COP8AME9
13.0 Power Saving Features
(Continued)
13.5.2 Low Speed Idle Mode
In the IDLE mode, program execution stops and power
consumption is reduced to a very low level as with the HALT
mode. However, the low speed oscillator, IDLE Timer (Timer
T0), and Clock Monitor continue to operate, allowing real
time to be maintained. The device remains IDLE for a se-
lected amount of time up to 2 seconds, and then automati-
cally exits the IDLE mode and returns to normal program
execution using the low speed clock.
The device is placed in the IDLE mode under software
control by setting the IDLE bit (bit 6 of the Port G data
register).
The IDLE Timer window is selectable from one of five values,
0.125 seconds, 0.25 seconds, 0.5 seconds, 1 second, and
2 seconds. Selection of this value is made through the ITMR
register.
The IDLE mode uses the on-chip IDLE Timer (Timer T0) to
keep track of elapsed time in the IDLE state. The IDLE Timer
runs continuously at the low speed clock rate, whether or not
the device is in the IDLE mode. Each time the bit of the timer
associated with the selected window toggles, the T0PND bit
is set, an interrupt is generated (if enabled), and the device
exits the IDLE mode if in that mode. If the IDLE Timer
interrupt is enabled, the interrupt is serviced before execu-
tion of the main program resumes. (However, the instruction
which was started as the part entered the IDLE mode is
completed before the interrupt is serviced. This instruction
should be a NOP which should follow the enter IDLE instruc-
tion.) The user must reset the IDLE Timer pending flag
(T0PND) before entering the IDLE mode.
As with the HALT mode, this device can also be returned to
normal operation with a Multi-Input Wake-up input.
The IDLE Timer cannot be started or stopped under software
control, and it is not memory mapped, so it cannot be read or
written by the software. Its state upon Reset is unknown.
Therefore, if the device is put into the IDLE mode at an
arbitrary time, it will stay in the IDLE mode for somewhere
between 30 µs and the selected time period.
In order to precisely time the duration of the IDLE state, entry
into the IDLE mode must be synchronized to the state of the
IDLE Timer. The best way to do this is to use the IDLE Timer
interrupt, which occurs on every underflow of the bit of the
IDLE Timer which is associated with the selected window.
Another method is to poll the state of the IDLE Timer pending
bit T0PND, which is set on the same occurrence. The Idle
Timer interrupt is enabled by setting bit T0EN in the ICNTRL
register.
Any time the IDLE Timer window length is changed there is
the possibility of generating a spurious IDLE Timer interrupt
by setting the T0PND bit. The user is advised to disable
IDLE Timer interrupts prior to changing the value of the
ITSEL bits of the ITMR Register and then clear the T0PND
bit before attempting to synchronize operation to the IDLE
Timer.
As with the HALT mode, it is necessary to program two
NOP’s to allow clock resynchronization upon return from the
IDLE mode. The NOP’s are placed either at the beginning of
the IDLE Timer interrupt routine or immediately following the
“enter IDLE mode” instruction.
For more information on the IDLE Timer and its associated
interrupt, see the description in
20006324
FIGURE 20. Multi-Input Wake-Up Logic
41
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
