NXP Semiconductors LPC24XX UM10237 - Manual (Page 67)

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User manual Rev. 04 — 26 August 2009 67 of 792
NXP Semiconductors
UM10237
Chapter 4: LPC24XX Clocking and power control
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
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 V
DD(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.