The AVR Butterfly has connections for the USI-interface. Figure 1 shows the pin-out for the USI. Through the USI interface other modules can be connected, and the AVR Butterfly can serve as a top-module card.
Figure 1. USI
connector
AVR Butterfly USI Connector:
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Pin 1 (USCK/SCL)
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Pin 2 (DI/SDA)
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Pin 3 (DO)
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Pin 4 (GND)