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Embedded Systems — Interview Questions

A curated collection of interview questions organized by difficulty, covering the full range of embedded systems topics.

Beginner

  1. What is an embedded system? Give three examples from your daily life.
  2. What is the difference between RAM and Flash memory on a microcontroller? Why do MCUs need both?
  3. Explain GPIO. How would you configure a pin as an output and toggle it?
  4. What is a register? How does a program interact with hardware registers?
  5. What does volatile do in C? Why is it important when programming hardware?
  6. What is an interrupt? Describe the sequence of events when an interrupt fires.
  7. What is a baud rate? How does it relate to UART communication?
  8. What is PWM? Give a practical application.
  9. Explain the difference between a microcontroller and a microprocessor.
  10. What is an ADC? If you have a 12-bit ADC with a 3.3V reference, what is the smallest voltage change it can detect?

Intermediate

  1. Compare SPI and I2C. When would you choose one over the other? Consider speed, wiring complexity, and device count.
  2. What is DMA? Describe a scenario where DMA significantly improves system performance.
  3. What is priority inversion? Explain with a three-task example and describe how priority inheritance solves it.
  4. Why should ISRs be kept short? What techniques can you use to defer processing?
  5. What is a watchdog timer? How would you implement multi-task monitoring in an RTOS-based system?
  6. Explain memory-mapped I/O. How does the CPU distinguish between a memory access and a peripheral register access?
  7. What is the NVIC? How does it handle nested interrupts on Cortex-M?
  8. Describe the boot process on an ARM Cortex-M. What are the first two values read from flash?
  9. What is a mutex in FreeRTOS? How does it differ from a binary semaphore?
  10. What is a bootloader? Describe a safe firmware update mechanism.

Advanced

  1. Design a system that reads temperature from an I2C sensor every 100ms and sends it over UART. Describe your task architecture, buffer management, and error handling in an RTOS.
  2. How would you implement OTA firmware updates for a battery-powered IoT device? Address security, atomicity, power-loss recovery, and rollback.
  3. Explain how you would profile and optimize the power consumption of an embedded device. What tools and techniques would you use?
  4. What is the worst-case interrupt latency on a Cortex-M4? What factors affect it?
  5. Describe how you would implement a lock-free single-producer, single-consumer ring buffer for passing data between an ISR and a task.
  6. You have a hard real-time system that must respond to an external event within 50µs. How would you guarantee this deadline architecturally and verify it?
  7. What is tickless idle in FreeRTOS? How does it work, and what are its limitations?
  8. Explain the ARM TrustZone security architecture. How would you use it to protect firmware intellectual property and secure communication keys?
  9. Compare CAN bus arbitration to CSMA/CD (Ethernet). Why is CAN better suited for real-time control systems?
  10. A field-deployed device is experiencing random reboots. Walk through your debugging methodology from data collection to root cause analysis.

Comparison Questions

  1. RTOS vs bare-metal: When would you use each? What is the overhead of an RTOS context switch?
  2. Polling vs interrupt-driven I/O: What are the trade-offs in terms of CPU usage, latency, and complexity?
  3. FreeRTOS heap schemes (heap_1 through heap_5): Which would you use for a safety-critical medical device?
  4. JTAG vs SWD: Why does ARM recommend SWD for Cortex-M debugging?
  5. Cooperative vs preemptive scheduling: What are the advantages of each in a hard real-time system?

Common Traps

  1. “I’d use malloc to allocate memory dynamically.” → Embedded firmware typically avoids dynamic allocation. Explain why and describe alternatives.
  2. “I’d put a printf in the ISR to debug it.” → ISRs must be fast and non-blocking. printf can deadlock. Explain proper ISR debugging techniques.
  3. “A semaphore and a mutex are the same thing.” → They are not. A mutex has ownership semantics and priority inheritance. A semaphore does not.
  4. “I’ll use a global variable to share data between the ISR and main loop.” → Without volatile and proper synchronization, the compiler may optimize away reads or reorder accesses.
  5. “I’ll set all tasks to high priority for best responsiveness.” → This defeats the purpose of priority-based scheduling. Only truly time-critical tasks should be at the highest priority.