Time slicing and preemption
Two tasks that never yield still share the processor a tick at a time, seen on the logic analyzer; a higher priority task cuts in on time; and one priority change starves a task completely.
Open it in the simulatorAll paths
Scheduling and RTOS
From a scheduler written by hand to FreeRTOS on the Uno: tasks and priorities, time slicing on the logic analyzer, queues, mutexes and priority inversion, starvation and the watchdog, and a four-task device.
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What you will build
2 projects, built step by step
7 lessons in 4 chapters, about 6 hours on a simulated Arduino Uno R3. Free, in your browser, every step checked by the simulator.
Starvation and the watchdog
A busy worker starves a heartbeat task, a watchdog task at the top catches it in the act, and splitting the work into chunks lets every task check in on time.
Open it in the simulatorThe map
Chapter by chapter
Each lesson opens when the one before it is done, and each chapter ends in a project.
Chapter 1
Scheduling by hand
Many jobs in one loop, and where that stops working.
- Open350 XP
A scheduler by hand
Three jobs at three rates in one loop with millis() and no delay(): a table of jobs, a loop that runs whichever is due, and the one rule that makes it work.
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Chapter 2
Tasks and the scheduler
Tasks, priorities, and a tick that takes the processor back.
- Locked350 XP
Tasks and priorities
The same lamps as FreeRTOS tasks: each one a loop of its own with a stack and a priority, vTaskDelay() counted in 16 ms ticks, and a report that outranks them.
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- ProjectLocked700 XP
Time slicing and preemption
Two tasks that never yield still share the processor a tick at a time, seen on the logic analyzer; a higher priority task cuts in on time; and one priority change starves a task completely.
0 of 6 steps
Chapter 3
Tasks that work together
Queues, mutexes, priority inversion and starvation.
- Locked270 XP
Queues
A sensor task hands readings to a logger task through a FreeRTOS queue: no shared variable, a receiver that sleeps until data arrives, and what happens when the queue fills up.
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- Locked320 XP
Mutexes and priority inversion
Three tasks and one shared bus: a mutex guards it, priority inversion makes the most important task wait for the least, and priority inheritance fixes it. Then break the fix and watch it happen.
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- ProjectLocked600 XP
Starvation and the watchdog
A busy worker starves a heartbeat task, a watchdog task at the top catches it in the act, and splitting the work into chunks lets every task check in on time.
0 of 5 steps
Chapter 4
Boss project
A temperature alarm built from four tasks.
- Locked380 XP
Boss: a multi-task device
A temperature alarm built from four tasks: a sensor overwrites a one-slot queue, a display owns the LCD, a button notifies the alarm, and a mutex keeps every serial line whole.
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Syllabus
Every lesson in the course
All 7 lessons, in order, each with what it covers. Any of them opens in the editor right away.
Scheduling by hand
- A scheduler by handThree jobs at three rates in one loop with millis() and no delay(): a table of jobs, a loop that runs whichever is due, and the one rule that makes it work.
Tasks and the scheduler
- Tasks and prioritiesThe same lamps as FreeRTOS tasks: each one a loop of its own with a stack and a priority, vTaskDelay() counted in 16 ms ticks, and a report that outranks them.
- Time slicing and preemptionTwo tasks that never yield still share the processor a tick at a time, seen on the logic analyzer; a higher priority task cuts in on time; and one priority change starves a task completely.
Tasks that work together
- QueuesA sensor task hands readings to a logger task through a FreeRTOS queue: no shared variable, a receiver that sleeps until data arrives, and what happens when the queue fills up.
- Mutexes and priority inversionThree tasks and one shared bus: a mutex guards it, priority inversion makes the most important task wait for the least, and priority inheritance fixes it. Then break the fix and watch it happen.
- Starvation and the watchdogA busy worker starves a heartbeat task, a watchdog task at the top catches it in the act, and splitting the work into chunks lets every task check in on time.
Boss project
- Boss: a multi-task deviceA temperature alarm built from four tasks: a sensor overwrites a one-slot queue, a display owns the LCD, a button notifies the alarm, and a mutex keeps every serial line whole.
Keep going
The next course on your track
Scheduling and RTOS is part of the embedded track, a ladder of courses from zero to expert.
Low power products
Make a battery last years: where an Uno’s current goes, sleep modes and wake sources, a power budget on a bare chip, measuring with a meter and an INA219, battery life checked against a draining cell, and a coin cell node.
Open the next courseEmbedded track
Arduino, ESP32, Raspberry Pi Pico and STM32: pins, timing, buses, motors, WiFi, registers and debugging, every lesson checked on a simulated board.
See every course on the trackKits
Build it on your bench too
Every lesson runs in the simulator with nothing to buy. When you want the real thing, these kits have the parts.
Find your kit
Pick the starter kit you own and run every lesson in its manual with the same parts on the same pins.
See every kitQuestions
About Scheduling and RTOS
Is Scheduling and RTOS free?
Yes. All 7 lessons of Scheduling and RTOS are free and run in your browser, with nothing to install and no parts to buy before you start.
Do I need a real Arduino Uno R3?
No. Every lesson runs on a simulated Arduino Uno R3 with its parts on the canvas, and the simulator checks each step as you go. When you want to build it for real, the same wiring and code work on a real board.
How long does Scheduling and RTOS take?
About 6 hours: 7 lessons in 4 chapters, each one a few short steps. Your progress is kept, so you can stop after any lesson and pick up where you left off.
What will I build?
Each chapter ends in a project that puts it together: Time slicing and preemption and Starvation and the watchdog.
Where do I start?
With "A scheduler by hand", and each lesson builds on the one before it. New to circuits? Electronics basics is the course that starts from zero.