
UM10413
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
User manual
Rev. 1 — 16 December 2011
56 of 268
NXP Semiconductors
UM10413
MPT612 User manual
10.12 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 previously mentioned 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 uses 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
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 (for example, 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 on-board module’s initialization is complete, the processor
is released to execute instructions if the external reset is de-asserted. 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 can 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 are operational before the beginning of program execution.
Any of the various resets can bring the MPT612 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.
To put the device in Power-down mode and allow activity on one or more of these buses
or lines to power it back up, software must reprogram the pin function to external interrupt,
select the appropriate mode and polarity for the interrupt, and then select Power-down
mode. At wake-up, software must restore the pin multiplexing to the peripheral function.
Fig 15. APB Divider connections
aaa-000579
PLL0
crystal oscillator or
external clock source
(f
osc
)
APB DIVIDER
processor clock
(CCLK)
APB clock
(PCLK)