
Resets and Interrupts
Low-Power Operation
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Resets and Interrupts
107
5.6  Low-Power Operation 
Both stop mode and wait mode suspend CPU operation until a reset or interrupt 
occurs. Wait mode suspends processing and reduces power consumption to an 
intermediate level. Stop mode turns off all on-chip clocks and reduces power 
consumption to an absolute minimum while retaining the contents of the entire 
RAM array. 
5.6.1  Wait Mode
The WAI opcode places the MCU in wait mode, during which the CPU registers are 
stacked and CPU processing is suspended until a qualified interrupt is detected. 
The interrupt can be an external IRQ, an XIRQ, or any of the internally generated 
interrupts, such as the timer or serial interrupts. The on-chip crystal oscillator 
remains active throughout the wait standby period. 
The reduction of power in the wait condition depends on how many internal clock 
signals driving on-chip peripheral functions can be shut down. The CPU is always 
shut down during wait. While in the wait state, the address/data bus repeatedly 
runs read cycles to the address where the CCR contents were stacked. The MCU 
leaves the wait state when it senses any interrupt that has not been masked. 
The free-running timer system is shut down only if the I bit is set to 1 and the COP 
system is disabled by NOCOP being set to 1. Several other systems also can be 
in a reduced power-consumption state depending on the state of 
software-controlled configuration control bits. Power consumption by the 
analog-to-digital (A/D) converter is not affected significantly by the wait condition. 
However, the A/D converter current can be eliminated by writing the ADPU bit to 0. 
The SPI system is enabled or disabled by the SPE control bit. The SCI transmitter 
is enabled or disabled by the TE bit, and the SCI receiver is enabled or disabled by 
the RE bit. Therefore, the power consumption in wait is dependent on the particular 
application. 
5.6.2  Stop Mode
Executing the STOP instruction while the S bit in the CCR is equal to 0 places the 
MCU in stop mode. If the S bit is not 0, the stop opcode is treated as a no-op (NOP). 
Stop mode offers minimum power consumption because all clocks, including the 
crystal oscillator, are stopped while in this mode. To exit stop and resume normal 
processing, a logic low level must be applied to one of the external interrupts (IRQ 
or XIRQ) or to the RESET pin. A pending edge-triggered IRQ can also bring the 
CPU out of stop. 
Because all clocks are stopped in this mode, all internal peripheral functions also 
stop. The data in the internal RAM is retained as long as V
DD
 power is maintained. 
The CPU state and I/O pin levels are static and are unchanged by stop. Therefore, 
when an interrupt comes to restart the system, the MCU resumes processing as if 
there were no interruption. If reset is used to restart the system, a normal reset 
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