SPI NOR flash driver

The code

Embedded C for Cortex-M. It compiles. There are 4 real bugs in it: the kind that pass a bench test and fail in the field.

uint8_t flash_read_status(void)
{
    gpio_write(FLASH_CS, 1);
    spi_transfer(0x05);
    uint8_t s = spi_transfer(0x00);
    gpio_write(FLASH_CS, 0);
    return s;
}

void flash_write_page(uint32_t addr, const uint8_t *d, size_t n)
{
    gpio_write(FLASH_CS, 0);
    spi_transfer(0x02);
    spi_transfer(addr >> 16);
    spi_transfer(addr >> 8);
    spi_transfer(addr);
    for (size_t i = 0; i < n; i++) {
        spi_transfer(d[i]);
    }
    gpio_write(FLASH_CS, 1);
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

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