from Bauke Siderius at Philips

The min high and min low times of the external clock are given in the AC electrical characteristics of the datasheets.
For the Rx2 they are 17ns min high and 17ns min low, depending on the speed you are running you can calculate the maximum.
The fastest you can clock the PCA would be in 6clk mode using a 20MHz clock source, the PCAclock will be devided down from the clock source to the part, devide by 2 is the fastest.
The devide by 2 will cause the PCAclock to be symetrical, since 1 clock period to the part is now the high/low time for the PCAclock, 50ns high 50 ns low (@20MHz).

Hope this helps,

Erik