I/O pins which are unused in a design typically do not need to be grounded or connected anywhere.
Some of the devices have global setting for pull-ups on I/O likes -Up, -Down or Bus-Hold, and others may have settings for each pin. By default, Lattice device inputs are either Pull-Up or Pull-Down. So long as the Pull mode is either -Up or -Down, or Bus-Hold, user can leave the I/O pins unconnected.
User must only tie-off the input pins if they intentionally disable the internal pull-up or pull-down. If not tied-off to VCC or GND, it may cause some additional leakage due to the floating state of the IO when device powers up. For this reason, it is recommended that user either ground them or connect them to VCC to avoid leakage current.
If the device is within the permitted junction temperature range, this leakage current will not have affect the functionality of the design. However, if this current is causing the device to go out of junction temperature range, then it may affect device performance.
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