About the problem of STM32 serial port idle interrupt IDEL

1. The idle interrupt is triggered when a byte of high level (idle) appears after receiving data. It does not mean that the idle interrupt will continue. To be precise, it should be a byte after the rising edge (stop bit). If it is always at a low level, the idle interrupt will not be triggered (it will trigger a break interrupt).

2. There are three situations to be prepared for the second point. In the datasheet,
"When an idle frame is detected, the processing steps are the same as receiving a normal data frame, but if the IDLEIE bit is set, an interrupt will be generated."
"The idle symbol is regarded as a complete data frame consisting entirely of '1', followed by the number of '1' bits of the start bit of the next frame of data, including the number of stop bits." The picture of the idle symbol is followed by this low level.
Some people understand that only when the start bit of the next data is received will the interrupt be triggered. This understanding is wrong. It should be triggered when there is an idle frame after the data.

3. The way to clear interrupts feels strange. The function USART_ClearITPendingBit( USART1, USART_IT_IDLE ) cannot clear interrupts. I use the 3.5 library. Looking at the function description, there is no IDLE in the @param parameter. In the @note below, it says:
"PE(Parity error),FE(Framing error),NE(Noise error),ORE(OverRun error) and IDLE(Idle line detected) pending bits are cleared by software sequence: a read operation to USART_SR register (USART_GetITStatus()) followed by a read operation to USART_DR register (USART_ReceiveData())."
I clear the IDLE interrupt through the statement "USART1->DR;".

Nowadays, many data processing methods require variable-length data, but the RXNE interrupt of the microcontroller serial port can only receive one byte of data at a time. There is no buffer and it is impossible to receive multiple data in one frame. Now two methods of using the serial port IDLE interrupt to receive a frame of data are provided. The methods are as follows:

Method 1: Implementation idea: Use the serial port 1 of STM32F103, configure it to the idle interrupt IDLE mode and enable DMA reception, and set the receiving buffer and initialize DMA at the same time. After the initialization is completed, when the external sends data to the microcontroller, assuming that the length of this frame of data is 200 bytes, then when the microcontroller receives a byte, the serial port interrupt will not be generated, but DMA will silently move the data to the specified buffer in the background. The serial port will only generate an interrupt after the entire frame of data is sent. At this time, the DMA_GetCurrDataCounter() function can be used to calculate the length of the data received this time, so as to process the data.

Application objects: Suitable for various serial port related communication protocols, such as MODBUS, PPI; also suitable for GPS data reception and analysis, serial port WIFI data reception, etc., which are all good application objects.

Method 2: Implementation idea: Directly use the RXNE and IDLE interrupts of stm32 to receive indefinite byte data.