Samsung KS57C2308 manual Clock Control Registers, Using a Subsystem Clock

OSCILLATOR CIRCUITS KS57C2308/P2308/C2316/P2316

Clock Control Registers

When the system clock mode control register, SCMOD, and the power control register, PCON, are both cleared to zero after RESET, the normal CPU operating mode is enabled, a main system clock of fx/64 is selected, and main system clock oscillation is initiated.

PCON is used to select normal CPU operating mode or one of two power-down modes — stop or idle. Bits 3 and 2 of the PCON register can be manipulated by a STOP or IDLE instruction to engage stop or idle power-down mode.

The system clock mode control register, SCMOD, lets you select the main system clock (fx) or a subsystem clock (fxt) as the CPU clock and to start (or stop) main or sub system clock oscillation. The resulting clock source, either main system clock or subsystem clock, is referred to as the CPU clock.

The main system clock is selected and oscillation started when all SCMOD bits are cleared to logic zero. By setting SCMOD.3, SCMOD.2 and SCMOD.0 to different values, CPU can operate in a subsystem clock source and start or stop main or sub system clock oscillation. To stop main system clock oscillation, you must use the STOP instruction (assuming the main system clock is selected) or manipulate SCMOD.3 to “1” (assuming the sub system clock is selected).

The main system clock frequencies can be divided by 4, 8, or 64 and a subsystem clock frequencies can only be divided by 4. By manipulating PCON bits 1 and 0, you select one of the following frequencies as CPU clock.

fx/4, fxt/4, fx/8, fx/64

Using a Subsystem Clock

If a subsystem clock is being used as the selected system clock, the idle power-down mode can be initiated by executing an IDLE instruction. The subsystem clock can be stopped by setting SCMOD.2 to “1”.

The watch timer, buzzer and LCD display operate normally with a subsystem clock source, since they operate at very slow speeds (122 µs at 32.768 kHz) and with very low power consumption.

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