How important is power consumption in embedded systems? On an individual piece of equipment every watt can be important. Reducing power consumption reduces heat, which may eliminate heat sinks, but more importantly improves component life expectancy. When you consider the effect of large quantities of small numbers the effect can be staggering. For example, there are about 10 million medical instruments using embedded processors shipped every year. A power savings of just one watt per new medical device shipped per year can save 10 megawatts – or 8% of the Indian Queens power station in Cornwall UK.
The impact of one watt can become tremendous – improving automotive fuel economy, reducing the need for new power generating station construction, extending product reliability, reducing pollution, minimizing power supply needs, controlling recycling requirements, and much more.
“Green” is fast becoming a critical factor in embedded systems design. Selecting the right set of design choices can mean lower systems power consumption – which may lead to smaller power supplies and smaller footprints, while maintaining high capability in the embedded application. One aspect of green design is determined by the power budget allotted to the embedded system.
Supplying power to embedded systems may be accomplished several ways:
- Individual mains-supplied power
- Cabled power supply from a central location
- Power over Ethernet (PoE)
- Local power supply (battery, solar, ultra-caps, specialty supplies) for remote systems
Mains-supplied power has few intrinsic technical restrictions concerning power efficiency, and centralized dedicated power cabling is similar. All too often mains powered embedded systems are powered by the lowest cost “wall wart” supply – which is often a lower efficiency supply. But systematic efficiency is of critical importance for devices powered either remotely or via Power over Ethernet (PoE) technology. PoE is important for those devices that can operate in the PoE environment because it defines a maximum permissible power consumption.
PoE embraces sources and devices. Power Sourcing Equipment (PSE) may be a device such as an Ethernet switch that provides power on the Ethernet cable, while Powered Devices (PD) use the power. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W (12.95W at the device), while IEEE 802.3at, offers 34.2 W (25.5 W at the device). Typical embedded systems that can use PoE include:
- Internet Protocol-based Intercom / Paging / Public address systems
- VoIP desk phones
- Master clock synchronized Wall clocks using Network Time Protocol
- Wireless access points
- Wireless Customer Premises Equipment (CPEs) used by wireless ISPs.
The power budget for PoE-based embedded systems can be tight, so saving power is critical.
Establishing the PoE configuration is achieved by a simple negotiation approach:
- PSE tests PD physically using 802.3af
- PSE powers up PD.
- PD sends to PSE: ID as a PD, max power = X, max power requested = X.
- PSE sends to PD: ID as a PSE, max power allowed = X.
- PD may now use the specified amount of power from the PSE.
The negotiation for power in a PoE environment is simple, but the negotiation must take place using less than 13W of power. This is critical for any system designed to operate on PoE – consuming too much power during negotiation will cause the PSE to withhold power from the device since it is a non-compliant device. Vendors of Intel® Atom™ processor-based embedded systems are starting to adopt PoE as one simplifying choice.
ADI Engineering’s (1) Cinnamon Bay Single Board Computer (SBC) incorporates an Intel® Atom™ processor with optional PoE capability. Power draw is determined in large part by the sleep modes employed by the board. Atom processors include a range of low power modes designated by ‘S’ numbers. Cinnamon Bay uses as little as 3.5W running (S0), sleep power of 500mW in S3, and 160mW in S4 and S5. The board is supported by Microsoft® Corporation (2) Embedded Windows operating system. The Cinnamon Bay board includes an optional backup power connection in the event that Ethernet power is interrupted.
IEI Technology Corp. (3) offers the AFL-057A panel PC using the Atom processor with a configuration that will work using PoE. The AFL-057A offers a unique feature that when connected to the company’s Lite-managed PoE Ethernet Switch (IPS-2042TX) , the switch will automatically send an alarm on device failure. The PoE switch will then either reboot or shut down the device depending on the user setting. Users of the PoE can also schedule the time to power on the attached devices with power scheduling settings. Scheduling the operation of AFL-057A attached devices is another approach to minimizing power consumption. The board operates using 12 W of power.
Special function systems calls are often part of an embedded operating system. For example, Microsoft Embedded Windows’ CPUEnterIdle call enables interrupts and puts the CPU, or the hardware platform, into a low-power state. When an interrupt occurs it will wake the system. OEMIdle performs a similar function and is called by the kernel to place the CPU in the idle state when there are no threads ready to run. Entering into other Atom-based low power modes can be accomplished by creating additional system functions to use the very low power options of Atom.
Developing green solutions using the Intel® Atom™ family of components provides alternatives for meeting systems requirements. Systems using the Atom processor with Input/Output Hub (IOH) give the most flexibility for peripheral choice. Third party specialized IOHs from ADI Engineering, Harman, and Oki Semiconductor(4) offer applications-specific IOHs that reduce the logic footprint of their embedded applications. Reducing logic means more than cost reduction. Reduced logic can also mean reductions in power consumption. Since the superfluous logic doesn’t necessarily cause state switch if unused, it doesn’t burn a lot of power, but every reduction helps when dealing with miserly power sources. One design philosophy for cost and power savings is detailed in ADI Enginerring’s “Thin Application” white paper. The ADI design offers another trade-off in systems design - permitting IOH design customization.
Applications software, operating system, and hardware configuration all can have a major impact on green designs. Of these, all classes of software represent a major opportunity for power savings.
Software developed for desktop computers often goes by a philosophy of “making it work” and largely ignoring the opportunity for power savings due to software optimizations. As a simple example, a linear search in a list requires n/2 compares on average, where n is the number of items in the ordered list. But a simple change can reduce the number of compares to log2(n). This in turn means that the optimized software needs to run fewer cycles to achieve the same result. For a list of 128 items the time is reduced by a factor of eight. The CPU can then be idled during the time that would have been taken to search in a brute force method.
Finding fruitful code for optimization in an application can be simplified by code analysis tools. For example, Green Hills Software (5) software tools provide programmers with information about where the software is spending the most amount of time. Wind River Systems(6) offers a similar execution time tool.
What combination of hardware and software choices can you make in your next project to “green up” the design?
1. ADI Engineering, Inc is an Associate member of the Intel Embedded Alliance
2. Microsoft® Corporation is an Associate member of the Intel Embedded Alliance
3. IEI Technology Corp. is an Associate member of the Intel Embedded Alliance
4. Oki Semiconductor Co Ltd is an Affiliate member of the Intel Embedded Alliance
5. Green Hills Software, Inc is an Affiliate member of the Intel Embedded Alliance
6. Wind River Systems is an Associate member of the Intel Embedded Alliance
Roving Reporter (Intel Contractor)
Intel® Embedded Alliance