rtos-patterns

name	rtos-patterns
description	RTOS task design — FreeRTOS/Zephyr scheduling, IPC mechanisms, resource management, and real-time guarantees
allowed-tools	["Read","Write","Grep"]
effort	high

When to activate

• Designing task architecture for RTOS-based firmware
• Choosing IPC mechanisms (queues, semaphores, events)
• Optimizing task priorities and stack sizes
• Debugging priority inversion, deadlocks, or stack overflows
• Migrating from bare-metal to RTOS

When NOT to use

• For bare-metal super-loop firmware (no RTOS)
• For Linux-based embedded systems
• For high-level application architecture

Instructions

1. Task decomposition. One task per responsibility. Sensor task, network task, UI task. Minimize task count (each = RAM overhead).
2. Priority assignment. Rate Monotonic: shorter period = higher priority. Critical tasks above all. Idle task for power saving.
3. IPC selection. Queues (producer/consumer), Semaphores (resource counting), Mutexes (shared resource), Events (flag signaling).
4. Stack sizing. Measure with watermark pattern (fill stack with known value, measure unused). Add 25% margin.
5. Avoid pitfalls. No dynamic allocation after init. No printf in ISR. Short ISRs (set flag, process in task). Priority inheritance for mutexes.
6. Power management. Tickless idle. vTaskSuspend unused tasks. configUSE_TICKLESS_IDLE = 1.
7. Debugging. Stack overflow detection. Task runtime monitoring. Idle task hook for CPU usage calculation.

Example

RTOS Task Design: Sensor Gateway

Tasks (by priority):
  1. RadioTask     (pri=5, stack=2KB, period=event-driven) — BLE receive/send
  2. SensorTask    (pri=4, stack=1KB, period=100ms) — BME280 read + filter
  3. DataTask      (pri=3, stack=1.5KB, period=event-driven) — aggregate + store
  4. CloudTask     (pri=2, stack=4KB, period=event-driven) — MQTT publish
  5. UITask        (pri=1, stack=2KB, period=500ms) — LED + button handling
  6. IdleTask      (pri=0) — WFI for power saving

IPC:
  SensorTask → DataTask: xQueue (sensor readings)
  RadioTask → DataTask: xQueue (commands from phone)
  DataTask → CloudTask: xQueue (batched data)
  All → UITask: xEventGroup (status flags)

Total RAM: 10.5KB tasks + 2KB system = 12.5KB (MCU has 64KB)