The right way to clear an interrupt flag on STM32

3
votes

I'm developping a bare-metal project on a STM32L4 and I'm starting from an existing code base.

The ISRs have been implemented the following way:

  1. read interrupt status in the peripheral to know what event(s) provoked the interrupt
  2. do something
  3. clear the flags that have read at the beginning.

Is it the right way to clear the flag ? Shouldn't the flags be cleared at the very beginning of the ISR ? My understanding is that, if the same peripheral event is happening a second time during step 2, it will not provoke a second IRQ so it would be lost. On the other hand if you clear the flag as soon as you can, this second event would pulse the interrupt whose state in the CPU would change to "pending and active": a second IRQ would happen.

PS: From STM32 Processor Programming Manual I read: "STM32 interrupts are both level-sensitive and pulse-sensitive".

1
arminterruptstm32isr

1 Answers

5
votes

Definitely at the beginning (unless you have special reasons in the program logic) as some time is needed the for actual write to the flag clear register to propagate through the buses.

If you decide for some reason to put it at the end of the interrupt you should leave some instructions, place the barrier instruction or read back the register before the interrupt routine return to make sure that the clear operation has propagated across the buses. Otherwise you may have a "phantom" duplicate routine calls.