Do You Need a Real-Time OS?
During a series of seminars I gave on microcontrollers, someone asked when they would need to use a real-time operating system. I bet engineers and system designers ask that question frequently, so more information will help them answer it. (For the seminar series, visit: http://www.designnews.com/lecture-calendar.asp.)
Basically, real time depends on your reference. The display on my dishwasher counts down from a preset time in minutes as it goes through various cycles. Eventually the display reaches 1 and a minute later the display turns off. But I have watched the display show a 1 for as long as five minutes! The dishwasher does not operate in "real time" and its display serves only to show about how long the dishwasher has to run. This type of control goes under the heading "soft real time" because an extra five minutes isn't something to worry about.
On the other hand, when pilots must change course, they want an immediate response from aircraft hydraulics as they move cockpit controls. A five-minute delay would seem like eternity. The aircraft controller operates in "hard real time," because it must respond within a specified time. This type of control requires a real-time operating system, while the dishwasher does not.
In essence, you must measure the worst-case response of your software and determine if that case meets the "deadline" described in product specifications. Those specs define "real time" for your software and hardware. If this worst-case timing cannot meet the specifications, you need an RTOS, and you might need to look for bottlenecks in your code and try to reduce its worst-case time. Keep in mind that timing depends upon the processor, its speed, the response time of peripherals, and the timing of external events. An RTOS also has its own worst-case response time, so keep that in mind, too.
So how can you measure the worst-case execution times (WCETs) of your code? You can use tools that perform a static analysis of your code, measure code-execution times, or run simulations. The paper, "The Worst-Case Execution Time Problem--Overview of Methods and Survey of Tools," from the ACM Transactions on Embedded Computing Systems, provides much useful information about software that performs these functions and how the analyses work. Find the paper at: http://www.cs.fsu.edu/~whalley/papers/tecs07.pdf.
Rapida Systems published a paper, "RapiTime Explained," which describes how WCET tools operate and the type of information they provide. Find the paper at: http://www.rapitasystems.com/downloads/rapitime_explained_white_paper. According to the company, "RapiTime 3.0 is compatible with virtually every 8, 16, and 32-bit microprocessor on the market. It can also be used with DSPs."
AbsInt also sells a WCET-analysis software package, aiT WCET Analyzers. Learn more at: http://www.absint.com/ait/analysis.htm. In 2010, NASA used the aiT analyzer to demonstrate the absence of timing-related software defects in the unintended acceleration investigation of Toyota vehicles.
The WCET tools can knock a hole in a project budget, but you must balance that cost with the need for real-time performance. So, the answer to whether or not you need a hard real-time OS depends on a thorough quantitative analysis of your system's timing. In many cases, MCU-based designs can use a simple scheduler, while processor-based equipment in large, complex designs demands an RTOS. MCU vendors can tell you which schedulers or RTOSs will run on their chips. Microchip, for example, provides a list of 3rd party RTOS vendors on its Web site. Just search for "RTOS." --Jon Titus
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