RS-485 transmit with DMA

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

static uint8_t tx_buf[64];

void rs485_send(const uint8_t *frame, uint16_t len)
{
    memcpy(tx_buf, frame, len);
    gpio_write(DE_PIN, 1);
    DMA1_Channel7->CCR &= ~1u;
    DMA1_Channel7->CMAR = (uint32_t)(uintptr_t)tx_buf;
    DMA1_Channel7->CNDTR = len;
    DMA1_Channel7->CCR |= 1u;
}

void DMA1_Channel7_IRQHandler(void)
{
    DMA1->IFCR = DMA_IFCR_CTCIF7;
    gpio_write(DE_PIN, 0);
    response_timer_start(MODBUS_TIMEOUT_MS);
}

void USART2_IRQHandler(void)
{
    if (USART2->ISR & USART_ISR_RXNE) {
        rx_push((uint8_t)USART2->RDR);
    }
}

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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