Embedded Systems Interview Guide: Linux Thread Scheduling, STM32 Configurations, SPI, DMA, Synchronization, and Debugging

Introduction: The author shares an embedded systems interview guide, noting the popularity and attractive salaries of embedded positions.

Self‑introduction advice: Emphasize background, education, and embedded‑related projects.

Linux thread scheduling

The kernel selects the next thread based on priority and scheduling policy.

Runnable threads are kept in a scheduling queue.

When a CPU core is idle, the highest‑priority runnable thread is assigned.

Threads run until their time slice expires, they block, or they sleep.

Blocked or sleeping threads are removed from the run queue and later re‑awakened.

Single‑core CPU execution order

Threads are scheduled using a time‑slice round‑robin algorithm, which is pre‑emptive; the kernel can interrupt a running thread when its slice ends, it blocks, or a higher‑priority thread becomes ready.

Thread selection strategies

First‑Come‑First‑Served (FCFS)

Round‑Robin

Priority scheduling

Pre‑emptive scheduling

Shortest Job Next

STM32 chip configurations

Examples:

STM32F103: 72 MHz CPU, 64–128 KB Flash, 20–48 KB RAM.

STM32F407: 168 MHz CPU, 512 KB–1 MB Flash, 192–256 KB RAM.

STM32L432: 80 MHz CPU, 256–512 KB Flash, 64–128 KB RAM.

Typical STM32 development areas

Embedded system design (e.g., smart home, robotics).

IoT applications with cloud integration.

Peripheral control (LCD, touch, LEDs, etc.).

Data acquisition and processing.

Smart vehicle or drone control.

Driver development examples

Display drivers

Network interface drivers

Audio drivers

USB device drivers

Storage drivers

SPI communication

Four lines: SCLK, MOSI, MISO, SS. Typical speeds range from hundreds of kHz to tens of MHz.

SPI modes

Mode 0 (CPOL = 0, CPHA = 0)

Mode 1 (CPOL = 0, CPHA = 1)

Mode 2 (CPOL = 1, CPHA = 0)

Mode 3 (CPOL = 1, CPHA = 1)

DMA overview

Direct Memory Access transfers data between peripherals and memory without CPU intervention. Typical flow: configure controller, request transfer, automatic data movement, completion interrupt.

Synchronization primitives

Mutex

Semaphore

Condition variable

Barrier

Read‑Write lock

Lock contention and priority inversion

When a low‑priority task holds a lock, higher‑priority tasks may block. Solutions include priority inheritance, priority ceiling, and careful use of semaphores or mutexes.

Debugging techniques for user‑space and runtime errors

Use debuggers (gdb, lldb).

Insert logging statements.

Memory analysis tools (Valgrind).

Static analysis (Clang Static Analyzer).

Core dump inspection.

Visual debugging environments.

Assertions and exception handling.

Program memory layout

Code (text) segment

Data segment

BSS segment

Heap

Stack

Special segments (shared libraries, etc.)

Heap vs. Stack

Heap memory is allocated and freed manually using functions such as malloc() , new , free() , and delete , while stack memory is managed automatically by the compiler.