Lpc2000

Dear lpc group,
how can i program my lpc2148 to change the internal crystal oscillator 
frequency? 
One a first step I would like a 30Mhz (fosc and cclk) without pll 
enable. 
One second step,I would like tu have the most speed possible. I have 
deal with the pll (with the suppose of 14745600 Mhz), it's work with 
more speed but I think it can be better with other params like 5x12Mhz 
= 60Mhz. 
I have no switch on my board, so is there a register to subscribe the 
new frequency (fosc)or anything else to do?

Is there a way to control the actual frequency of the internal crystal 
oscillator?

Thanks for any help
Cyril