Embedded Systems Principles

Guidelines for resource-constrained and real-time system development.

When to Invoke

• Developing for microcontrollers or constrained devices
• Real-time requirements (hard or soft)
• Hardware abstraction layer design
• Memory-constrained environments

Resource Constraints

Memory

1. Static allocation preferred — avoid malloc/free in real-time paths.
2. Stack size budgets — calculate max stack depth per task.
3. Memory pools for fixed-size allocations.
4. No heap fragmentation — use fixed-block allocators.

Power

1. Sleep modes — enter lowest power state when idle.
2. Peripheral clock gating — disable unused peripherals.
3. Batch operations — reduce wake cycles.

Real-Time Patterns

Priority-Based Scheduling

1. Priority inversion protection — use priority inheritance mutexes.
2. Deadline monotonic — shortest deadline = highest priority.
3. Avoid priority ties — every task has unique priority.

Interrupt Handling

1. ISRs must be short — set flags, defer processing to task context.
2. No blocking calls in ISRs — no malloc, no mutex locks.
3. Volatile for shared variables between ISR and task.

Hardware Abstraction Layer (HAL)

Application Layer
├── Middleware (protocols, file systems)
├── HAL (hardware-agnostic interfaces)
└── Board Support Package (BSP — hardware-specific)

1. Interface per peripheral type — GPIO, UART, SPI, I2C, Timer.
2. BSP implements HAL — swap BSP to port to new hardware.
3. No hardware registers in application code.

RTOS Patterns

1. Tasks for concurrent activities — each with dedicated stack.
2. Queues for inter-task communication — type-safe message passing.
3. Semaphores for synchronization — binary for signaling, counting for resources.
4. Mutexes for shared data — always with timeout to prevent deadlock.

Testing

For universal testing principles, see .agents/rules/testing-strategy.md. Below: language-specific patterns only.

1. Unit test on host — test logic without hardware.
2. Hardware-in-the-loop (HIL) — automated tests with real hardware.
3. Mock HAL interfaces — inject test implementations.
4. Static analysis required — MISRA C/C++ compliance if safety-critical.

Safety-Critical Considerations

• MISRA C/C++ compliance for automotive, medical, aerospace
• Watchdog timers for fault recovery
• CRC/checksums for data integrity
• Redundancy for critical paths

Related

• C++ Idioms @.agents/skills/cpp-idioms/SKILL.md
• Resources and Memory Management @.agents/rules/resources-and-memory-management-principles.md
• Concurrency and Threading Principles @.agents/rules/concurrency-and-threading-principles.md