Hello everyone,
I'm currently working on a UART communication protocol using STM32 USART in multiprocessor mode (MME / Address Mark) over RS485. My goal is to receive variable-length data frames only when the address matches, and use DMA + IDLE interrupt to detect the end of each frame.
:wrench:Frame format:
- First byte: 9-bit address word (MSB = 1)
- Followed by N data words (9-bit, MSB = 0)
:white_heavy_check_mark:Setup:
- USART configured in 9-bit data mode (no parity)
- MME (multiprocessor mode) enabled
- Address match set via ADD[7:0]
- DMA reception via `HAL_UARTEx_ReceiveToIdle_DMA(...)`
- `__HAL_UART_ENABLE_IT(&huart, UART_IT_IDLE)` enabled
- IDLE handling expected in `HAL_UARTEx_RxEventCallback()`
:cross_mark:Issue:
- When MME is **disabled**, IDLE interrupt works correctly, and the RxEventCallback is triggered on frame end.
- When MME is **enabled**, the USART correctly detects the address and wakes up from mute mode, and DMA begins receiving data.
- But **IDLE interrupt never triggers** even after long idle times, unless the DMA buffer is filled entirely.
- It appears that **hardware no longer triggers IDLE** after address wake-up when in MME mode.
:books:I've reviewed the reference manual (STM32F4 series), which says IDLE or Address Match should both wake up the USART. But it seems after address match, IDLE detection may remain disabled or blocked.
:question_mark:Has anyone faced this behavior? Is this expected? Are there known limitations when using MME + IDLE together with DMA?
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**PS:**
I know that CAN bus would be a more suitable solution for addressed variable-length frames, but due to hardware constraints, I’m limited to USART over RS485.
Thanks in advance for any insights!
