Samsung KS57C2308 manual Elapsed Machine Cycles During CPU Clock Switch

KS57C2308/P2308/C2316/P2316OSCILLATOR CIRCUITS

Table 6-6. Elapsed Machine Cycles During CPU Clock Switch

	AFTER			SCMOD.0 = 0				SCMOD.0 = 1
BEFORE		PCON.1 = 0	PCON.0 = 0	PCON.1 = 1	PCON.0 = 0	PCON.1 = 1	PCON.0 = 1
	PCON.1 = 0	N/A		1 MACHINE CYCLE		1 MACHINE CYCLE
	PCON.0 = 0
SCMOD.0 = 0	PCON.1 = 1	8 MACHINE CYCLES		N/A		8 MACHINE CYCLES		N/A
	PCON.0 = 0
	PCON.1 = 1	16 MACHINE CYCLES		16 MACHINE CYCLES		N/A		fx/4fxt
								MACHINE
								CYCLE
	PCON.0 = 1
SCMOD.0 = 1		N/A		N/A		fx/4fxt (M/C)		N/A

NOTES:

1.Even if oscillation is stopped by setting SCMOD.3 during main system clock operation, the stop mode is not entered.

2.Since the XIN input is connected internally to VSS to avoid current leakage due to the crystal oscillator in stop mode, do not set SCMOD.3 to "1" or STOP instruction when an external clock is used as the main system clock.

3.When the system clock is switched to the subsystem clock, it is necessary to disable any interrupts which may occur during the time intervals shown in Table 6-6.

4.“N/A” means “not available”.

5.fx: Main–system clock, fxt: Sub–system clock, M/C: Machine Cycle. When fx is 4.19 MHz, and fxt is 32.768 kHz.

+PROGRAMMING TIP — Switching Between Main System and Subsystem Clock

1. Switch from the main system clock to the subsystem clock:

MA2SUB	BITS	SCMOD.0	; Switches to subsystem clock
	CALL	DLY80	; Delay 80 machine cycles
	BITS	SCMOD.3	; Stop the main system clock
	RET
DLY80	LD	A,#0FH
DEL1	NOP
	NOP
	DECS	A
	JR	DEL1
	RET

2. Switch from the subsystem clock to the main system clock:

SUB2MA BITR	SCMOD.3	; Start main system clock oscillation
CALL	DLY80	; Delay 80 machine cycles
CALL	DLY80	;	Delay 80 machine cycles
BITR	SCMOD.0	;	Switch to main system clock
RET

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