APPENDIX B POWER SAVING
S1C33L26 TECHNICAL MANUAL
Seiko Epson Corporation
AP-B-1
Appendix B Power Saving
Current consumption depends, to a large degree, on the CPU operating mode, operating clock frequency, and the
peripheral circuits to be activated. This chapter summarizes the control to save power.
The following shows the clock systems that can be controlled with software and power saving control methods. For
details of control registers and control methods, see the chapter for each module.
System sleep (disabling all clocks)
• Executing the slp instruction
Execute the slp instruction if all of the system can be stopped. In SLEEP mode, the CPU stops operating and
the CMU stops supplying a clock to each functional module. Therefore, all peripheral circuits (except the
OSC1 oscillator circuit and RTC) stop operating.
The CPU is reawaken from SLEEP mode by initial reset, RTC interrupt, #NMI signal, or other interrupt from
an external device (port input interrupt).
System clock
• Selecting the clock source (CMU module)
Either OSC3, PLL, or OSC1 can be selected as the system clock source. If the application can process the
task with a low-speed clock, select OSC1 as the system clock source to reduce current consumption.
• Disabling the OSC3 oscillator circuit (CMU module)
Using OSC1 for the system clock and disabling the OSC3 oscillator circuit achieves more reduction of cur-
rent consumed.
• Selecting a low system clock (CMU module)
The CMU module provides a clock divider to set the system clock speed to 1/1 to 1/32 of the source clock
(OSC3, PLL, OSC1). By running the S1C33L26 with the lowest speed required for the application’s task,
current consumption can be reduced.
CPU clock (CCLK)
• Executing the halt instruction
Execute the halt instruction if there is no task to be processed by the CPU such as when the display on the
LCD is only required or when the CPU is waiting an interrupt. Although the CPU enters HALT mode and
stops operating, the peripheral modules keep the status when the halt instruction is executed. So the LCD
controller and the peripheral modules used to generate an interrupt can be made to be run. Power saving ef-
fect will be enhanced by disabling the unnecessary oscillator and peripheral modules before executing the
halt instruction. The CPU reactivates from HALT mode by an interrupt from the ports or peripheral modules
that are being operated in HALT mode.
Peripheral clocks
• Disabling peripheral clocks (CMU and PSC modules)
The peripheral clock supply can be disabled if the peripheral modules listed below can be placed in standby
state.
1 Peripheral Modules and Operating Clocks
Table B.
Clock
Clock enable bit
Peripheral modules
PCLK1
PCLK1_EN/CMU_CLKCTL register
• Prescaler (PSC Ch.0)
• 8-bit programmable timer Ch.0, 2, 4, 6 (T8 Ch.0, 2, 4, 6)
• 16-bit PWM timer Ch.0, 1 (T16A5 Ch.0, 1)
• 16-bit audio PWM timer (T16P)
• Universal serial interface (USI)
• Serial interface Ch.0 (FSIO Ch.0)
• A/D converter (ADC10)
• I
2
S (I2S)
• Misc registers (MISC)