7 Steps to Writing a Simple Cooperative Scheduler

Free

A step-by-step guide to building a lightweight cooperative scheduler for embedded systems

FreeFree tier

Type

Open Source

Visit Website GitHub (40 stars)

About 7 Steps to Writing a Simple Cooperative Scheduler

This article on EDN provides a step-by-step guide to writing a simple cooperative scheduler for embedded systems. It explains how a cooperative scheduler differs from a preemptive RTOS, emphasizing its simplicity, ease of debugging, and suitability for many microcontroller applications where task preemption is not required. The scheduler uses a background periodic timer to create a system tick and executes tasks at periodic intervals, with no priorities – if two tasks are due, the one higher in the list runs first. The article highlights that cooperative schedulers can achieve soft real-time behavior and, with interrupts, can meet hard real-time needs as well.

Key Features

Periodic task scheduling using a background system tick

No task preemption – tasks run to completion or yield voluntarily

Task list executed in order when multiple tasks are due

Simpler to debug compared to preemptive RTOS

Suitable for many MCU applications where an RTOS is overkill

Pros & Cons

Pros

Much simpler to implement and debug than an RTOS
Lower overhead since there is no context switching or priority management
Can meet hard real-time deadlines when combined with interrupts
Good fit for many microcontroller applications that do not require preemption

Cons

No task preemption – a long-running task can block others
Only periodic task execution; not suited for sporadic high-priority tasks
Task ordering is static and cannot be adjusted dynamically
Not suitable for applications with hard real-time requirements on multiple concurrent tasks

Best For

MCU-based applications with periodic, non-critical tasksSoft real-time systems where deterministic preemption is not requiredEducational projects to understand scheduler fundamentalsSmall embedded projects with limited memory and processing requirements

FAQ

What is the main difference between a cooperative scheduler and an RTOS?

A cooperative scheduler does not preempt tasks; tasks run to completion or yield voluntarily. An RTOS can temporarily suspend a task to switch to a higher priority task (preemption). Cooperative schedulers are simpler and easier to debug but cannot handle hard real-time requirements as naturally as an RTOS.

Is a cooperative scheduler suitable for hard real-time applications?

Cooperative schedulers provide soft real-time behavior by default, but through the use of interrupts and other mechanisms, they can also meet hard real-time needs.

How are tasks scheduled in a cooperative scheduler?

A background periodic timer creates a system tick. The scheduler checks which tasks are due to run at the current time and executes them sequentially in the order they appear in the task list.