Why do IO Cell registers in my design show as being unused in the Resource Property Editor even though they are being used?

Description

Due to a problem in the Quartus® II software versions 12.0 Service Pack 1 and earlier, IO Cell registers may appear to be unused (gray color) in the Resource Property Editor even if they are being used.

Resolution

The control and data lines connected to a used register are correctly highlighted (blue color) which indicates whether the register is used or not.

This problem is fixed beginning with the Quartus II software version 12.1.