UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 66 of 792
NXP Semiconductors UM10237
Chapter 4: LPC24XX Clocking and power control
whenever any of the aforementioned functions are turned off for any reason. Since the
oscillator and other functions are turned off during Power-down mode, any wakeup of the
processor from Power-down mode makes use of the Wakeup Timer.
The Wakeup Timer monitors the crystal oscillator as the means of checking whether it is
safe to begin code execution. When power is applied to the chip, or some event caused
the chip to exit Power-down mode, some time is required for the oscillator to produce a
signal of sufficient amplitude to drive the clock logic. The amount of time depends on
many factors, including the rate of VDD(3V3) ramp (in the case of power on), the type of
crystal and its electrical characteristics (if a quartz crystal is used), as well as any other
external circuitry (e.g. capacitors), and the characteristics of the oscillator itself under the
existing ambient conditions.
Once a clock is detected, the Wakeup Timer counts a fixed number of clocks (4096), then
sets the flag (OSCSTAT bit in the SCS register) that indicates that the main oscillator is
ready for use. Software can then switch to the main oscillator and, if needed, start the
PLL. Refer to the Main Oscillator description in this chapter for details.