UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 45 of 792
NXP Semiconductors UM10237
Chapter 4: LPC24XX Clocking and power control
register) so that software can determine when the oscillator is running and stable. At that
point, software can control switching to the main oscillator as a clock source. Prior to
starting the main oscillator, a frequency range must be selected by configuring the
OSCRANGE bit in the SCS register.
2.3 RTC oscillator
The RTC oscillator can be used as the clock source for the RTC, and/or the watchdog
timer. Also, the RTC oscillator can be used to drive the PLL and the CPU.
3. Register description
All registers, regardless of size, are on word address boundaries. Details of the registers
appear in the description of each function.
[1] Reset Value reflects the data stored in used bits only. It does not include reserved bits content.
3.1 Clock source selection multiplexer
Several clock sources may be chosen to drive the PLL and ultimately the CPU and
on-chip peripheral devices. The clock sources available are the main oscillator, the RTC
oscillator, and the Internal RC (IRC) oscillator.
The clock source selection can only be changed safely when the PLL is not connected.
For a detailed description of how to change the clock source in a system using the PLL
see Section 4–3.2.14 “PLL setup sequence”.
Note the following restrictions regarding the choice of clock sources:
Table 41. Summary of system control registers
Name Description Access Reset
value[1] Address
Clock source selection
CLKSRCSEL Clock Source Select Register R/W 0 0xE01F C10C
Phase Locked Loop
PLLCON PLL Control Register R/W 0 0xE01F C080
PLLCFG PLL Configuration Register R/W 0 0xE01F C084
PLLSTAT PLL Status Register RO 0 0xE01F C088
PLLFEED PLL Feed Register WO NA 0xE01F C08C
Clock dividers
CCLKCFG CPU Clock Configuration Register R/W 0 0xE01F C104
USBCLKCFG USB Clock Configuration Register R/W 0 0xE01F C108
IRCTRIM IRC Trim Register R/W 0xA0 0xE0 1F C1A4
PCLKSEL0 Peripheral Clock Selection register 0. R/W 0 0xE01F C1A8
PCLKSEL1 Peripheral Clock Selection register 1. R/W 0 0xE01F C1AC
Power control
PCON Power Control Register R/W 0 0xE01F C0C0
INTWAKE Interrupt Wakeup Register R/W 0 0xE01F C144
PCONP Power Control for Peripherals Register R/W 0x03BE 0xE01F C0C4