STM32F4 interrupt status register gotcha

I have been trying to implement UART line idle detection. Idle condition is considered, if no control bits are received within time frame of next byte.

Seemed reasonably simple - enable RXNE (receive buffer not empty) and IDLE interrupts. Sometimes it would work, and sometimes it wouldn't. All Most of the time I had USART peripheral memory reading enabled in debugger and it would show, that the IDLE bit is set in status register, but code would read the value without the bit. And, of course, that would break all the ISR logic. After banging my head against it for 2 days, I realized, that to reset the IDLE flag, one has to read the register. And then it dawned on me, that my debugger is reading the register before code gets there, and, hence, it is reset.


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