Managing power in embedded applications using dual operating systems

Loc Truong

June 28, 2011

Loc TruongJune 28, 2011

In this product how-to article, TI’s Loc Truong describes how to use inter-processor communication and state machine design to reduce the overall system power in a heterogeneous dual-core system based on the company’s OMAP-L138 C6-Integra DSP + ARM processor running its in-house dual DSP/BIOS RTOS.

Energy consumption is becoming more of a concern as it is receiving an increasingly larger percentage of the overall operating costs. Imagine superstores with lines and lines of check-out lanes, each with a cash register, a credit-card reader, a scanner and a weight measuring station.

It is a waste if these equipments are not designed to be energy efficient with abilities to power down between customers or during non-operating hours. When multiplied by the number of stores, the number of cities and the operating life of the product, the total accumulated portion of the energy bill that could be saved is in the millions of dollars.

Many of today’s operating systems, like Linux, come with power management support. The features have been available on the mainstream kernel since Linux made headways to lower power portable devices like smart phones, tablets and ebook readers. So even though your design is a plugged-in appliance, you can embrace the “go green” initiative from the ground up by taking advantage of the power management features that are already in place and incorporate them.

In this article I will first review power savings techniques available with today’s powered (i.e. plugged-in) system-on-chip (SoC)-based embedded systems and quickly move on to the discussion of how two operating systems (OSes), each with its own power methodologies, can cooperate at the system level to provide power management services.

Chip and system hardware issues
There are two different components to the power equation from a silicon process stand-point: static, sometimes referred to as standby), and active. Static power is affected by leakage mainly and increases with temperature and supply voltage. Since leakage is a natural phenomenon that comes with shrinking process technology, the only way to really eliminate it is to shut that component down. Within the SoC, tactics employed so far include power islands, enabling part of the SoC to completely shut down.

On the other hand, active power, which does increase with supply voltage, but not temperature, depends on chip activity. Strategies here include:

1 - Dynamic voltage and frequency scaling (DVFS), where the voltage and frequency can be dynamically adjusted to adapt to the performance required

2 - Clock domain to gate off unused peripheral

3 - Dynamic power switching (DPS), where software can switch between power modes based on system activity. The “software” is usually part of the operating system

4 - Adaptive voltage scaling (AVS), a closed-loop, hardware and software cooperative strategy to maintain performance while using the minimum voltage needed based on the silicon process and temperature

From the system standpoint, operations needed for power management include the ability to:

1 - Go to standby (user-application- or system-initiated system service)

2 - Hibernate to memory or storage (user-application- or system-initiated system service)

3 - Suspend and resume (user-application-initiated system service)

4 - Transition to different power profiles (user application condition or state, system initiated and controlled)

Power can also be affected how the application code is designed. For example, input/output (I/O) buffers at the pin, memory controllers and especially double data rate (DDR) need to drive current. Unnecessarily moving data in and out of the SoC can waste energy.

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