Linux Power Optimization Features
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SPRAC74A–February 2017–Revised March 2017
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AM335x Low Power Design Guide
Multiple messages can be written, one after another, if more than one address needs to be written.
3.1.4 Device Tree Configuration
Device tree is a tree structure with nodes and properties; it is used by Linux as a way of describing the
hardware the Linux kernel is running on without having to modify kernel source code. Nodes can be used
to describe entire platforms such as the AM335x evm, processors, external peripherals, and internal
modules. Each node has a set of key, value pair properties that can be defined by the system designer,
and is interpreted by Linux. Therefore, device tree can be used to describe almost every aspect of the
hardware in the SOC, and as long as Linux has a defined behavior for the device specified, device tree
can be used to control the configuration of hardware on a system. Unused peripherals can be disabled in
the device tree. Certain interfaces may require a different pin configuration when in system sleep or
suspend, these different pin configurations can be controlled through the device tree pinmux. Finally,
OPPs can be defined to enable more clocking speeds or combinations than exist in the default device
tree.
3.1.4.1 Disabling Peripherals in Device Tree
When designing for low power, it is always a good idea to use only the peripherals necessary to support
your specific application. However, simply ignoring the other peripherals on the device is not a good idea
since Processor SDK leaves many peripherals turned on by default in order to showcase the full
functionality of the device. Even when idle, most peripherals consume 2 mW or more simply from being
clocked, thus, it is important to tell Linux to turn off power and clock gate unused peripherals.
In general, all that is needed to turn off a module on the AM335x is to mark its status in device tree as
"disabled". As long as the default reset mode of the module is to be in idle, then the module is never
powered on during the Linux startup sequence. Devices such as USB that do not default to a sleep state
require further steps to turn the module off. For the default power and clocking settings for each module,
see to the device-specific TRM.
Device tree also allows for an include functionality where additional nodes can be described in Device
Tree Source Include files. Be sure to check the am33xx.dtsi for other peripherals that are not described in
the am335x-evm.dts file.
LCD Panel Device Tree Fragment
panel {
compatible = "ti,tilcdc,panel";
status = "okay";
pinctrl-names = "default";
pinctrl-0 = <&lcd_pins_s0>;
panel-info {
...
};
display-timings {
800x480p62 {
...
};
};
};
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