AM335x Power Management User Guide

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Overview

This article provides a description of the example applications under the Power page of the Matrix application that comes with the Sitara SDK.  This page is labled "Power" in the top-level Matrix GUI. The location of the Power icon on the main Matrix app list may be different than shown here, depending on screen size. (Screen shots from SDK 06.00)



matrix_app_launcher.png


PLEASE NOTE: cpufreq may cause I2C lockups on AM335x EVM boards. Beaglebone is not affected. This is a known issue related to the CPLD firmware. If the CPLD firmware on your EVM is detected to be the wrong version, the Matrix application output will inform you of the version mismatch and continue.
Once updated CPLD firmware is available, this documentation will be updated to teach users how to upgrade their CPLD if necessary/desired. This procedure will require an Altera programming pod.


Power Examples

Several power examples exist to provide users the ability to dynamically switch the CPU clock frequency. The frequencies shown are those available for your system. Upon making a selection, you will be presented a confirmation page. The readout number "BogoMIPS" will confirm the new clock frequency. Please note that the frequency will read out with a slight margin compare to the intended frequency. For example, if you select 1GHz, you may see a number like 998.84 (in MHz). This is normal. After reviewing the confirmation page, press the Close button to return to normal Matrix operation.

Other power examples are provided which may be useful for power management developers and power users. These have been included in Matrix in part to make users aware that these valuable debugging tools exist, in addition to the convenience of executing each application from the GUI. In depth descriptions for each application follow. Similar descriptions are also given via description pages in Matrix, which will be displayed when clicking the button. Where appropriate, the documentation will point out the corresponding command line operation.

The Suspend/Resume button demonstrates the ability to put the machine into a suspended state. See below for complete documentation of this feature.

Please note that the order of applications which appear on your screen may differ from the picture below, due to devices with different screen sizes, and differences between different versions of Matrix. Screen shot is from SDK 06.00.



AM335x_Power_screen.png


Set Frequency

This command opens up another screen from which you choose the frequency based on the available frequencies on the board. Here is a picture of the screen you will see:



AM335x_matrix_set_frequency.png


The following are the Linux command line equivalents for selecting the operating frequency. Please note that changing the frequency also changes the MPU voltage accordingly. The commands are part of the "cpufreq" kernel interface for selecting the OPP (operating performance point). Cpufreq provides an opportunity to save power by adjusting/scaling voltage and frequency based on the current cpu load.

 (command line equivalent)
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
(view options, select one for next step)
echo <selected frequency, in KHz> > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
cat /proc/cpuinfo 

Suspend/Resume

 (command line equivalent)
mkdir /debug
mount -t debugfs debugfs /debug
echo mem > /sys/power/state 

This command sequence will put the platform into suspend mode. The final command initiates the suspend.

IMPORTANT NOTE: When running this from Matrix, the system will only properly resume if the user sends a keypress to the UART. If the user presses the touchscreen or a button on the EVM, resume will not complete normally. This issue will be fixed in a future release. If you run these commands from the terminal - all of the normal wakeup events (UART keypress, touchscreen press, EVM keypad press) will operate correctly.

SmartReflex

SmartReflex is an active power management technique which optimizes voltage based on silicon process ("hot" vs. "cold" silicon), temperature, and silicon degradation effects. In most cases, SmartReflex provides significant power savings by lowering operating voltage.

On AM335x, SmartReflex is enabled by default in Sitara SDK releases since 05.05.00.00. Please note that the kernel configuration menu presents two options: "AM33XX SmartReflex support" and "SmartReflex support". For AM33XX SmartReflex, you must select "AM33XX SmartReflex support", and ensure that the "SmartReflex support" option is disabled. The latter option is intended for AM37x and OMAP3 class devices.

The SmartReflex driver requires the use of either the TPS65217 or TPS65910 PMIC. Furthermore, SmartReflex is currently supported only on the ZCZ package. Please note that SmartReflex may not operate on AM335x sample devices which were not programmed with voltage targets. To disable SmartReflex, type the following commands at the target terminal:

mkdir /debug
mount -t debugfs debugfs /debug
cd /debug/smartreflex   ==> NOTE: You may not see 'smartreflex' node if you have early silicon.  In this case SmartReflex operation is not possible.
echo 0 > autocomp
(Performing "echo 1 > autocomp" will re-enable SmartReflex)

On AM335x, to compile SmartReflex support out of the kernel, follow this procedure to modify the kernel configuration:

cd <kernel source directory>
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- menuconfig
<the menuconfig interface should appear>
Select "System Type"
Select "TI OMAP Common Features"
Deselect "AM33xx SmartReflex Support"
Select "Exit" until you are prompted to save the configuration changes, and save them.
Rebuild the kernel.

