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Introduction to RTOS

What is "Real-Time"?

• Myth - The Real-Time Computing is equivalent to fast computing (X)

  Truth - The Real-Time Computing is equivalent to predictable computing (O)

• A real-time system guarantees timeliness execution of given tasks (i.e., response time is guaranteed), NOT the raw speed of execution. Having more processors, more RAM, faster bus interfaces does NOT make a system real-time.

• In a real-time system, the correctness of the computations depends upon both of the followings:

  • The logical correctness of the computation

  • The time at which the result is produced

  Note that failing to meet the timing constraints will also be considered a FAILURE in a real-time system.

What is a Real-Time Application (RTA)?

• RTAs are not necessarily the fast-executing applications.

• RTAs are time-deterministic applications, which means that their response time to events is almost constant.

• Soft real-time applications (Tolerant to small delays)

  There could be a small deviation in response time, in terms of milliseconds or seconds, and it will not be considered a failure.

  e.g., VoIP application, Stock market website

• Hard real-time applications (Not tolerant to any delays)

  MUST complete within a given time limit and failure to do so will result in an absolute failure of the system.

  e.g., Airbag deployment system, Anti-lock breaking system, Missile guidance and control system

• To run a real-time application, a real-time operating system is necessary.

What is a Real-Time Operating System (RTOS)?

• An RTOS is an OS specially designed to run applications with very precise timing and a high degree of reliability.

• To be considered as "real-time", an OS must have a known maximum time for each of the critical operations that it performs. Some of these operations include:

  • Handling of interrupts and internal system exceptions

  • Handling of critical sections

  • Scheduling mechanisms, etc.

    • In general, RTOS scheduling policy favors high-priority tasks.

RTOS and Multitasking

• A task in a real-time system is a unit of individual functionality, that is, a piece of code which can be scheduled/dispatched on the CPU to execute.

  e.g., Possible tasks in a temperature monitoring application can be:

  • Sensor data gathering

  • Updating display

  • User input processing

    ...

• There are different ways to achieve multitasking.

  • In a single-core system, multitasking can be achieved by using a scheduler. (Concurrency)

  • In a multi-core system, multiple tasks can run on multiple cores simultaneously. (Parallelism)

  In an embedded environment, single-core system widely used so task scheduling is very important. Effective and fast scheduling gives a user an illusion that multiple tasks are running simultaneously.

References

Nayak, K. (2022). Mastering RTOS: Hands on FreeRTOS and STM32Fx with Debugging [Video file]. Retrieved from https://www.udemy.com/course/mastering-rtos-hands-on-with-freertos-arduino-and-stm32fx/