Is it safe to use an I/O pin as a clock input? I need my design to be synced up to some external logic running at a variable clock speed. Of 0.1 to 2MHz.

Sorry @Tim Edwards would you mind comment on this?

Anything up to approximately 50MHz is fine. We use the same kind of GPIO pad for the CMOS clock input to the chip, which can be up to 40MHz.

Thank you. That actually solves quite a few problems for me.