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Shawn Schaffert’s ratcheting scheduler

Real Time Operating Systems?

Even though this project is about a smart home dashboard that doesn’t necessarily have any time-critical tasks, it very easily could. What if a cloud covered the solar panels, and I wanted to dim the lights in response. I don’t want my software to wait for all the temperature sensors to finish updated before changing the lighting. To avoid this potential problem, I want a “Real Time Operating System” for my project.

Systems using embedded controllers are often characterized by the need to exhibit real-time computing behavior. It’s important to understand what this means and how it affects the choice of software to use in a project. No one would argue that computers have been getting faster and faster over the years. Hard drive, memory, video card, and motherboard speeds and clock rates have been steadily increasing with each new generation of hardware. This has meant that the user can run complex applications without having to wait unreasonable amounts of time, and that simple applications perform their functions in vanishingly small time increments. For most desktop applications, small delays in program execution are unnoticeable and insignificant. But what if part of your application monitors the emergency-stop button for your home-built CNC milling machine? You need the emergency-stop button to shut the machine down within a guaranteed amount of time. Not having this determinism could create a very dangerous situation if, say, a background task like playing music or saving a file have momentarily taken over the computer bandwidth. You press and release the emergency-stop and watch in horror as your mill bit keeps going right through your part.

It doesn’t take a powerful or fast CPU to give a timely response to a single bit change in a peripheral I/O register. Anyone who has programmed a port interrupt in a simple 8-bit microcontroller can figure out how long it will take for the 8-bit processor to capture the I/O interrupt and call the code to respond to the event. With PC operating systems like OSX, Windows, and Linux, there is some ability to implement “real-time” threads, but this capability is often implemented by overriding the operating system’s normal behavior by using an operating system add-on to achieve this capability. These operating systems were designed to be effective general purpose tools for running a wide variety of applications and interacting with numerous third-party hardware devices, but they were not designed to have real-time, deterministic behavior.

A true real-time operating system (RTOS) is designed with features such that a program can be split into tasks which run on a fixed schedule, and asynchronous events, such as the CNC mill emergency-stop button press, will have a guaranteed maximum latency before they execute. You can write a program to toggle one I/O pin at 5 kHz and another at 1 kHz with minimal jitter, or program the emergency stop button response to shut off your CNC mill motors within a guaranteed 1 millisecond.

Check out a list of real time operating systems and find out about Windows Embedded CE 6.0 R2 here.


This SPARK Your Imagination Make: Windows Embedded project series is sponsored by Microsoft Corporation.

Kipp Bradford

Kipp Bradford is a technology consultant and entrepreneur with a passion for making things. He is the Senior Design Engineer and Lecturer in Engineering at Brown University, where he teaches several engineering design and entrepreneurship courses. Kipp is also on the Technical Advisory Board for Make Magazine.


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