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R8C Family General RTOS Concepts


Why RTOS for Real-Time Application

RTOS is not a required component of all real-time application in embedded systems. An embedded system in a simple electronic rice cooker does not require RTOS. But as the complexity of applications expands beyond simple tasks, benefits of having an RTOS far outweigh the associate costs.

Embedded systems are becoming more complex hardware-wise with every generation. And as more features are put into them in each iteration, application programs running on the embedded system platforms will become increasingly complex to be managed as they strive to meet the system response requirements. An RTOS will be effective to allow the real-time applications to be designed and expanded more easily whilst meeting the performances required.

Figure 2 Real-Time Embedded System with RTOS


Classification of RTOS

RTOS’s are broadly classified into three types, namely, Hard Real Time RTOS, Firm Real Time RTOS and Soft Real Time RTOS as described below:

  • Hard real-time: degree of tolerance for missed deadlines is extremely small or zero. A missed deadline has

catastrophic results for the system

  • Firm real-time: missing a deadline might result in an unacceptable quality reduction

  • Soft real-time: deadlines may be missed and can be recovered from. Reduction in system quality is acceptable

    • 2.4

      Misconception of RTOS

      • a)

        RTOS must be fast

The responsiveness of an RTOS depends on its deterministic behavior and not on its processing speed. The ability of RTOS to response to events within a timeline does not imply it is fast.

b) RTOS introduce considerable amount of overhead on CPU An RTOS typically only require between 1% to 4% of a CPU time.

c) All RTOS are the same RTOS are generally designed for 3 types of real-time systems (i.e. hard, firm & soft). In addition, they are further classified according to the types of hardware devices (e.g. 8-bit, 16-bit, 32-bit MPU) supported.


January 2010

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