but I think the RTC he uses is CMOS and the capacitances involved may be high. I think it advisable to put a network anyway to eliminate that possibility

The OP, as so many others, totally ignore that, today, there is no such thing as a "89c51" and, for that reaason, simply states 89C51 which say NOTHING, instead of the full 'name' of his chip. Thus my comments are made with the assumption that he actually uses a plain vanilla e.g. P89C51, not a SILabs or a RD2 or .... .

Without repeating the exceptions mentioned in my previous post, the "brief strong pullup" should be more than sufficient to overcome the capacitances (I, naturally, assume a well made PCB),

Now one more exceptiont: if the peripheral chip is open collector (or open drain) pullups may be needed, however, in most cases, I would not suggest them when using 'plain vanilla' derivatives.

An admission: when I started using the 100MHz 1 clock SILabs chips, I missed the fact that the 'speedos' do not have a strong pullup and thus will need pullup resistors in the cases where push-pull can not be enabled for whatever reason (e.g. keypad scan)- well even old dogs can learn new tricks..

Erik

PS: anticipating a post "why can you not use push-pull for keypad scan" just visualize two keys pressed simultanesously.