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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
56 of 808
NXP Semiconductors
UM10360
Chapter 4: LPC17xx Clocking and power control
8.10 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.
8.11 Power domains
The LPC17xx provides two independent power domains that allow the bulk of the device
to have power removed while maintaining operation of the Real Time Clock.
The VBAT pin supplies power only to the RTC domain. The RTC requires a minimum of
power to operate, which can be supplied by an external battery. Whenever the device core
power is present, that power is used to operate the RTC, causing no power drain from a
battery when main power is available.
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.