
APPENDIX B POWER SAVING
S1C17705 TECHNICAL MANUAL
Seiko Epson Corporation
AP-B-1
Appendix B Power Saving
Current consumption will vary dramatically, depending on CPU operating mode, operation clock frequency, and the
peripheral circuits being operated. Listed below are the control methods for saving power.
Clock Control Power Saving
B.1
Figure B.1.1 illustrates the S1C17705 clock system.
CCLK
OSC3
OSC1
IOSC
FOUTA
output circuit
OSC3
oscillator
(8.2 MHz)
IOSC
oscillator
(2.7 MHz)
OSC1
oscillator
(32.768 kHz)
Clock gear
(1/1–1/8)
OSC
controller
Gate
S1C17 Core
CT, SWT, WDT
OSC3
OSC4
System
clock
EXOSC3
FOUTA
FOUTB
output circuit
FOUTB
OSC1
OSC2
SLEEP, wakeup
HALT
CLG
T16, SPI, I2CM,
I2CS, P, MISC,
VD1, ADC, REMC
PCLK
256 Hz
Gate
LCD, SVD, RFC,
T16A, UART
IOSC
divider
OSC3
divider
OSC1
divider
1.1 Clock System
Figure B.
This section describes clock systems that can be controlled via software and power-saving control details. For more
information on control registers and control methods, refer to the respective module sections.
System SLEEP (All clocks stopped)
Execute the slp instruction
Execute the slp instruction when the entire system can be stopped. The CPU enters SLEEP mode and the
system clocks stop. This also stops all peripheral circuits using clocks. Starting up the CPU from SLEEP
mode is therefore limited to startup using a port (described later).
System clocks
Select a low-speed clock source (CLG module)
Select a low-speed oscillator for the system clock source. You can reduce current consumption by selecting
the OSC1 clock when low-speed processing is possible.
Disable unnecessary oscillator circuits (CLG module)
Operate the oscillator comprising the system clock source. Where possible, stop the other oscillators. You can
reduce current consumption by using OSC1 as the system clock and disable the IOSC and OSC3 oscillators.
CPU clock (CCLK)
Execute the halt instruction
Execute the halt instruction when program execution by the CPU is not required—for example, when only
the display is required or for interrupt standby. The CPU enters HALT mode and suspends operations, but the
peripheral circuits maintain the status in place at the time of the halt instruction, enabling use of peripheral
circuits for timers and interrupts. You can reduce power consumption even further by suspending unneces-
sary oscillator and peripheral circuits before executing the halt instruction. The CPU is started from HALT
mode by an interrupt from a port or the peripheral circuit operating in HALT mode.
Select a low-speed clock gear (CLG module)
The CLG module can reduce CPU clock speeds to between 1/1 and 1/8 of the system clock via the clock gear
settings. You can reduce current consumption by operating the CPU at the minimum speed required for the
application.