XC886/888CLM
Power Saving Modes
User’s Manual
8-2
V1.3, 2010-02
Power Saving Modes, V 1.0
8.1
Functional Description
This section describes the various power saving modes, their operations, and how they
are entered and exited.
8.1.1
Idle Mode
The idle mode is used to reduce power consumption by stopping the core’s clock.
In idle mode, the oscillator continues to run, but the core is stopped with its clock
disabled. Peripherals whose input clocks are not disabled are still functional. The user
should disable the Watchdog Timer (WDT) before the system enters the idle mode;
otherwise, it will generate an internal reset when an overflow occurs and thus will disrupt
the idle mode. The CPU status is preserved in its entirety; the stack pointer, program
counter, program status word, accumulator, and all other registers maintain their data
during idle mode. The port pins hold the logical state they had at the time the idle mode
was activated.
Software requests idle mode by setting the bit PCON.IDLE to 1.
The system will return to active mode on occurrence of any of the following conditions:
•
The idle mode can be terminated by activating any enabled interrupt. The CPU
operation is resumed and the interrupt will be serviced. Upon RETI instruction, the
core will return to execute the next instruction after the instruction that sets the IDLE
bit to 1.
•
An external hard reset signal (RESET) is asserted.
8.1.2
Slow-Down Mode
The slow-down mode is used to reduce power consumption by decreasing the internal
clock in the device.
The slow-down mode is activated by setting the bit SD in SFR PMCON0. The bit field
CMCON.CLKREL is used to select a different slow-down frequency. The CPU and
peripherals are clocked at this lower frequency. The slow-down mode is terminated by
clearing bit SD.
The slow-down mode can be combined with the idle mode by performing the following
sequence:
1. The slow-down mode is activated by setting the bit PMCON0.SD.
2. The idle mode is activated by setting the bit PCON.IDLE.
There are two ways to terminate the combined idle and slow-down modes:
•
The idle mode can be terminated by activation of any enabled interrupt. CPU
operation is resumed, and the interrupt will be serviced. The next instruction to be
executed after the RETI instruction will be the one following the instruction that had
set the bit IDLE. Nevertheless, the slow-down mode stays enabled and if required
termination must be done by clearing the bit SD in the corresponding interrupt service
*