I looked into this further and found this LPC932 Users Manual. The part has an onboard system clock divider which you can program at will from 0 to 255. This register, called DIVM, permits the F_osc to be divided by a factor from 1 to 256 which allows operation at quite a range of frequencies. If you are using the on-board 7.3728 MHz RC oscillator you could program this to obtain operation at say 3.6864 or 2.4576 MHz.

This would manage CPU speed and corresponding power management. The ring oscillator can only be modified slightly by programming different values into the TRIM register which intended mainly for calibration purposes.

Michael Karas