SC9308
Operating mode control
SC9308 work in three modes: normal mode, sleep mode, and data maintained mode. Normal mode can be
divided into high-speed mode and low-speed mode. Their difference is the system clock from different sources.
The high speed mode system clock comes from internal RC oscillator or high speed oscillator extended by PC4/5,
while the low speed mode system clock comes from external low speed oscillator.
After power on, CPU clock is provided by internal RC oscillator. When this oscillator is working, the internal
reference module is needed to open. Select clock frequency or switch to the external low speed clock (75KHz) by
setting system control register and clock control register.
The MCU enters sleep mode by setting SLEEP flag of MCU sleep control register MCU_SLP as 1. After the MCU
enters sleep mode, external 75KHz oscillator still works, and CPU clock will be turned off. But it can still response
to internal timer interrupt or external IO interrupt. The circuit enters sleep mode which can reduce the system
standby power dissipation (75KHz crystal oscillator operating, when RTC enables at 3V operating voltage, typical
power dissipation current <5μA). Reset, internal or external interrupt can wake up CPU and make it enter work in
full speed.
When system clock comes from external low speed oscillator, if this oscillator is closed by program, the circuit will
enter data save mode. In this mode, RAM data is saved. The dissipation current is reduced to less than 1uA.
External IO interrupt or reset can make the oscillator work again.
Note: The clock should be switch to 75KHz before entering the sleep mode. Turn off the power reference circuit
and other relative circuits to reduce the power dissipation.
Clock and Reset
SC9308 supports many selectable operating clocks. The clock source can be external 75KHz crystal oscillator,
and can also be the clock generated by internal RC oscillator. When need high precision and high speed
application, it can extend high frequency external oscillator through PC4/5. The structure is as follows:
The system reset occurs when: internal watchdog counting overflows; external Reset pin input low level. The
circuit jumps to the start address of the program from reset address FFFAH and begins working normally.
SC9308 has built in RC oscillator, and frequency of RC oscillator is about 3.2MHz when supply voltage is 3V. If
you need to fast response to internal and external events without high precision, adopt RC oscillator as CPU
clock to make the system process the events quickly. Due to RC oscillator will vary following the voltage and
temperature, PC4/PC5 must be used to extend high-speed oscillator to satisfy the system requirements under
the conditions that need high-precision timing or asynchronous communication, etc..
RC oscillator begins to work after reset, and CPU clock selects the RC oscillator.
HANGZHOU MICROELECTRONICS CO.,LTD
REV:1.1
2008.10.20
Http://www.silan.com.cn
Page 10 of 17
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