
iiiPeriPheraLModuLes1(systeM):CLoCkManageMentunit(CMu)
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iii-1-33
III
CMU
iii.1.13Power-downControl
The amount of current consumed on the chip varies significantly with the CPU operation mode, operating clock
frequency, and peripheral circuit to be operated. The following summarizes points on how to reduce power
consumption on the chip.
1.reducingtheoperatingclockfrequencyasmuchaspossible
The CMU allows one of the three available clock sources to be selected, and the system clock frequency to be
set. (See III.1.4 to III.1.10.)
Reduce the operating clock frequency to as low a frequency as permitted for the intended processing content of
the system.
When the OSC1 clock can suffice, turn off the PLL and OSC3, and use only OSC1 to operate the system.
When the OSC3 clock can suffice (although high-speed processing is needed), turn off the PLL and use
OSC3 to operate the system.
When the PLL is needed, use it with as small a frequency multiplication rate as possible.
The divide ratios with which to generate MCLK from the source clock can be selected. Select the lowest
divide ratios permitted for the intended processing content.
Some peripheral circuits may use the prescaler, while others have an exclusive clock control function
incorporated in the module. For details, refer to the description of each peripheral circuit.
2.turningoffunnecessaryclocksupply
The CMU allows clock supplies to be turned on or off independently for each functional module. (See III.1.9.)
Turn off clock supplies for unused functional modules.
Some peripheral circuits may use the prescaler, while others have an exclusive clock control function
incorporated in the module. For details, refer to the description of each peripheral circuit.
3.PlacingtheCPuinstandbymode
Place the CPU in standby mode by executing the halt or slp instruction as much as possible when, for example,
waiting for key input. (See III.1.11.)
Optimum power saving effects may be obtained by placing the CPU in SLEEP mode whenever OSC3 oscillation
is turned off. In such case, however, basically only the RTC can be used without other peripheral circuits.
To wake CPU quickly from SLEEP mode with no oscillation stabilization time inserted, enter SLEEP mode
after setting OSC3 so that it does not stop in SLEEP mode. The core and peripheral circuits other than RTC
and OSC3 cell unit enter power-down state.
When peripheral circuits must be operated, use the halt instruction to place the CPU in HALT mode. In
HALT mode, the CPU and A0RAM all stop operating. Furthermore, the circuit for which the clock supply is
automatically stopped in HALT mode (see Section III.1.9.2) stops operating.
4.turningoffunnecessaryexternalportpull-ups
When input ports are driven to low level, their pull-up resistors consume some amount of current. Use the pull-
up control register to turn off unnecessary pull-ups. However, care should be taken to prevent the input ports
from becoming left open (floating state).