Power Management
15-39
Clock Generation and System Reset Management
The OMAP5910 device exits from the power-saving modes by means of a
reset or any interrupt (with or without additional external control from the
CHIP_nWAKEUP pin). A wake-up interrupt must be enabled (not masked off)
to bring the OMAP5910 device out of the power-saving mode.
15.3.6 ULPD Power Management State Machine
The power management function of the ultralow-power module (ULPD),
handles the high-frequency oscillator on/off sequences. It is a state machine
that can stop the oscillator and restart it on a wake-up signal. Set-up times of
the oscillator are taken into account in order to stop/restart internal clocks in
a clean manner. The ULPD state-machine uses the 32-kHz clock.
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The ULPD DPLL and APLL for the 48-MHz USB clock is handled in the
ULPD module.
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ULPD DPLL is a x4 digital PLL.
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APLL is a x48 analog PLL. The input clock ref is 1 MHz, based on either
a 12-MHz system clock divided by 12 or optionally on a 13-MHz system
clock divided by 13.
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The switch between DPLL and APLL is controlled by software through a
TIPB register of ULPD.
15.3.6.1
Gauging the 32-kHz Oscillator
As the 32-kHz oscillator exact frequency is unknown, it can be gauged by
comparing the 32-kHz oscillator with a high-frequency clock (12-MHz oscilla-
tor, ULPD DPLL, or external clock) during any active period. Gauging is only
necessary in specific applications where it is important to know the exact
frequency of the 32-kHz oscillator.
There is a software limitation: the counter is not resynchronized on the TIPB
strobe. Therefore, the value is not readable while the counter is running (when
gauging is enabled). You must first disable the gauging (GAUG-
ING_CTRL_REG[0] = 0), then read the high-frequency counter and the
32-kHz counter value.