Interview Experience with SanDisk’s SD Card Team (C Programming Focus)

Table of Contents#

Pre-Interview Preparation 1.1 Company & Role Research 1.2 Technical Prep: C & Embedded Systems 1.3 SD Protocol & Storage Concepts
Interview Rounds Overview
Detailed Round Breakdown with Questions & Solutions 3.1 Phone Screen: C Basics & Problem-Solving 3.2 Onsite Round 1: Advanced C Programming 3.3 Onsite Round 2: SD Protocol & Firmware Implementation 3.4 Onsite Round 3: System Design & Debugging 3.5 Behavioral & HR Round
Key Takeaways & Best Practices
References

1. Pre-Interview Preparation#

1.1 Company & Role Research#

SanDisk’s core focus: Flash memory innovation, SD/MMC protocol leadership, and embedded storage solutions.
Role expectations: Develop low-level firmware for SD cards, optimize memory usage, debug hardware-software interactions, and ensure compliance with SD Association specs.

1.2 Technical Prep: C & Embedded Systems#

Master basics: Pointers, memory management, bit manipulation, and standard library functions.
Deep dive into embedded C: volatile keyword, const correctness, interrupt handling, and memory-constrained optimization.
Practice: Solve embedded C problems on platforms like LeetCode (focus on pointers and bitwise operations) and review open-source embedded firmware projects.

1.3 SD Protocol & Storage Concepts#

Study official SD Association specs: SPI vs SDIO modes, command structures (CMD/ACMD), block operations, and CRC algorithms.
Understand flash memory fundamentals: Wear leveling, block erasure, and bad block management.

2. Interview Rounds Overview#

The interview process consisted of 5 rounds:

Phone Screen (30 mins): C basics + SD protocol trivia.
Onsite Round 1 (60 mins): Advanced C programming (memory, pointers, optimization).
Onsite Round 2 (60 mins): SD protocol design + firmware implementation.
Onsite Round 3 (60 mins): System design + debugging.
Behavioral/HR Round (30 mins): Past projects, teamwork, and cultural fit.

3. Detailed Round Breakdown with Questions & Solutions#

3.1 Phone Screen: C Basics & Problem-Solving#

Question 1: Reverse a Null-Terminated String in Place Using Pointers#

Interviewer: Write a C function to reverse a null-terminated string in place using only pointers (no array indexing). Explain your approach.

Answer & Explanation: Use two pointers to swap characters from the start and end of the string until they meet in the middle. Validate input to avoid undefined behavior.

Common Pitfalls:

Forgetting to handle NULL input or empty strings.
Reversing the string back by looping beyond the middle.

Best Practices:

Defensive programming: Always validate inputs.
Document edge cases (single-character strings, NULL).

Example Code:

void reverse_string(char *str) {
    if (str == NULL || *str == '\0') return;
 
    char *start = str;
    char *end = str + strlen(str) - 1;
    char temp;
 
    while (start < end) {
        temp = *start;
        *start = *end;
        *end = temp;
        start++;
        end--;
    }
}

Question 2: SPI vs SDIO Modes for SD Cards#

Interviewer: Explain the difference between SPI and SDIO modes. Which is more common in embedded systems, and why?

Answer:

SPI Mode: Simple 4-wire protocol (CS, SCK, MOSI, MISO) compatible with most microcontrollers. Slower but easy to implement.
SDIO Mode: High-speed protocol with up to 8 data lines. Supports interrupts and advanced features but requires dedicated hardware.
Common Use: SPI mode dominates low-cost embedded systems due to its simplicity and wide compatibility.

3.2 Onsite Round 1: Advanced C Programming#

Question 1: Volatile Keyword in Embedded C#

Interviewer: Explain when to use volatile in C, with an example relevant to SD card firmware.

Answer: volatile tells the compiler that a variable’s value can change unexpectedly (e.g., hardware registers modified by the SD card). It prevents the compiler from optimizing away read/write operations.

Example:

// SD card status register (hypothetical hardware address)
volatile const uint8_t *SD_STATUS_REG = (volatile const uint8_t *)0x40001000U;
 
// Wait until data is ready (compiler won't optimize this loop)
while (!(*SD_STATUS_REG & (1 << 3))) {
    // Busy wait
}

Common Pitfalls: Overusing volatile (unnecessary performance overhead) or forgetting it for hardware registers (stale data).

Question 2: Bit Manipulation for SD Commands#

Interviewer: Write a function to construct a CMD17 (Read Single Block) packet. Use big-endian format as required by SD specs.

Answer: CMD17 is a 48-bit packet with:

Byte 0: Command index (17) + start/transmit bits.
Bytes 1-4: 32-bit block address (big-endian).
Byte 5: CRC7 value.

