Power consumption is a critical factor in choosing a platform for embedded computing in a portable or handheld device. But what if the system plugs into the mains supply? It's a commonly held belief that if a system or device plugs into the mains, then power consumption just isn't an issue. But with the sharply rising cost of electricity, plus the cost to the environment, designers would do well to take power consumption into account for all systems.
Consider a point of sale terminal as an example. Since it's plugged into the mains, it may make sense initially to use a low-end PC as a computing platform. The PC is reasonably inexpensive, and provides a familiar setup that is easy to program. However, it uses 115 Watts of power from the mains. If the terminal were in a petrol station running 24 hours a day, it would cost around £97 in electricity to run a single terminal for a year (Table 1). Generating this much electricity releases 592 kg of CO2 per year into the atmosphere.
To reduce the power consumption in a smaller form factor, the designer may consider using a single board computer. By comparison, a typical single board computer might use 10.5 Watts, equivalent to £8.91 of electricity or 54 kg of CO2 emissions per year. A single board computer (SBC) is an off-the-shelf solution that is based on a PC processor, with standard connectors.
Many applications developed on a single board computer could also be developed using an embedded core module. Modules like this have much lower power consumption, and compact form factors are available which are usually smaller than their SBC counterparts. An embedded core module is not based on a PC processor but has a specially designed ARM 9 processor for embedded applications. It runs at 533 MHz, making it ideal for point of sale terminals. A simple comparison between this module and a couple of typical SBCs is shown in Table 1. The Digi ConnectCore 9M_2443's processor is based on the Samsung S3C2443 ARM9 microprocessor and features a multilayer memory bus architecture that allows simultaneous data transfer between processor, memory and peripherals (Fig. 1). This eliminates the traditional bus bandwidth bottlenecks that are common on other platforms. For example, updating graphical information through the LCD controller and retrieving data from memory at the same time can now be done, without compromising overall performance.
Maximum operating power of the ConnectCore 9M_2443 module is just 1.29 Watts. If WiFi is required, the power goes up, but it's still an order of magnitude below the single board computer. Even with continuous use, these modules would use just £1 of electricity in a 1-year period. The CO2 associated with generating this electricity is just 6.6 kg. This Digi module, like many embedded solutions on the market, features a 'suspend' mode which reduces the power consumed when not in use; this mode consumes 0.38 Watts.
The module features a full set of peripheral and multimedia connectivity options including an integrated LCD controller, USB 2.0 support and support for SD Memory cards. It can run Microsoft Windows Embedded CE 6.0 or Linux. Most embedded core modules have better temperature range performance than a single board computer would, especially at the critical high end for industrial applications. The module operates in the range -40 to +85°C. A comparison is in Table 1.
Lower power means lower junction temperature for the processor, which reduces the potential for failure. The result is an increased MTTF (mean time to failure). Also, the board does not need a fan. This is in direct contrast to PCs and single board computers, whose ball-bearing type fans have an expected lifetime of about 60000 hours before they begin to fail. They are prone to a tell-tale ambient noise increase before finally failing, leading to potentially catastrophic overheating of the system.
The downside to using an embedded core module is the design effort required. It is certainly true that creating this bespoke solution requires design of your own baseboard. The resulting solution would be tailored exactly for your application and as we have seen, it would be extremely power-efficient. A good design-in distributor should be able to give you the required technical support, or recommend a design services company that can do it for you.
The cost associated with designing a bespoke baseboard means it is not suited to prototyping or very low volume applications. However, the embedded module hardware itself costs less than a typical single board computer. If the design is going to be produced in sufficient volume, it may be more economical to use an embedded module. The price of a single board computer ranges from approximately £150 to £300. Taking into account the cost of designing a bespoke baseboard, if we compare the module to an SBC from the low end of the price range, the break-even point is reached if more than 600 systems are to be produced. If we considered an SBC at the top end of the price range, the bespoke baseboard is more economical when the volume passes just 100 units.
It is the responsibility of today's engineers to design systems with power consumption in mind, even if there is an apparently 'limitless' supply of energy available from the plug. Using an embedded module can save power, save money and drastically reduce your system's carbon footprint. The break-even point when compared to a PC or single board computer, even when including custom development of a baseboard for a module, is at very low volume, well below 1000 units.
Richard Bethell is Technical Director at Solid State Supplies in Paddock Wood, Kent, UK. www.sssplc.com. www.digi.com