Archive for February, 2009

RTOS Myth #5: You Need One

Monday, February 23rd, 2009 Michael Barr

The Myth: You need a real-time operating system (RTOS) to make your embedded software easy to implement and maintain.

The Truth: Three positive implications of the use of a preemptive priority-based RTOS must be weighed against ten negative implications. An RTOS works well in some scenarios, but overly complicates the design of many other systems.

Annual surveys of the readers of EETimes and Embedded Systems Design generally find that about half (50%) of embedded software developers work on projects that utilize a commercial RTOS, such as VxWorks or uC/OS-II. Due to a sampling bias stemming from the correlation between big teams and RTOS use, the number of new products containing an RTOS is likely much lower than 50%.

Contrary to popular belief, a real-time operating system (RTOS) is not the answer to all of your design problems. When chosen for the wrong reasons, the presence an RTOS can make the firmware design more complicated rather than less. In addition, preemption increases the opportunity for race condition and non-reentrancy bugs. Finally, the inclusion of an RTOS has other costs, such as additional RAM and ROM/Flash.

By my calculations, preemption has three principal positive implications and ten negative implications. For example, one positive is that an RTOS can help separate the timing (i.e., which code is running on the CPU when?) from the application-level algorithmics. The problem is decomposed into a less “fragile”/more maintainable firmware design.

However, the use of separately-coded parallel tasks also complicates the software. Without the RTOS, the only possible race conditions occur between communicating interrupt service routines and the background loop in main(). Additional tasks increase the need for communication and synchronization factorially.

Perhaps the most interesting tradeoff concerns responsiveness to interrupts. Although what an RTOS does is to divide up the spare CPU time not used by any interrupt service routine (ISR) between pseudo-parallel running tasks (i.e., a set of C functions that don’t return), one negative side effect is slower CPU response to interrupts. The interrupt latency is higher with an RTOS than without.

Don’t get me wrong, sometimes an RTOS is a valuable tool. But don’t go using one simply to put its name on your resume. You may instead find yourself languishing in an overly-complicated and buggy embedded software design at your present job.

The “RTOS Alternatives” course by Netrino Institute includes full coverage of these tradeoffs, including details of each of preemptions three positives and all ten of the negative implications of a commercial RTOS.

Go back to RTOS Myth #4.

Embedded Systems Conference 25% Discount Code CTDSS15

Monday, February 23rd, 2009 Michael Barr

As a speaker and track chair for the “Designing Safer Systems” track at the Embedded Systems Conference Silicon Valley, I am able to offer conference attendees a 25% registration discount. To receive the discount you must register with the promotional code CTDSS15.

The complete conference program can be found at

Requirements vs. Design

Wednesday, February 4th, 2009 Michael Barr

Over the years, I have found that many engineers (as well as their managers) struggle to separate the various elements or layers of firmware engineering. For example, we are barraged with requests for “design reviews” that turn out to be “code reviews” because the customer is confused about the meaning of “design”.

In the hopes of clearing this up, I propose a concise set of definitions and an architectural analogy.

The requirements are the WHAT of the system. A set of requirements is a list of statements each of which begins “The system shall…” Each such statement must be objective and testable. The requirements should not unnecessarily restrict the HOW of the architecture, design, or implementation.

The architecture of a system is the outermost layer of HOW. The architecture is a block diagram. The architecture of a system describes dataflow and workflow partitioning at the hardware vs. software level. The architecture of firmware features subsystem-level blocks such as device drivers, middleware, RTOS, etc. The architecture does not include function or variable names. It should be extensible in the direction of anticipated future changes.

Analogy: An architect describes a new building very broadly. A scale model and drawings show the outer dimensions, foundation, and number of floors. The number of rooms and their specific uses are not included at this level.

The design of a system is the middle layer of HOW. A firmware design document identifies finer structural details, such as the names and responsibilities of tasks within the specific subsystems or device drivers, the brand of RTOS (if one is used), and the various interfaces between subsystems. The design does include class, task, function, and variable names that must be agreed upon by all implementers.

Analogy: A designer describes the interior and exterior of the new building in finer detail than the architect. He locates and names the rooms and gives them purposes. The location of pipes and vents and outlets are not included at this level.

An implementation is the lowest layer of HOW. There is no document, other than the source code or schematics, to describe the implementation details. If the interfaces are defined sufficiently at the design level above, individual engineers are able to begin implementation in parallel.

Analogy: The carpenter, plumber, and electrician work in parallel and apply their own judgement about the finer details of component placement.

Constructive feedback is welcome via the blog comments or e-mail.