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RTC-Only Mode
195
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Power Management
9.7
RTC-Only Mode
In real-time clock (RTC)-only mode, the RTC is powered on and the rest of the device is completely
powered off (all supplies except the RTC supply are removed). In this mode, the RTC is fully functional
and keeps track of date, hours, minutes, and seconds. In this mode, the overall power consumption would
be significantly lower, as voltage from the rest of the core and I/O logic can be completely removed,
eliminating most of the active and static power of the device, except for what is consumed by the RTC
module, running at 32 kHz.
NOTE:
To put the device in RTC-only mode, there is no software control sequence. You can put the
device in the RTC-only mode by removing the power supply from all core and I/O logic,
except for the RTC core logic supply (RTC_CVDD). During wake up, all power sequencing
requirements described in the device-specific data manual must be followed.
Some limitations apply in the RTC-only mode. First, the RTC_ALARM pin is not available as an option for
use as a control to signal an external power supply to reapply power to the rest of the device. This is
because the RTC_ALARM pin is powered by the I/O supply that is powered down in RTC-only mode.
Second, in RTC-only mode, only the RTC register contents are preserved, all other internal memory and
register contents are lost. Mobile DDR and DDR2 contents can be preserved through the use of self-
refresh (see
). However, software must be in place to restore the context of the device, for
example, reinitialize internal registers, setup cache memory configurations, interrupt vectors, etc.
9.8
Dynamic Voltage and Frequency Scaling (DVFS)
Dynamic voltage and frequency scaling (DVFS) consists of minimizing the idle time of the system. The
DVFS technique uses dynamic selection of the optimal frequency and voltage to allow a task to be
performed in the required amount of time. This reduces the total power consumption of the device while
still meeting task requirements. DVFS requires control over the clock frequency and the operating voltage
of the device elements. By intelligently switching these elements to their optimal operating points, it is
possible to minimize the power consumption of the device for a given task.
For reasons related to the device (clock architecture, process, etc.), DVFS is used only for a few discrete
steps, not over a continuum of voltage and frequency values. Each step, or operating performance point
(OPP), is composed of a voltage and frequency pair. For an OPP, the frequency corresponds to the
maximum frequency allowed at a voltage, or reciprocally; the voltage corresponds to the minimum voltage
allowed for a frequency. See your device data manual for a list of the OPPs supported by the device.
When applying DVFS, a processor or system always runs at the lowest OPP that meets the performance
requirement at a given time. You determine the optimal OPP for a given task and then switch to that OPP
to save power.
9.8.1 Frequency Scaling Considerations
The operating frequency of the device is controlled through its two PLL controllers (PLLC0 and PLLC1).
Through a series of multipliers and dividers you can change the frequencies of various clocks throughout
the device. See the
Device Clocking
chapter for information on the clock architecture of the device and
see the
Phase-Locked Loop Controller (PLLC)
chapter for information on the PLL controllers. A few things
must be noted when changing the various internal frequencies of the device:
•
Changing the SYSCLK frequency
The PLL_VCO (PLLOUT) frequency can be programmed through a PLL multiplier. A series of dividers
divide PLLOUT to generate the various device SYSCLKs.
To change the SYSCLK frequency you can change the PLL multiplier or you can change the SYSCLK
divider ratio. When changing the PLL multiplier, you must put the PLL controller in bypass mode while
the PLL multiplier value is modified and a lock on the new frequency is reached. The lock time is given
in the device data manual. When changing the divider ratios it is not required to put the PLL controller
in bypass mode.
Changing the SYSCLK frequency through the dividers is faster as there is no need to reprogram the
PLL. However, the SYSCLK frequency will depend solely on the divider ratios used.