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CLOCK CIRCUITS S3C9228/P9228
7-6
SWITCHING THE CPU CLOCK
Data loadings 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 system 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, or 16.
For example, you are using the default system 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 OSCCON.0
to "1", take a delay, and OSCCON.3 to "1" sequently. 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 system clock oscillation. Then, after a certain number of machine cycles has elapsed, select the
main clock by setting OSCCON.0 to "0".
++PROGRAMMING TIP — Switching the CPU clock
1. This example shows how to change from the main clock to the sub clock:
MA2SUB OR OSCCON,#01H ; Switches to the sub clock
CALL DLY16 ; Delay 16ms
OR OSCCON,#08H ; Stop the main clock oscillation
RET
2. This example shows how to change from sub clock to main clock:
SUB2MA AND OSCCON,#0F7H ; Start the main clock oscillation
CALL DLY16 ; Delay 16 ms
AND OSCCON,#0FEH ; Switch to the main clock
RET
DLY16 LD R0,#20H
DEL NOP
DEC R0
JR NZ,DEL
RET