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Xilinx Data Book-‘94
Page 9-4, “Unused Pins”
Modern CMOS devices have extremely low input-leakage current, perhaps only a few nanoamps. Left disconnected, such an input could therefore float to any voltage [between Vcc and ground]. An input voltage close to the threshold value 1.2V for TTL level-compatibility, 2.5V for CMOS level-compatibility will turn the input buffer partially on, thus creating a static current path from Vcc to ground and causing static power dissipation. Such a biased buffer also acts as a fairly high gain amplifier, making the circuit very susceptible to noise, crosstalk, ground-bounce and other undesirable disturbances. It is, therefore, advisable to force unused inputs to a proper logic level. <..>
=>Configure as active output driven by an internally defined signal
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