Introduction
Embedded systems interviews can be intimidating due to the combination of tough hardware and intricate software architecture. Whether you’re dealing with the architecture of microcontrollers, scheduling of RTOS, or writing drivers for devices, the key is having a clear understanding of how software interfaces with the silicon.
This ultimate guide will cover all the important Embedded Systems interview questions and answers, starting from the basics to the real-time system level. Looking to prepare in an organized way? Check out our embedded systems course syllabus.
Embedded Systems Interview Questions and Answers for Freshers
1. What is an embedded system?
An embedded system is a dedicated computer system based on either a microcontroller or a microprocessor to do a particular job in a large mechanical or electrical system under certain constraints, such as power, memory, and time.
2. Differentiate between a microcontroller and a microprocessor.
The microcontroller is a combination of CPU, memory (RAM/ROM), and I/O peripherals in one chip for a particular control operation, while the microprocessor has only the CPU unit.
3. Define WDT.
Watchdog Timer is a hardware countdown timer that helps to reset the microcontroller in case of a hang or an infinite looping condition of the software.
4. Explain the difference between RISC and CISC architecture.
RISC (Reduced Instruction Set Computer) has simple and single-cycle instructions, hence many codes of software, whereas CISC (Complex Instruction Set Computer) has complicated instructions that have multiple cycles.
5. Explain the use of the volatile keyword in the C language?
Volatile is used to indicate to the compiler that optimization should not be performed on the variable as its value may change unexpectedly due to some reason outside the scope of the current code, which the compiler cannot see, e.g., a hardware interrupt or peripheral register.
6. Explain what an interrupt is. How is an interrupt processed?
An interrupt is a hardware/software signal that stops the execution of the main program. The CPU saves its state at the current time and transfers control to a particular subroutine called the Interrupt Service Routine (ISR).
7. Is it possible to pass arguments or return values in an ISR?
No. ISR does not take arguments nor return values as it gets executed randomly through hardware signals, which means it is not a call from the software, and therefore, there is no fixed caller or stack frame.
8. Discuss the basic differences between UART and SPI communication.
UART is an asynchronous communication protocol that requires two lines of transmission, TX and RX, while SPI is a synchronous master-slave communication protocol that uses four lines.
9. Explain I2C communication.
It is a synchronous multi-master and multi-slave serial bus protocol using only two lines, i.e., Serial Data (SDA) and Serial Clock (SCL). It uses a unique 7-bit or 10-bit addressing scheme to identify each particular device on the bus.
10. What is a Pull-up Resistor?
The pull-up resistor makes the connection between the input pin and the power supply voltage (V_{CC}). As a result, the input pin can always be in a stable logical state “High” when there is no active circuit.
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11. Define RAM and ROM in embedded systems?
RAM is Volatile Memory that holds the temporary data/variables during run time. On the other hand, ROM or Flash is Non-Volatile Memory to hold the permanent firmware code.
12. Define RTOS.
Real-Time Operating System (RTOS) is a special type of operating system used for managing the hardware resources and executing application programs with accurate timing.
13. What is a deadlock in RTOS?
Deadlock arises in the case where there are two or more tasks that are stuck forever because each of them has a lock on the resource required by the other, thus forming a circular dependency.
14. Define priority inversion in RTOS.
Priority inversion takes place when a low-priority task blocks access to the resource by a high-priority task and is preempted by a medium-priority task.
15. Compare the hard real-time system and the soft real-time system.
In the case of a hard real-time system, missing a deadline is considered a failure in the whole system operation (e.g., pacemakers and airbags). On the other hand, in a soft real-time system, missing a deadline is acceptable but may lead to reduced performance.
Embedded Systems Interview Questions and Answers for Experienced Candidates
1. Designing an efficient dynamic memory allocation scheme in a safety-critical embedded system where fragmentation is not allowed?
For safety-critical systems, conventional malloc() and free() functions are prohibited owing to heap fragmentation and unpredictable execution times. In such scenarios, a fixed-size block memory pool allocator must be implemented.
