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Embedded Linux Applications: An Overview

By Darrick Addison
2005-04-30

Real-Time Embedded Linux Applications
One of the most important issues with embedded systems is the need for a real-time operating system. The definition of real-time here varies quite a bit. To some people, real-time means responding to an event in the one-microsecond range, to others it is 50 milliseconds. The hardness of real-time also varies quite a bit. Some systems need hard real-time response, with short deterministic response latencies to events. However, on many systems, when analyzed closely, we see a response time requirement that is actually near real-time. Often the real-time requirement is a tradeoff of time and buffer space. With memory getting cheaper, and CPUs getting faster, near real-time is now more typical than hard real-time and many commercial operating systems that claim to be real-time are far from being hard real-time. Usually, when you get into the detailed design of these systems, there are warnings that the drivers' interrupts and applications must be very carefully designed in order to meet real-time requirements.

RT-Linux (Linux with real-time extensions) contains time critical functions to provide precise control over interrupt handling, through the use of an interrupt manager, and does a good job of making sure that critical interrupts get executed when needed. The hardness of this approach depends mostly on the CPU interrupt structure and context-switch hardware support. This approach is sufficient for a large range of real-time requirements. Even without the real-time extensions, Linux does pretty well at keeping up with multiple streams of events. For example, a Linux PC system on a low end Pentium is able to keep multiple 10BaseT interfaces executing effectively, while simultaneously running character-level serial ports at a full 56KBPS without losing any data.

Some real-time hardware and software Linux APIs to consider are RTLinux, RTAI, EL, and Linux-SRT. RTLinux is a hard real-time Linux API originally developed at the New Mexico Institute of Technology. RTAI (DIAPM) is a spin-off of the RTLinux real-time API that was developed by programmers at the Department of Aerospace Engineering, Polytechnic Politecnico di Milano (DIAPM). EL/IX is a proposed POSIX-based hard real-time Linux API being promoted by Red Hat. And Linux-SRT is a soft real-time alternative to real-time APIs, which provides performance-enhancing capabilities to any Linux program without requiring that the program be modified or recompiled.

See the Resources section later in this article for information on the above and for some Web sites offering different flavors of software extensions, development tools, support, and training courses for the standard Linux operating system.

Short deterministic response latencies
Some real-time embedded systems need to respond quickly to external events in order to accomplish a specific task. A custom microcontroller embedded inside a missile, for example, needs to respond quickly to external events such as moving targets, weather, humans, etc., before instructing the missile to target a specific object in its surrounding environment. Short deterministic response latencies mean that the embedded system can determine the time it will take to respond to an external event.



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First published by IBM DeveloperWorks
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