Parameters such as list price per unit, energy consumption, performance, form factor, operating system, required peripherals and so on all come into play. The final selection is entirely dependent on what is needed for that specific project.
Until just a year or two ago, it was easy to choose a CPU for embedded systems, but with Intel’s return to the embedded scene with the Atom, the ARM processors now face strong competition on the market.
Energy consumption and performance
In spite of Intel’s revival, ARM remains the preferred CPU architecture for a wide range of embedded systems and devices. In particular, ARM has held on tenaciously in consumer electronics such as mobile phones and PDAs, with a market share of over 90 per cent, and ARM processors are also starting to appear in small netbook devices.
“The reason for ARM’s dominance in small electronic products is quite simply the matter of energy consumption versus performance,” explains Gilad Mizrahi, hardware development manager for Data Respons Denmark. “An ARM-based system typically uses as little as 2 watts, whereas a fully optimised Intel Atom solution uses 5 or 6 watts. This is incredibly significant for the battery lifetime in small mobile devices.”
Another reason why ARM is so popular in small electronic devices is that one can reduce the solution’s total size much more with the ARM architecture than with x86 CPUs at present.
Form factor, cost per unit and price
With the Intel Atom, the x86 architecture is now starting to catch up with ARM’s form factor. But there are still some major differences that count in favour of the ARM architecture.
“In terms of dimensions, the ARM architecture still has a clear advantage over, say, the Intel Atom. This is because ARM has developed the processor core and licensed it to other chip manufacturers. These then add various features to their specific solutions. This means that we can find exactly the chip with the features we need without having to make adjustments around the CPU, which allows us to reduce the solution’s total size considerably,” says Mizrahi.
Because multiple features are added directly to the ARM MCU, the solution’s total unit cost is reduced. And if there is considerable pressure on price, dimensions and unit cost for a particular project, the ARM core can be integrated into an FPGA so that a proper system-on-chip can be developed with a CPU, DSP and other peripheral components on one chip.
With a reduced unit cost, ARM offers a considerably cheaper total solution than the x86 architecture can at present, and as price is one of the most important criteria when choosing a CPU, typically an ARM solution is the preferred choice.
On the other hand, if there are particular requirements regarding performance, easy integration with peripherals such as displays, scalability or a need to use commercial OSes such as Windows XP or Windows 7, the outcome may be quite different.
Standard, scalable and easy integration
One of the major advantages of the x86 architecture highlighted by Mizrahi is that standard commercial software, including Windows XP and now Windows 7, is often designed to run on x86-based CPUs. The same is true for many plug-and-play peripherals such as printers, keyboards, monitors and so on.
“If you’re looking to build a system, and energy consumption and dimensions are not absolutely critical, but performance and easy integration with a wide range of standard peripherals are, an x86 solution is often a good choice. This is why we are now seeing the x86 architecture – and in particular the Intel Atom – taking the lead in standard industrial box PCs,” says Gilad Mizrahi. He goes on to explain that scalability is an important factor in the choice of CPU: “Say you have a system that needs to monitor a certain amount of data, and the Atom processor can do the job, that’s what you use. However, if you need to monitor a much greater amount of data using the same system, the Atom processor can be replaced with a more powerful x86 processor like the Core Duo without there being any need to redevelop the software from scratch.”
It would not be quite so simple with an ARM processor. Every ARM processor is individually developed with specific features, which means that software must be specifically adapted. For this reason it is not so straightforward to scale an ARM solution up or down.
“When we are developing an ARM-based solution, it is important that we know exactly what the embedded solution needs to be able to do and what requirements will be made of it. Then we develop a dedicated solution that does exactly what is required at an optimal unit cost – but the downside is that you lose some of the flexibility that an x86 solution offers,” concludes Mizrahi.