There are two reasons.

1. The pins on Cyclone II chips can supply only around 8mA of current.
2. On the DE2 board, the expansion header pins are connected to the FPGA through a 33 ohm resistor.

If your expansion device draws too much current, the voltage will drop.

There are two reasons.

1. The pins on Cyclone II chips can supply only around 8mA of current.
2. On the DE2 board, the expansion header pins are connected to the FPGA through a 47 ohm resistor.

If your expansion device draws too much current, the voltage will drop.