page.
The sensor on the breakout requires 3V power. Since many customers have 5V microcontrollers like Arduino, we tossed a 3.3V regulator on the board. Its ultra-low dropout so you can power it from 3.3V-5V just fine.
Vin - this is the power pin. Since the chip uses 3 VDC, we have
included a voltage regulator on board that will take 3-5VDC and safely
convert it down. To power the board, give it the same power as the logic
level of your microcontroller - e.g. for a 5V micro like Arduino, use
3Vo - this is the 3.3V output from the voltage regulator, you can grab up to 100mA from this if you like
GND - common ground for power and logic
SCL - I2C clock pin, connect to your microcontrollers I2C clock line.
SDA - I2C data pin, connect to your microcontrollers I2C data line.
IRQ and ADDR Pins
ADDR is the I2C address select pin. By default this is pulled down to ground with a 100K resistor, for an I2C address of 0x5A. You can also connect it to the 3Vo pin for an address of 0x5B, the SDA pin for 0x5C or SCL for address 0x5D
IRQ is the Interrupt Request signal pin. It is pulled up to 3.3V on the breakout and when the sensor chip detects a change in the touch sense switches, the pin goes to 0V until the data is read over i2c