Samsung KS57C2308 manual System Clock Mode Register Scmod, FB7H SCMOD.3 SCMOD.2 SCMOD.0 Scmod

KS57C2308/P2308/C2316/P2316 OSCILLATOR CIRCUITS

SYSTEM CLOCK MODE REGISTER (SCMOD)

The system clock mode register, SCMOD, is a 4-bit register that is used to select the CPU clock and to control main and sub-system clock oscillation. SCMOD is mapped to the RAM address FB7H.

When main system clock is used as clock source, main system clock oscillation can be stopped by STOP instruction or setting SCMOD.3 (not recommended).

When the clock source is subsystem clock, main system clock oscillation is stopped by setting SCMOD.3. SCMOD.0, SCMOD2, and SCMOD.3 cannot be simultaneously modified. Sub-oscillation goes into stop mode only by SCMOD.2. PCON which revokes stop mode cannot stop the sub-oscillation. The stop of sub-oscillation is released only by reset.

RESET clears all SCMOD values to logic zero, selecting the main system clock (fx) as the CPU clock and starting clock oscillation. The reset value of the SCMOD is 0.

SCMOD.3, SCMOD.2, and SCMOD.0 bits can be manipulated by 1-bit write instructions (In other words, SCMOD.0, SCMOD.2, and SCMOD.3 cannot be modified simultaneously by a 4-bit write). Bit 1 is always logic zero.

FB7H

SCMOD.3

SCMOD.2

"0"

SCMOD.0

SCMOD

A subsystem clock (fxt) can be selected as the system clock by manipulating the SCMOD.3 and SCMOD.0 bit settings. If SCMOD.3 = "0" and SCMOD.0 = "1", the subsystem clock is selected and main system clock oscillation continues. If SCMOD.3 = "1" and SCMOD.0 = "1", fxt is selected, but main system clock oscillation stops.

If you have selected fx as the CPU clock, setting SCMOD.3 to "1" will stop main system clock oscillation. But this mode must not be used. To stop main system clock oscillation safely, main oscillation clock should be stopped only by a STOP instruction in main system clock mode.

Table 6-3. System Clock Mode Register (SCMOD) Organization

SCMOD Register Bit Settings			Resulting Clock Selection
SCMOD.3	SCMOD.2	SCMOD.0	fx Oscillation	fxt Oscillation	CPU Clock (note)
0	0	0	On	On	fx
0	1	0	On	Off	fx
0	0	1	On	On	fxt
1	0	1	Off	On	fxt

NOTE: CPU clock is selected by PCON register settings.

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