Software Power Management with the OSD335x Family

Published On: July, 11, 2017 By: Neeraj Dantu | Updated: August 22, 2018 by Cathleen Wicks

In a previous article, we discussed the hardware aspects associated with power management and power budgeting for the OSD335x. However, to optimize the power performance of your design you must also fine tune the power usage of the AM335x inside the OSD335x through software. Using firmware to control the power management of the OSD335x leads to significant power savings in some applications. This document introduces software power management techniques for the OSD335x in Linux and provides examples to demonstrate the advantages of these techniques.

A PDF of this Application Note can be found here.

1. Power saving features of AM335x

The AM335x processor inside the OSD335x allows you to control different aspects of its functionality to optimize power consumption for your application. The following list outlines the main features used to minimize power consumption (Source: Linux Power Management User Guide).

1. Clock gating: Involves dynamic enabling/disabling clocks of peripherals to reduce dynamic power consumption. See section 8.1.3.2 in the technical reference manual for more information.
2. Clock domain transitions: Involves enabling/disabling a clock domain (which consists of a group of peripherals fed by clock signals controlled by the same clock manager) to reduce dynamic power consumption. See section 8.1.3.3 in the technical reference manual for more information.
3. Power domain transitions: Involves managing 4 functional power domains of the processor (WAKEUP, MPU, PER and RTC). See section 8.1.4.1 for more information on power domains.
4. Dynamic voltage and frequency scaling: Involves managing the Operational Performance Points (OPPs) of the processor. This is discussed in detail below.
5. Low power modes: Involves using one of the 4 low power modes of the processor based on application to optimize power consumption. See section 8.1.4.3 for more information on power modes.

You can use bare-metal (No OS) programming to utilize all these features and have complete control of power management of the device. However, under Linux, some of these features are leveraged automatically while others are accessible through simple abstractions.  Linux provides constructs that enable you to easily minimize power consumption.  The rest of this article will explore controlling the two most common features, Operational Performance Points, and Low Power Modes, through Linux.  If you are interested in how to control the other features, please see the Technical Reference Manual of AM335x.

2. Operational Performance Points (OPPs)

The TPS65217C Power Management IC (PMIC), inside the OSD335x, supplies the different voltages to each of the many voltage domains inside the AM335x, such as VDD_MPU and VDD_CORE.  These voltages are changed by pragmatically communicating with the PMIC over the I2C interface.

The AM335x also contains multiple clock domains, such as MPU PLL and Core PLL, that operate independently and are fully managed within the SoC.

Operational Performance Points (OPPs) consist of a set of predefined voltage ranges and maximum frequencies.  These OPPs set the operating voltages for the voltage domains and the frequencies for the clock domains of the AM335x System-on-Chip (SoC).  For example, to run the Cortex A8 processor within the AM335x at 1GHz, the VDD_MPU voltage domain must be set between 1.272V and 1.378V.  This operating condition is delineated as OPP “Nitro” for VDD_MPU.  OPPs affect current draw and device lifetime (power-on-hours).  They are discussed in detail in AM335x datasheet, but, specifics for the DDR version and AM335x revision inside the are discussed below.

OPPs are set for two of the AM335x voltage domains, VDD_CORE and VDD_MPU.  Selecting and using the appropriate OPP for your application is critical to optimizing the power consumption of the OSD335x. The different OPPs for the two domains, their voltage ranges, and frequencies of operation are given in the tables below.  (This information can be found in section 5.4 (Operating Performance Points) of the AM335x datasheet and is provided here for convenience).

Using VDD_CORE OPP50 will disable the DDR3L memory present inside the OSD335x.

There are also a valid set of combinations of VDD_CORE and VDD_MPU OPPs. Those are listed in the following table.

2.1 Controlling OPPs in Linux

Linux has a built-in framework for the user to make use of the OPPs. How to use this functionality is demonstrated below.  The Linux image: bone-debian-8.4-lxqt-4gb-armhf-2016-05-13-4gb.img (Beagleboard.org Latest Images) was used on the OSD3358 SBC Reference Design board for the demonstration.

2.1.1 Linux Governors

The Linux construct for controlling OPPs, specifically the VDD_MPU OPP, is CPUfreq governors.  They can be used to change the frequency and voltage of the processor on the fly. They manage the OPPs of the processor while providing a user friendly construct.  For a full explanation of governors see: kernel.org Governors documentation. The following command can be used to view all the available governors.

Each governor uses a different set of criteria to dynamically set the processor to optimize performance.  For example, when the powersave governor is used, the processor is set to the lowest defined frequency. When the ondemand governor is used the frequency is automatically adjusted to match the load on the processor. The governor for the CPU is set by writing its value to the to the scaling_governor file.

The following command sets the active governor to ondemand.

