Linux Power Optimization Features - Texas Instruments AM335 Series Design Manual

Linux Power Optimization Features

For situations where current or voltage is sourced from the multimeter as opposed to an on-board power
supply, four wire sensing is required to ensure accuracy of the reading, especially for the low currents and
resistances encountered in the power measurement shunts. Even when current or voltage is not sourced
from the multimeter, remote sensing is going to be more accurate than two terminal measurement.
However, if only current or only voltage is being measured, not both, it is generally acceptable to use
local, or two terminal sensing.
3 Linux Power Optimization Features
The previous section outlined the hardware design practices and techniques that can lower power in an
AM335x system. However, system power consumption is highly dependent on how software configures
the underlying hardware. Once the board level optimizations such as DDR and PMIC selection are done,
the rest of the power savings possible is determined by how well the software controls the processor and
the on-die peripherals. For example, scaling the processor speed can have large power implications, not
only does the switching rate increase, but the voltage supply must increase to support the higher
frequency. This means that power consumption is not linear with clock speed, and also implies that if
lower clock speeds are used, the power savings can be significant. In this section, optimization is possible
using the TI Linux Processor SDK v3.0.1 that is discussed, however, many of these concepts are
transferable to another Linux distribution, or even to other operating systems.
In this section configuration to the Kernel, U-boot, and device tree that must be done prior to booting the
device is discussed. The next section illustrates how some runtime configuration choices such as
choosing an operating performance point can have significant impacts on power consumption. The last
segment discusses how to diagnose power consumption issues on a Linux based platform.
3.1 Pre-Deployment Optimizations
This section explains how to enable and disable desired peripherals in the Linux Device Tree, as well as
how to customize and enable features such as dynamic voltage and frequency scaling (DVFS), and the
CPU Idle driver. All these features require modifications to either the Linux kernel, U-boot, or device tree,
and must be performed before the system is powered on. Runtime tweaks are useful for quick
experimentation and discussed in the next section.
3.1.1 Hardware and Software Alignment
For optimal results, the hardware and software must be configured to the same specification to take
advantage of the power saving features designed into the hardware as well as to avoid leakage current.
3.1.1.1 Pinmux Tool
The pinmux tool generates relevant sets of files, header files for AM335x starterware, and pin control
device tree entries. These files contain the necessary data to configure the instantiated pins in the design.
It is still up to the user to define behavior for unused pins.
For inputs ensure that an external pull does not conflict with an internal pull, as it would provide a current
path if they are opposite.
3.1.1.2 Clock Tree Tool
The clock tree tool
allows users to visualize the clock tree, interactively modify the clock tree elements and view the effect on
Power, Reset, and Control Management (PRCM) registers. Additionally, they can use the tool to modify
PRCM registers and view their effects on the clock tree, and track the impact of user defined changes in
the device clock tree. The clock tree tool also allows for import and export of the current clock tree settings
as a text file.
In terms of power optimization, clock tree tool can help determine the register writes required to attain a
certain clocking frequency. Certain peripherals have timing constraints that prevent them from having their
clock frequency reduced, but other peripherals can be configured to run at lower clock speeds, which can
help save power. Determining the necessary Phase Locked Loop (PLL) parameters is also needed when
creating custom Operating Performance Points (OPPs) in the Device Tree.
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AM335x Low Power Design Guide
is a standalone tool that allows for configuration of the AM335x clock tree. The tool
Copyright © 2017, Texas Instruments Incorporated
SPRAC74A – February 2017 – Revised March 2017
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