Sat May 9 02:00:30 CEST 2015

STM32F level triggered interrupts

EXTI is all edge triggered.  This is bad for some applications when it
is possible to miss an edge during setup.

The basic problem is that it is possible that an event arrives right
before a clear pending operation.

It seems possible to work around this by performing some extra reads..

[1] https://electronics.stackexchange.com/questions/158749/how-to-use-level-triggered-interrupts-with-stm32f1xx?rq=1