Calculation of Bit frequency

Bit Frequency = pclk/ (I2CSCLH + I2CSCLL)

eg: Since the maximum speed the PCF8574 could interface to the LPC2106 is100 KHz
100KHz= 2.5MHz/(I2CSCLH + I2CSCLL)
Therefore
I2CSCLH + I2CSCLL=25
We select
I2CSCLH=13
&
I2CSCLL=12