COP8ACC5 Datasheet, PDF (18/41 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 4k Memory and High Resolution A/D

Power Save Modes (Continued)
The IDLE timer window is selectable from one of five values,
4k, 8k, 16k, 32k or 64k instruction cycles. 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 instruction 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 inter-
rupt is enabled, the interrupt is serviced before execution 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 instruction.) 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 reset, or with a Multi-Input Wakeup
input. Upon reset the ITMR register is cleared and the ITMR
register selects the 4,096 instruction cycle tap of the Idle
Timer.
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 arbi-
trary time, it will stay in the IDLE mode for somewhere be-
tween 1 and the selected number of instruction cycles.
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 IT-
SEL bits of the ITMR Register and then clear the TOPND bit
before attempting to synchronize operation to the IDLE
Timer.
Note: 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 the Timers section.
Multi-Input Wakeup
The Multi-Input Wakeup feature is used to return (wakeup)
the device from either the HALT or IDLE modes. Alternately
Multi-Input Wakeup/Interrupt feature may also be used to
generate up to 4 edge selectable external interrupts.
Figure 12 shows the Multi-Input Wakeup logic.
The Multi-Input Wakeup feature utilizes the L Port. The user
selects which particular L port bit (or combination of L Port
bits) will cause the device to exit the HALT or IDLE modes.
The selection is done through the register WKEN. The regis-
ter WKEN is an 8-bit read/write register, which contains a
control bit for every L port bit. Setting a particular WKEN bit
enables a Wakeup from the associated L port pin.
The user can select whether the trigger condition on the se-
lected L Port pin is going to be either a positive edge (low to
high transition) or a negative edge (high to low transition).
This selection is made via the register WKEDG, which is an
8-bit control register with a bit assigned to each L Port pin.
Setting the control bit will select the trigger condition to be a
negative edge on that particular L Port pin. Resetting the bit
selects the trigger condition to be a positive edge. Changing
an edge select entails several steps in order to avoid a
Wakeup condition as a result of the edge change. First, the
associated WKEN bit should be reset, followed by the edge
select change in WKEDG. Next, the associated WKPND bit
should be cleared, followed by the associated WKEN bit be-
ing re-enabled.
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