NXP Semiconductors MC9S08PA4, Reference Manual

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/* the following code segment demonstrates initializing external oscillator */
/* supposing external 32768Hz crystal is installed in low power mode */
ICS_OSCSC = 0xA0; /* low-range, low-power, oscillator required, ERCLK enabled in stop mode */
while (ICS_OSCSC_OSCINIT == 0); /* waiting until oscillator is ready */
8.4 1 kHz low-power oscillator (LPO)
The 1 kHz low-power oscillator acts as a standalone low-frequency clock source in all
run, wait, and stop3 modes.
8.5 Peripheral clock gating
This device includes a clock gating system to manage the bus clock sources to the
individual peripherals. Using this system, the user can enable or disable the bus clock to
each of the peripherals at the clock source, eliminating unnecessary clocks to peripherals
that are not in use, thereby reducing the overall run and wait mode currents.
Out of reset, all peripheral clocks will be enabled. For lowest possible run wait currents,
user software should disable the clock source to any peripheral not in use. The actual
clock will be enabled or disabled immediately following the write to the Clock Gating
Control registers (SCG_Cx). Any peripheral with a gated clock cannot be used unless its
clock is enabled. Writing to the registers of a peripheral with a disabled clock has no
effect.
Note
User software should disable the peripheral before disabling the
clocks to the peripheral. When clocks are re-enabled to a
peripheral, the peripheral registers need to be re-initialized by
user software.
In stop modes, the bus clock is disabled for all gated peripherals, regardless of the setting
in SCG_Cx registers.
8.6 ICS control registers
1 kHz low-power oscillator (LPO)
MC9S08PA4 Reference Manual, Rev. 5, 08/2017
166 NXP Semiconductors