General Description: Why do the internally created asynchronous global clocks in XPLA1 devices disable pins for use as inputs?
If a global clock is created internally by an equation, the associated pin will be disabled for use as an external input. In other words, the clock signal will not be treated as a node which would have permitted an external signal to drive the device on the pin associated with that macrocell.
The internal clock circuitry samples the signal between the macrocell output buffer and the pin. The architecture is set up this way to allow an external clock to drive the pin while an internal signal can use the macrocell as a node. This prevents wasted resources.