Wake-Up Timer - Philips LPC2101 User Manual

Philips Semiconductors
Volume 1

3.12 Wake-up timer

The purpose of the wake-up timer is to ensure that the oscillator and other analog
functions required for chip operation are fully functional before the processor is allowed 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
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 4096 clocks, then enables the on-chip
circuitry to initialize. When the onboard modules initialization is complete, the processor is
released to execute instructions if the external Reset has been deasserted. In the case
where an external clock source is used in the system (as opposed to a crystal connected
to the oscillator pins), the possibility that there could be little or no delay for oscillator
start-up must be considered. The wake-up timer design then ensures that any other
required chip functions will be operational prior to the beginning of program execution.
Any of the various Resets can bring the microcontroller out of Power-down mode, as can
the external interrupts EINT2:0 and the RTC interrupt if the RTC is operating from its own
oscillator on the RTCX1-2 pins. When one of these interrupts is enabled for wake-up and
its selected event occurs, an oscillator wake-up cycle is started. The actual interrupt (if
any) occurs after the wake-up timer expires, and is handled by the Vectored Interrupt
Controller.
However, the pin multiplexing on the LPC2101/02/03 (see
Section 7.4 on page
Power-down mode. The pin-function pairing allows interrupts from events relating to RI1 /
EINT2.
User manual
crystal oscillator or
external clock source
(F )
OSC
Fig 11. APB divider connections
66) was designed to allow other peripherals to bring the device out of
Rev. 01 — 12 January 2006
Chapter 3: System control block
PLL0
APB DIVIDER
ramp (in the case of power on), the type of crystal
DD
Section 6.2 on page 61
UM10161
processor clock
(CCLK)
APB clock
(PCLK)
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
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