2.1.2 Manually setting OSD335x frequencies

Manually setting the frequency of the processor requires setting the governor to userspace.  This allows the user to switch between pre-defined OPPs.  These OPPs are defined in the device tree which is loaded during boot. To see the frequencies (in kHz) that are supported by the OSD335x look at the scaling_available_frequencies file. The following command can be used to see them:

Now that the available frequencies are known, you can set them using the following command.

This command sets the frequency of the CPU to 720 MHz.  Simply change the 720000 to your desired frequency to make the CPU operate at that frequency.

In addition to setting the frequency, the voltage rails of the processor can also be monitored using a similar approach. The following command returns the output voltage of regulator 2 (VDDS_DDR).

Several CPU frequency statistics can be found by running ‘cpufreq-info’ in command line.

2.1.3 OPPs effect on Power Consumption

Operating performance points can be leveraged to reduce overall current consumption of the board.  Table 4 shows a comparison between the current consumption of the board in different OPPs running the same example application.  The example application involves the processor gathering sensor data to make decisions.

As you can see, the average current consumption of the example application drops as the OPP frequency changes from 1GHz to 300MHz.  One thing to note about the example application is that it is I/O limited and not CPU limited.  This means that the CPU frequency can be reduced without reducing the overall performance of the application.  In the case of a CPU limited application, care needs to be taken that the change in performance from modifying the OPP does not cause real-time deadlines to be missed.

3. Low power modes

Often, the system can be put in a low power mode when there is nothing to do to save power. This is especially useful in battery powered applications. While the AM335x has 4 low powered states, 3 are supported by the OSD335x. The unsupported RTC-Only mode is due to the version of the PMIC inside the SiP (ie the OSD335x uses the TPS65217C). The low power states and their corresponding power consumption are listed in the following table taken from AM335x datasheet.

A list of resources for power management including implementation of various power modes are given below:

1. AM335x Power Management User Guide
2. Linux Core Power Management User’s Guide
3. AM335x Linux Power Management User Guide
4. AM335x Power Management Standby User’s Guide

Note that some of the features in the above resources are specific to TI’s Linux distributions and are not available in all Linux images.  Please make sure your image supports the functions you wish to use.

3.1 Setting Power Modes

Setting processor power modes is straight forward. To see all the available low power states, look at the state file using the following command

While the power state names are not the same as modes listed in Table 5, some of them do map to the processors low power modes. The mem power state corresponds to deepsleep0 state of the processor described in Table 5.  Similarly, the standby power state corresponds to the standby state of the processor. More information on these low power modes can be found in the references.

Depending on the version of the Linux image used, some of the low power modes might not be supported.

To put the processor in the desired sate, just write the power mode you wish to operate in to the state file. This command will put the processor in the standby power state:

Waking the processor up from a low power mode requires an interrupt sent through one of the wakeup sources. Availability of this feature by default depends on the Linux image version on the device. Please check the documentation of the image to see all the wakeup sources and if they are enabled by default.

3.2 Maximizing power on time using low power modes

This section illustrates how power consumption can be optimized in an example application scenario. We are using the OSD3358 SBC Reference Design running bone-debian-8.6-lxqt-4gb-armhf-2016-11-06-4gb (Beagleboard.org Latest Images) as a data collector in this example.

Let us assume the OSD335x SBC reference design is being used to collect, process and write data to the eMMC on board.  The data is being collected over an I2C bus from sensors that have their own power source. After receiving the data, the processor will perform some basic arithmetic operations on it to determine a course of action such as turning one of the GPIO pins on/off. It will then take the action required and write the data to the on board eMMC.

To save power when the board is not collecting any data, it can be put into standby mode as shown earlier. It is setup to wake up using the UART0_RX pin. So, the board has two states of operation. The current consumption of each state and the percentage of total time spent in these states are shown below. Some field experimentation may be required to determine the percentage of time spent in a state.

If the board is being powered by a 3000 mAh battery, the board should be able to run for 31 hours before the battery needs to be changed. This is in comparison to 10.3 hours with no standby mode. This scenario illustrates the important nature of low power modes especially in event driven systems.

4.    Conclusion

Along with hardware power budgeting, power management through software can play a major role in reducing power consumption especially in application driven scenarios. These power savings are achieved through leveraging various features of the PMIC and the AM335x processor inside the OSD335x.

5.    References

1. TPS65217C datasheet
2. TL5209 LDO datasheet
3. AM335x datasheet
4. Powering the AM335x with TPS65217C
5. Processor SDK Linux kernel performance guide
6. AM335x power consumption summary
7. AM335x Power Management User Guide
8. Linux Core Power Management User’s guide
9. AM335x Power Management User Guide
10. AM335x Power Management Standby User’s Guide