UM10360 Datasheet, PDF (66/835 Pages) NXP Semiconductors – LPC17xx User manual

NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
9. Wake-up timer
The LPC17xx 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 to begin
quickly. If the main oscillator or one or both PLLs are 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 wake-up 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 wake-up of the
processor from Power-down mode makes use of the Wake-up Timer.
The Wake-up 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(REG)(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 Wake-up Timer counts a fixed number of clocks (4,096),
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 start any required
PLLs. Refer to the Main Oscillator description in this chapter for details.
UM10360_1
User manual
Rev. 01 — 4 January 2010
© NXP B.V. 2010. All rights reserved.
66 of 835