// Example of a fixed-size block control structure
typedef struct Block {
struct Block* next;
} Block;
typedef struct {
Block* free_list;
uint8_t pool_storage[NUM_BLOCKS * BLOCK_SIZE];
} PoolAllocator;
It ensures O1 memory allocation and deallocation time and also ensures no fragmentation as reusable uniform chunks are utilized.
2. What is the issue of cache coherence in multi-core embedded processors? Explain how it is solved using both hardware and software approaches.
Cache coherence happens when several cores have copies of the same main memory location inside their respective caches. When Core A writes to its cache, Core B has outdated data stored inside its cache.
The cache coherence problem is solved through hardware techniques that include snooping protocols, like MESI protocol, in which caches observe a common bus. In software technique, data is written back from the cache into RAM, or invalidation of data is done from cache whenever needed, especially in DMA and shared memory locations.
3. Explain what is meant by a Bootloader and how to create a secure firmware over-the-air (FOTA) update solution?
A bootloader is a piece of code that runs on startup and initializes the hardware, updates or loads the firmware. For secure FOTA, create at least two partitions (Dual-Bank) in the flash memory.
| Memory Bank | Purpose |
| Bootloader | Verifies integrity, controls rollback, and executes the image |
| Bank 0 (Active) | Runs the current stable firmware version |
| Bank 1 (Download) | Receives and decrypts the new image via TLS/AES |
The bootloader verifies the new signature with public keys before switching the active banks. In case of failure to boot the new firmware, the system rolls back to Bank 0.
4. Explain how the compiler lays out memory in the case of structure padding and how you would optimize the structures for constrained microcontrollers?
Compiler pads the structures so that each element is aligned with the architecture word size (i.e., 4-byte boundary in case of ARM Cortex-M). This avoids inefficient, misaligned memory access.
// Unoptimized: 12 bytes due to padding
struct Unoptimized {
char a; // 1 byte + 3 bytes padding
int b; // 4 bytes
short c; // 2 bytes + 2 bytes padding
};
// Optimized: 8 bytes
struct Optimized {
int b; // 4 bytes
short c; // 2 bytes
char a; // 1 byte + 1 byte padding
};
Arranging variables from largest to smallest naturally reduces the padding without any performance loss.
5. Compare Inline Functions, Macros, and standard functions in terms of memory overhead and performance.
Preprocessor macros (or #define) simply substitute the text during the preprocessor stage, which results in no overhead because of function calls, but code expansion can cause some performance overhead and possible bugs due to loss of type-safety. Inline functions (using the keyword inline) tell the compiler to substitute function code into place, maintaining type-safety and omitting the overhead of context switching.
Functions keep one memory footprint, but require putting the registers on the stack, making a jump, and popping the stack from it, which involves minor overhead in terms of runtime.
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6. Describe how you would implement a lock-free and thread-safe Circular Buffer (also known as Ring Buffer) for a single-producer/single-consumer case.
To make the circular buffer lock-free and thread-safe in the single-producer (ISR), single-consumer case, we need to make both head and tail atomic values, so that we will not need any mutexes.
volatile uint32_t head = 0;
volatile uint32_t tail = 0;
uint8_t buffer[BUFFER_SIZE];
void push(uint8_t data) {
uint32_t next = (head + 1) % BUFFER_SIZE;
if (next != tail) { // Not full
buffer[head] = data;
head = next; // Atomic write updates state instantly
}
}
As the producer changes head only and the consumer changes tail only, using memory barriers, we can avoid race condition issues and also disable interrupts.
7. Compare the physical layers, the bandwidth capabilities, and the use cases for CAN, I2C, SPI, and RS-485.
The selection of the appropriate protocol depends greatly upon the surrounding environment, the limitations of distance, and the number of devices involved.
| Protocol | Physical Layer | Max Speed | Topology | Primary Use Case |
| I2C | Open-drain (2 wires) | 3.4 Mbps | Multi-master | Local on-board sensors |
| SPI | Push-pull (4+ wires) | 50+ Mbps | Master-Slave | High-speed flash/displays |
| CAN | Differential pair | 1-5 Mbps | Multi-master | Automotive/Industrial noise resistance |
| RS-485 | Differential pair | 10+ Mbps | Multi-drop | Long-distance (1200m) automation |
8. Explain how DMA is configured and optimized for processing high-speed ADC sampling while avoiding CPU starvation.
To process high-speed ADC samples without causing CPU starvation, the DMA must be set up for Ping-Pong(Double-buffer) mode using an ADC sample triggered by the timer.
