The C8051F975 does not appear to have a pinout assignment for XTAL3 and XTAL4 as in the higher pin-count C8051F973/4. Does the C8051F975 have the same RTC functionality as the C8051F973/4? What can I use as an oscillator source for the RTC?
On the smaller packages available in the C8051F97x family (QFN24), the XTAL3 and XTAL4 pins are not bonded out, so it is not possible to use the RTC with an external crystal as the oscillator source. There may still be a couple of other options, however:
1) Recommended: You can use the integrated low frequency oscillator (LFO) as the timebase to the RTC, as described in section 25.2.3 on page 258 of the revision 1.0 data sheet. The LFO will provide a low power, relatively accurate timebase for the RTC and requires neither external components nor internal shorting of the XTAL3 and XTAL4 pins. You can again verify the RTC frequency by routing the signal to a pin. (Note: The RTC timebase can be selected as the system clock and routed to a port pin. See Section “24. Clocking Sources” on page 242 for information on selecting the system clock source and Section “26. Port I/O (Port 0, Port 1, Port 2, Port 3, Port 4, Port 5, Port 6, Crossbar, and Port Match)” on page 278 for information on how to route the system clock to a port pin.)
2) It may still be possible to use the RTC oscillator circuit in self-oscillate mode as described in section 25.2.2 on page 258 of the C8051F97x data sheet (revision 1.0). Note that the frequency generated by this mode is a clock, but is not accurate.