Andi:
I could point you to an application note that Dallas has for their DS1307 RTC chip. This App Note #95 has 8051 assembly language source code to interface the I2C bus to the operate the DS1307 chip. The Philips PCF8563 part you are looking at is very similar in operation. With a bit of simple modification you should be able to use most of the code. Here is the link to Dallas App Note #95.

Michael Karas