Dynamic Frequency Scaling

This feature, which can be enabled via patch to the SDK, enables scaling frequency INDEPENDENT of voltage. It is also referred to as DFS (as in DVFS without the 'V').

Media:0001-Introduce-dynamic-frequency-scaling.patch

Discussion

Certain systems are unable to scale voltage, either because they employ a fixed voltage regulator, or use the ZCE package of AM335x. Without being able to scale voltage, the power savings enabled via DVFS are lost. This is because the current version of the omap-cpufreq driver requires a valid MPU voltage regulator in order to operate. The purpose of this DFS feature is to enable additional power savings for systems with these sort of limitations.

When using the ZCE package of AM335x, the CORE and MPU voltage domains are tied together. Due to Advisory 1.0.22, you are not allowed to dynamically modify the CORE frequency/voltage because the EMIF cannot support it. However, to achieve maximum power savings, it may still be desirable to use a PMIC which supports dynamic voltage scaling, in order to use Adaptive Voltage Scaling (aka SmartReflex or AVS). This implementation of DFS does not affect the ability of AVS to optimize the voltage and save additional power.

Using the patch

The patch presented here has been developed for and tested on the SDK 05.07. It modifies the omap-cpufreq driver to operate without requiring a valid MPU voltage regulator. From a user perspective, changing frequency via cpufreq is accomplished with exactly the same commands as typical DVFS. For example, switching to 300 MHz is accomplished with the following command:

echo 300000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed

After applying the patch, the user must modify the kernel defconfig in order to enable the DFS feature. You should also configure the "Maximum supported DFS voltage" (shown below) to whatever the fixed voltage level is for your system, in microvolts. For example, use the value 1100000 to signify 1.1V. The software will use the voltage level that you specify to automatically disable any Operating Performance Points (OPPs) which have voltages above that level.

On AM335x, first apply the patch, then follow this procedure to modify the kernel configuration:

cd <kernel source directory>
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- menuconfig
<the menuconfig interface should appear>
Select "System Type"
Select "TI OMAP Common Features"
Select "Dynamic Frequency Scaling"
Configure "Maximum supported DFS voltage (in microvolts)" (default is 1100000, or 1.1V)
Select "Exit" until you are prompted to save the configuration changes, and save them.
Rebuild the kernel.

Power Savings

  • Tested on a rev 1.2 EVM, running Linux at idle.
  • The delta between power consumption at 300MHz and 600MHz, with voltage unchanged, is approximately 75mW.

Static CORE OPP 50

Configuring the AM335x system to CORE OPP50 frequency and voltage is an advanced power savings method that can be used, provided that you understand the tradeoffs involved.

This patch, which was developed against the u-boot source tree from the SDK 05.07, configures the bootloader to statically program the system to CORE OPP50 voltage (0.95V) and frequencies. It also configures the MPU to OPP50 voltage (0.95V) and frequency (300MHz). DDR2 is configured with optimized timings to run at 125MHz.

Apply the following patch to your u-boot source tree and rebuild both MLO and u-boot.img. (Refer to AM335x_U-Boot_User's_Guide#Building_U-Boot)

Media:0001-Static-CORE-OPP50-w-DDR2-125MHz-MPU-300MHz.patch

Caveats

  • According to section 5.5.1 of the AM335x datasheet, operation of the Ethernet MAC and switch (CPSW) is NOT supported for CORE OPP50.
  • Note that MPU OPP50 operation is not supported for the 1.0 silicon revision (silicon errata Advisory 1.0.15).
  • Also be aware of Advisory 1.0.24, which states that boot may not be reliable because OPP100 frequencies are used by ROM at OPP50 voltages.
  • DDR2 memory must be used (as on the AM335x EVM up to rev 1.2). DDR2 memory timings must be modified to operate at 125MHz.

Power Savings

  • On an EVM (rev 1.2), active power consumption when Linux is idle for CORE and MPU rails was measured at 150mW. Using the out-of-the-box SDK at OPP100 (MPU and CORE), the comparable figure is 334mW.
  • Further savings are possible by disabling Ethernet drivers in the Linux defconfig. Refer to AM335x_CPSW_(Ethernet)_Driver's_Guide#Driver_Configuration and disable "Ethernet driver support" to acheive additional power savings.

Power Management Reference

Refer to this page for Linux specific information on AM335x devices.

The Power Estimation Tool (PET) provides users the ability to gain insight in to the power consumption of select Sitara processors.

This document discusses the power consumption for common system application usage scenarios for the AM335x ARM® Cortex™-A8 Microprocessors (MPUs).

Standby for AM335x is a inactive (system suspended) power saving mode in which the power savings achieved would be lesser than that achieved through DeepSleep0 mode but with lesser resume latency and additional wake-up sources.