LPCOpen Platform for LPC18XX/43XX microcontrollers  18XX43XX
LPCOpen Platform for the NXP LPC18XX/43XX family of Microcontrollers

The clock driver has options that configure it's operation at build-time.

MAX_CLOCK_FREQ
This macro defines the maximum frequency supported by the Chip [204MHz for LPC43xx 180MHz for LPC18xx]. API Chip_SetupXtalClocking() and Chip_SetupIrcClocking() will use this macro to set the CPU Core frequency to the maximum supported.
To set a Core frequency other than the maximum frequency Chip_SetupCoreClock() API must be used. Using this macro to set the Core freqency is not recommended.