The USB transceiver inside the 32U4 allows us to add USB connectivity on-board and do away with bulky external USB interface. Supported by Arduino IDE V1.0.1. This tiny little board does all of the neat-o Arduino tricks that you're familiar with: 4 channels of 10-bit ADC, 5 PWM pins, 12 DIOs as well as hardware serial connections Rx and Tx.
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