You have assigned CEX0 to P0.0, so you cannot use it as I/O pin anymore. Change XBR0 to 0x00. Use the Configuration wizard in the IDE to determine port pinout; it's a very handy tool.

In push-pull mode, the I/O pin is driven high when you set it '1' or low when you set it '0'. In open drain mode, the pin is only driven low and will float when set '1'. In this mode you can use it as input.

If you have more questions about Cygnal's devices, check out the user forum on their website.

Regards,
Rob.