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Clocks and Power Control
5-20
MPC801 USER’S MANUAL
MOTOROLA
5
The system responds quickly to an asynchronous interrupt. The wake-up time from normal
low, doze high/low, and sleep mode due to an asynchronous interrupt is only 3
to
4 clocks
of maximum system frequency. In a 40MHz system, this wake-up takes 75ns to 100ns. The
asynchronous wake-up interrupt from the interrupt controller is level sensitive. Therefore, it
is negated only after the cause of the interrupt in the interrupt controller is cleared. The
real-time clock, periodic interrupt timer, timebase, or decrementer interrupts set status bits
in the PLPRCR. The clock module views this interrupt as a pending asynchronous interrupt
as long as the TMIST bit is set. Therefore, the TMIST status bit should be cleared before
entering any low-power mode other than normal high mode.
The wake-up time due to synchronous interrupt sources from the interrupt controller is
measured in actual system clocks. It takes 2 to 4 system clocks from the interrupt event
before the system reaches normal high mode. In a 50MHz system where DFNL=111
(divided by 256), the wake-up time is 12.8 to 25.6
μ
s. In normal and doze modes, if the
PLPRCR
CSRC
bit is set, the system toggles between low frequency (defined by the DFNL
bit) and high frequency (defined by DFNL/DFNH) states. The conditions to switch from
normal low mode to normal high state are:
A pending interrupt from an interrupt controller must occur
The POW bit in the machine state register must be cleared (normal mode)
If none of these conditions exist, the PLPRCR
CSRC
bit is set, the asynchronous interrupt
status bits are reset, and the system switches back to normal low mode. A pending interrupt
from the interrupt controller transfers the system from doze mode to normal high mode. An
exit from deep sleep mode is caused by:
An asynchronous wake-up interrupt from the interrupt controller
Real-time clock, periodic interrupt timer, timebase, or decrementer interrupts (if
enabled)
In deep sleep mode, the PLL is disabled and the wake-up time from this mode is a maximum
of 500 PLL input frequency clocks. In 1:1 mode the wake-up time can be up to 1,000 PLL
input frequency clocks. If the PLL input frequency is 32KHz the wake-up time is less than
15.6ms and if it is 4MHz the wake-up time is less then 125
μ
s.
An exit from power-down mode is accomplished with a hard reset that should be asserted
by external logic in response to the TEXPS bit and TEXP pin assertion. The TEXPS bit is
asserted by an enabled real-time clock, periodic interrupt timer, timebase, or decrementer
interrupt. The hard reset takes longer than the time it takes the power supply to wake up, in
addition to the time it takes the PLL to lock. Hard reset assertion when the TEXPS bit and
TEXP pin are clear, sets the bit and the pin values, thus causing an exit from power-down
low-power mode. For more details on power-down mode, refer to
Section 5.10.1
Configuration
.