Samsung C8278X ユーザーズマニュアル

Data loading in the oscillator control register, OSCCON, determine whether a main or a sub clock is selected as 
the CPU clock, and also how this frequency is to be divided by setting CLKCON. This makes it possible to switch 
dynamically between main and sub clocks and to modify operating frequencies. 

OSCCON.0 select the main clock (fx) or the sub clock (fxt) for the CPU clock. OSCCON.3 start or stop main clock 
oscillation, and OSCCON.2 start or stop sub clock oscillation. CLKCON.4

−.3 control the frequency divider circuit, 

and divide the selected fxx clock by 1, 2, 8, 16. 

For example, you are using the default CPU clock (normal operating mode and a main clock of fx/16) and you 
want to switch from the fx clock to a sub clock and to stop the main clock. To do this, you need to set CLKCON.4-
.3 to "11", OSCCON.0 to “1”, and OSCCON.3 to “1” simultaneously. This switches the clock from fx to fxt and 
stops main clock oscillation. 

The following steps must be taken to switch from a sub clock to the main clock: first, set OSCCON.3 to “0” to 
enable main clock oscillation. Then, after a certain number of machine cycles has elapsed, select the main clock 
by setting OSCCON.0 to “0”.   

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⎯ Switching the CPU clock 

1.  This example shows how to change from the main clock to the sub clock: 

 MA2SUB 

LD 

OSCCON,#01H 

; 

Switches to the sub clock 

 

 

 

 

;  Stop the main clock oscillation 

 RET 

2.    This example shows how to change from sub clock to main clock: 

 

SUB2MA 

AND 

OSCCON,#07H 

;  Start the main clock oscillation 

 

 

CALL 

DLY16 

;  Delay 16 ms 

 

 

AND 

OSCCON,#06H 

;  Switch to the main clock 

  

RET 

 DLY16  SRP 

#0C0H 

  

LD 

R0,#20H 

 DEL 

NOP 

  

DJNZ  R0,DEL 

  

RET