UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 65 of 792
NXP Semiconductors UM10237
Chapter 4: LPC24XX Clocking and power control
[1] LPC247x only.

3.4.9 Power control usage notes

After every reset, the PCONP register contains the value that enables selected interfaces
and peripherals controlled by the PCONP to be enabled. Therefore, apart from proper
configuring via peripheral dedicated registers, the user’s application might have to access
the PCONP in order to start using some of the on-board peripherals.
Power saving oriented systems should have 1s in the PCONP register only in positions
that match peripherals really used in the application. All other bits, declared to be
"Reserved" or dedicated to the peripherals not used in the current application, must be
cleared to 0.
4. Power domains
The LPC2400 provides two independent power domains that allow the bulk of the device
to have power removed while maintaining operation of the Real Time Clock and the
Battery RAM.
The VBAT pin supplies power only to the RTC and the Battery RAM. These two functions
require a minimum of power to operate, which can be supplied by an external battery.
When the CPU and the rest of chip functions are stopped and power removed, the RTC
can supply an alarm output that may be used by external hardware to restore chip power
and resume operation. Details may be found in Section 26–8.
Note: The RTC and the battery RAM operate independently from each other. Therefore,
the battery RAM can be accessed at any time, regardless of whether the RTC is enabled
or disabled via a dedicated bit in the PCONP register.
5. Wakeup timer
The LPC2400 begins operation at power-up and when awakened from Power-down mode
by using the 4 MHz IRC oscillator as the clock source. This allows chip operation quickly
in these cases. If the main oscillator or the PLL is needed by the application, software will
need to enable these features and wait for them to stabilize before they are used as a
clock source.
When the main oscillator is initially activated, the wakeup timer allows software to ensure
that the main oscillator is fully functional before the processor uses it as a clock source
and starts to execute instructions. This is important at power-on, all types of Reset, and
27 PCI2S I2S interface power/clock control bit. 0
28 PCSDC SD card interface power/clock control bit. 0
29 PCGPDMA GP DMA function power/clock control bit. 0
30 PCENET Ethernet block power/clock control bit. 0
31 PCUSB USB interface power/clock control bit. 0
Table 63. Power Control for Peripherals register (PCONP - address 0xE01F C0C4) bit
description
Bit Symbol Description Reset
value