The DMA will automatically send the samples to Buffer A. Once Buffer A is full, the DMA hardware generates a half-transfer interrupt and immediately starts writing to Buffer B. The CPU can process the samples in Buffer A at the same time as the DMA is writing to Buffer B.
9. Describe bit-banging. Under what circumstances will you need it? What are its performance consequences?
Bit-banging is the software simulation of a hardware communication protocol (like I2C or SPI) by manually switching off the GPIO pins through the use of carefully timed loops.
Use it if your microcontroller doesn’t have built-in hardware peripherals or all of its hardware pins are already assigned. However, bit-banging adversely affects performance, as it makes the CPU engage in busy polling loops and ties up 100% of its processing cycles.
10. Describe how you would solve the priority inversion problem in your RTOS using Priority Inheritance Protocol vs. Priority Ceiling Protocol.
Priority inversion happens if a low-priority task holds a mutex required by a high-priority task, but there’s a medium-priority task preemption.
- Priority Inheritance: The priority of a low-priority task is dynamically boosted to a high-priority task when the latter tries to acquire the mutex, then restored to its initial value after release.
- Priority Ceiling: A high priority is assigned to a mutex, which any task locking it will inherit.
11. What is the distinction between Preemptive, Co-operative, and Time-Slicing scheduling in an RTOS?
Preemptive scheduling involves the RTOS scheduler stopping a running task immediately once a higher-priority task is available. In Co-operative scheduling, the tasks are responsible for yielding control to the scheduler through blocking operations; one misbehaving task will block the entire system. With Time-Slicing, equal CPU time slices are allocated to all tasks of the same priority level, and switching occurs every hardware timer tick.
12. How would you troubleshoot a stack overflow problem happening sporadically in an RTOS task?
To find the problem of a sporadic stack overflow, you need to enable stack-overflow checking within the RTOS and initialize RTOS tasks’ stacks with the same hex value (e.g., 0xDEADBEEF). After that, a system crash dump is taken, and a debugger starts reading memory downwards from the top of the stack.
The point where the pattern stops is the maximum stack usage; it indicates whether the stack size needs to be increased or the array declared locally.
13. What techniques do you use for reducing power consumption in your battery-operated IoT device to micro-amps?
- Micro-Amps power consumption needs the hardware-software co-design approach:
- Disable GPIO pins that are not being used and set them in analog configuration to prevent any current leakage.
- Place the CPU in the Deep Sleep/Stop mode, which will turn off the high speed oscillators within the CPU.
- Use either an external interrupt or an RTC to wake up your device, which is ultra low power consuming.
- Disconnect the external sensors with MOSFET switches to ensure that they will be consuming no energy when idle.
14. Explain the differences between hardware and software interrupts, and how are nested interrupts handled?
Hardware interrupt is an asynchronous event caused by some external physical component like GPIO edge detection or Timer expiry. Software interrupt is a synchronous event caused by CPU instructions like SVC on ARM, and can also be used to change the execution flow from the user mode to kernel mode.
Nested interrupts are handled through a hardware interrupt controller such as the NVIC; they can preempt each other in case of different priorities.
15. Explain how to create the automated Hardware Abstraction Layer (HAL) tests for an embedded system using HIL tests.
Automated HIL testing is done by connecting the Target Microcontroller to an automated Test Rig (based on an FPGA or DAQ card) via the Test Rig itself. The test host, which is running Python/PyTest, instructs the Test Rig to emulate certain physical inputs, such as analog voltages or signals for I2C communication.
The HAL code is executed by the Target microcontroller, while the digital outputs are tested by the Test Rig in real time.
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Conclusion
This is the way to get ahead in the world of jobs that are highly competitive if you are either entering the field or even at a level of senior designer. Your technical interview would measure your capability of connecting software logic with actual hardware implementation, and it is only through preparation that technical complexity becomes absolute confidence.
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