Touch controller interrupt

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.

static volatile int16_t touch_x, touch_y;

void EXTI9_5_IRQHandler(void)
{
    EXTI->PR1 = EXTI_PR1_PIF7;
    uint8_t buf[4];
    HAL_I2C_Mem_Read(&hi2c1, TP_ADDR << 1, TP_REG_XY,
                     I2C_MEMADD_SIZE_8BIT, buf, 4, 100);
    uint16_t rx = (uint16_t)((buf[0] << 8) | buf[1]);
    uint16_t ry = (uint16_t)((buf[2] << 8) | buf[3]);
    touch_x = (int16_t)(rx * 480 / 4096);
    touch_y = (int16_t)(ry * 272 / 4096);
}

bool touch_get(int16_t *x, int16_t *y)
{
    *x = touch_x;
    *y = touch_y;
    return true;
}

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.

Companies that test this

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