Example Code:

#define CMD17 17
 
static uint8_t crc7(const uint8_t *data, uint8_t len) {
    uint8_t crc = 0;
    for (uint8_t i = 0; i < len; i++) {
        crc ^= data[i];
        for (uint8_t j = 0; j < 8; j++) {
            if (crc & 0x80) {
                crc = (crc << 1) ^ 0x09;
            } else {
                crc <<= 1;
            }
        }
    }
    return (crc >> 1) & 0x7F;
}
 
void construct_cmd17(uint8_t packet[6], uint32_t block_addr) {
    packet[0] = 0x40 | CMD17; // Start + transmit bits + command index
    packet[1] = (block_addr >> 24) & 0xFF;
    packet[2] = (block_addr >> 16) & 0xFF;
    packet[3] = (block_addr >> 8) & 0xFF;
    packet[4] = block_addr & 0xFF;
    packet[5] = crc7(packet, 5) | 0x01; // CRC7 + start bit
}

3.3 Onsite Round 2: SD Protocol & Firmware Implementation#

Question 1: SD Card Initialization in SPI Mode#

Interviewer: Walk through the initialization sequence for an SD card in SPI mode. What are common failure points?

Answer:

Power-on: Send 80 clock cycles to synchronize the card.
CMD0: Put the card into idle state (response 0x01).
CMD8: Check SDHC/SDXC compatibility (optional but recommended).
ACMD41: Repeat until the card responds with 0x00 (ready).
CMD2/CMD3: Request CID and assign a relative address (RCA).

Common Failures:

Not sending enough clock cycles after power-on.
Skipping CMD55 before ACMD41 (ACMD requires a prefix CMD55).
Incorrect CRC values for commands.

Question 2: Read Block Firmware with Error Handling#

Interviewer: Design a function to handle CMD17 (Read Single Block) with error handling for timeouts and CRC errors.

Answer: Steps include validating the block address, sending CMD17, waiting for the start token, reading data, and verifying CRC.

Example Code Outline:

typedef enum { SUCCESS, ERROR_TIMEOUT, ERROR_CRC } ErrorCode;
 
ErrorCode sd_read_block(uint32_t block_addr, uint8_t *data_buf) {
    // Validate block address
    if (block_addr >= get_card_capacity() / 512) return ERROR_INVALID_ADDR;
 
    // Send CMD17 (using construct_cmd17 function)
    uint8_t packet[6];
    construct_cmd17(packet, block_addr);
    spi_send(packet, 6);
 
    // Wait for response (timeout after 100ms)
    uint8_t resp = spi_receive_timeout(100);
    if (resp != 0x00) return ERROR_TIMEOUT;
 
    // Wait for start token (0xFE)
    if (spi_receive_timeout(100) != 0xFE) return ERROR_TIMEOUT;
 
    // Read 512 bytes of data
    spi_receive(data_buf, 512);
 
    // Verify CRC16
    uint16_t received_crc = (spi_receive() << 8) | spi_receive();
    uint16_t computed_crc = compute_crc16(data_buf, 512);
    if (received_crc != computed_crc) return ERROR_CRC;
 
    return SUCCESS;
}

3.4 Onsite Round 3: System Design & Debugging#

Question 1: Basic Wear Leveling Algorithm#

Interviewer: Design a basic wear leveling algorithm for an SD card. What data structures and trade-offs do you consider?

Answer: Wear leveling extends flash lifespan by distributing writes evenly across blocks. Key components:

LBA-to-PBA Mapping: Array that maps logical block addresses (LBAs) to physical block addresses (PBAs).
Erase Count Tracking: Array that records write counts for each physical block.
Free List: Tracks available blocks for new writes.

Trade-offs:

Memory Overhead: Mapping tables use SRAM (compress or store in flash for large cards).
Performance: Adds write latency due to mapping lookups. Optimize by batching writes.

Question 2: Debugging Corrupt Read Data#

Interviewer: A user reports intermittent corrupt read data. Walk through your debugging process.

Answer:

Reproduce: Confirm if corruption is consistent or intermittent.
Check CRC: Verify if the SD card reports CRC errors.
Hardware Validation: Use a logic analyzer to check bus signal integrity and clock speed.
Firmware Review: Look for buffer overflows, incorrect pointers, or timeout issues.
Flash Health: Use CMD9 to check bad blocks and wear level.

Example Fix: If corruption is due to high clock speed, reduce the SPI baud rate to the card’s specified limit.

3.5 Behavioral & HR Round#

Question: Debugging a Critical Intermittent Issue#

Interviewer: Tell me about a time you debugged a hard-to-find embedded firmware issue.

Answer: "In a previous project, we faced intermittent data corruption in a flash module. I:

Collected logs to identify patterns (corruption occurred during high write loads).
Used a logic analyzer to find unacknowledged write commands.
Discovered the busy wait loop was too short, leading to premature command sends.
Fixed it by increasing the timeout and adding retries.

I learned that systematic debugging (tools + logging) beats guesswork, and defensive programming is critical for embedded systems."

4. Key Takeaways & Best Practices#

Master Embedded C: Focus on volatile, const, and memory optimization.
SD Protocol Expertise: Study official specs to understand command structures and initialization.
Debugging Skills: Practice using logic analyzers, oscilloscopes, and logging tools.
Defensive Programming: Validate inputs, handle errors, and implement retries for hardware interactions.
Behavioral Prep: Prepare stories about project failures, teamwork, and problem-solving.

5. References#

SD Association Official Specifications
The C Programming Language (2nd Edition) by Kernighan & Ritchie
Embedded C Programming and the Atmel AVR by Richard Barnett
Western Digital Flash Memory Technical Docs
Debugging Embedded Systems by Lynn Fuller