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Dynamic Voltage and Frequency Scaling (DVFS)
196
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Power Management
•
SYSCLK domain fixed ratios
Certain SYSCLK domains need to operate at a fixed ratio with respect to the ARM clock. Care should
be taken to ensure that these fixed ratios are maintained. For additional details, see the
Device
Clocking
chapter.
•
PLLC0 bypass clock
When switching the PLL multiplier, the PLL controller must be placed in bypass mode. Bypassing the
PLL sends a bypass clock instead of the PLL VCO output (PLLOUT) to the system clock dividers of
the PLL controller.
For PLLC0 the bypass clock is selected from either the PLL reference clock (OSCIN) or
PLL1_SYSCLK3. For PLLC1, the bypass clock is always OSCIN. The OSCIN frequency is typically, at
most, up to 50 MHZ.
You can use the OSCIN bypass mode to reduce the core and module clock frequencies to very low
maintenance levels without using the PLL during periods of very low system activity.
It may be desirable for the bypass clock to not revert to OSCIN in some situations to preserved
bandwidth during frequency scaling transitions. For this reason, the PLLC0 bypass clock can be set to
PLL1_SYSCLK3. This selection is made through the EXTCLKSRC bit in the PLLCTL register of
PLLC0.
•
Peripheral immunity from ARM clock frequency changes
Peripherals that are clocked by the PLL0_AUXCLK are immune to changes in the PLL0 frequency.
The PLL0_AUXCLK is derived from OSCIN.
Peripherals in the ASYNC3 domain are clocked off from either PLL1_SYSCLK2 or PLL0_SYSCLK2.
Furthermore, PLL0_SYSCLK2 must always be /2 of the ARM clock frequency. To keep these
peripherals immune from changes in PLL0 frequency (such as when the ARM frequency is modified),
you can configure the ASYNC3 domain to be clocked from PLL1_SYSCLK2. PLL1 is mainly used to
clock the DDR2/mDDR memory controller.
When peripherals are immune to changes in the ARM clock frequency, their internal clock dividers do
not have to be adjusted for changes in their input clock frequencies.
9.8.2 Voltage Scaling Considerations
The operating voltage of the device must be totally controlled through mechanisms outside the device. I2C
ports on the device can be used to communicate with external power management chips. A few things
must be noted when changing the operating voltage of the device:
•
Voltage ramp rate: The ramp rate of the operating voltage must be observed during operating
performance point (OPP) transitions. See the device data manual for ramp rate specifications.
•
Switching to a lower voltage: When switching to a lower voltage, the maximum operating frequency
changes. Care must be taken such that the maximum operating frequency supported at the new
voltage is not violated. For this reason, it is recommended to change the operating frequency before
switching the operating voltage.
9.9
Deep Sleep Mode
This device supports a Deep Sleep mode where all device clocks are stopped and the on-chip oscillator is
shut down to save power. Registers and memory contents are preserved, thus, upon recovery, the
program may continue from where it left off with minimal overhead involved.
The Deep Sleep mode is initiated when the DEEPSLEEP pin is driven low. The device wakes up from
Deep Sleep mode when the DEEPSLEEP pin is driven high. The DEEPSLEEP pin can be driven by an
external controller or it can be driven internally by the real-time clock (RTC). The RTC method allows for
automatic wake-up at a programmed time.
NOTE:
Due to pin multiplexing, the DEEPSLEEP pin can only be driven by an external controller or
its internal real-time clock (RTC). The DEEPSLEEP pin cannot be driven by both an external
controller and its internal real-time clock at the same time.