You should also put a 0.1uf ceramic cap between the Vcc and Gnd pins of the PCF' (generally, you need to do this for any digital integrated circuit unless the datasheet specifies otherwise), and this cap should be right next to the chip, as close as possible. Without it, under some conditions, the PCF' may unexpectedly reset or otherwise misbehave - the sort of unpredictable behavior this results in is extremely difficult to debug - including the cap is a very cheap defense against wasting hours debugging intermitant misbehavior due to insufficient decoupling.