
R8C/3GM Group
32. Reducing Power Consumption
R01UH0284EJ0100 Rev.1.00
Aug 09, 2011
32. Reducing Power Consumption
32.1
Overview
This chapter describes key points and processing methods for reducing power consumption.
32.2
Key Points and Processing Methods for Reducing Power Consumption
Key points for reducing power consumption are shown below. They should be referred to when designing a system
or creating a program.
32.2.1
Voltage Detection Circuit
If voltage monitor 1 and comparator A1 are not used, set the VCA26 bit in the VCA2 register to 0 (voltage
detection 1 circuit disabled). If voltage monitor 2 and comparator A2 are not used, set the VCA27 bit in the
VCA2 register to 0 (voltage detection 2 circuit disabled).
If the power-on reset and voltage monitor 0 reset are not used, set the VCA25 bit in the VCA2 register to 0
(voltage detection 0 circuit disabled).
32.2.2
Ports
Even after the MCU enters wait mode or stop mode, the states of the I/O ports are retained. Current flows into
the output ports in the active state, and shoot-through current flows into the input ports in the high-impedance
state. Unnecessary ports should be set to input and fixed to a stable electric potential before the MCU enters
wait mode or stop mode.
32.2.3
Clocks
Power consumption generally depends on the number of the operating clocks and their frequencies. The fewer
the number of operating clocks or the lower their frequencies, the more power consumption decreases.
Unnecessary clocks should be stopped accordingly.
Stopping low-speed on-chip oscillator oscillation: Set the CM14 bit in the CM1 register to 1 (low-speed on-chip
oscillator off) and the OCD2 bit in the OCD register to 0
(XIN clock selected).
Stopping high-speed on-chip oscillator oscillation: Set the FRA00 bit in the FRA0 register to 0.
32.2.4
Wait Mode, Stop Mode
Power consumption can be reduced in wait mode and stop mode. Refer to 9.7 Power Control for details.
32.2.5
Stopping Peripheral Function Clocks
If the peripheral function f1, f2, f4, f8, and f32 clocks are not necessary in wait mode, set the CM02 bit in the
CM0 register to 1 (peripheral function clock stops in wait mode). This will stop the f1, f2, f4, f8, and f32 clocks
in wait mode.
32.2.6
Timers
If timer RA is not used, set the TCKCUT bit in the TRAMR register to 1 (count source cutoff).
If timer RB is not used, set the TCKCUT bit in the TRBMR register to 1 (count source cutoff).
If timer RC is not used, set the MSTTRC bit in the MSTCR register to 1 (standby).
32.2.7
A/D Converter
When the A/D converter is not used, power consumption can be reduced by setting the ADSTBY bit in the
ADCON1 register to 0 (A/D operation stops (standby)) to shut off any analog circuit current flow.
32.2.8
Clock Synchronous Serial Interface
When the SSU or the I2C bus is not used, set the MSTIIC bit in the MSTCR register to 1 (standby).