SN8F2270B Series
USB 2.0 Low-Speed 8-Bit Micro-Controller
4.5 SYSTEM LOW CLOCK
The system low clock source is the internal low-speed oscillator built in the micro-controller. The low-speed oscillator
uses RC type oscillator circuit. The frequency is affected by the voltage and temperature of the system. In common
condition, the frequency of the RC oscillator is about 24KHz.
The internal low RC supports watchdog clock source and system slow mode controlled by CLKMD.
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Flosc = Internal low RC oscillator (24KHz).
Slow mode Fcpu = Flosc / 4
There are two conditions to stop internal low RC. One is power down mode, and the other is green mode of 24KHz
mode and watchdog disable. If system is in 24KHz mode and watchdog disable, only 24KHz oscillator actives and
system is under low power consumption.
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Example: Stop internal low-speed oscillator by power down mode.
B0BSET
FCPUM0
; To stop external high-speed oscillator and internal low-speed
; oscillator called power down mode (sleep mode).
Note: The internal low-speed clock can’t be turned off individually. It is controlled by CPUM0, CPUM1 (24
KHz, watchdog disable) bits of OSCM register.
4.5.1 SYSTEM CLOCK MEASUREMENT
Under design period, the users can measure system clock speed by software instruction cycle (Fcpu). This way is
useful in RC mode.
Example: Fcpu instruction cycle of external oscillator.
B0BSET
P0M.0
; Set P0.0 to be output mode for outputting Fcpu toggle signal.
@@:
B0BSET
B0BCLR
JMP
P0.0
P0.0
@B
; Output Fcpu toggle signal in low-speed clock mode.
; Measure the Fcpu frequency by oscilloscope.
Note: Do not measure the RC frequency directly from XIN; the probe impendence will affect the RC
frequency.
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