In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

I suppose that if you want a truly external clock source, you would be feeding a pulse through Osc #1 pin.  You might need a driver chip that would incur a small time delay from the original clock unless the driver provides parallel outputs to clock all devices in sync.

Other options are to [1] provide a pulse directly to the RTCC which is counted according to software decisions and then does a roll over to interrupt the main program and divert to an interrupt service routine [a specialized subroutine] at a rate less than the actual clock or [2] establish an interrupt via any, some or all of the Port B pins which will divert to an interrupt service routine which again is less than the actual clock.  

Read section 1.9 of Gunther's text.

The RTCC pin is not a true external oscillator port.  It is an interrupt/counter.  Nonetheless, it is useful.  For example, it can count pulses and the total can be compared againts another timer tied to the Osc to determine an actual pulse rate or frequency rate.

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