@robert-hh Thanks for your reply. Yes "P10" is connected to the Gnd by a simple push button. But I had no idea that the system was logging an interrupt every time I went to Gnd. Question: what solution to adopt? - debouncing hardware or software? What is the solution adopted on the expansion board 3.1 for the "S1" button ? Indeed I only use uart reception. it works great, however i will heed your suggestion of external resistance.