Accidental EXTI-Interrupts on STM32H735
Hello,
maybe someone has an idea or can share an similar experience. Since I'm currently in the evaluation phase for selecting an MCU for our new industrial embedded project. I want to inspect and analyze this problem, to be sure the used MCU is working fine and i can use it (STM32H735IGK6).
Issue:
Pushbuttons on different EXTI-Interrupt randomly fires twice or more times. Some GPIOs are effected, but not all, i have tested.
Setup:
Own PCB-Layout, 6 Pushbuttons, every one is debounced with 10kOhm and 10nF. Signal looks fine on scope, no bouncing can be seen with scope, absolutely clean signal.
Test 1:
apply a 200mHz sine-wave directly to the button pin. Expected behavior is: one interrupt on rising one interrupt on falling signal (due to the 200mV hysteresis on every input pin, as mentioned in datasheet). Behavior in RealLife: as soon as the voltage level on the pin reaches the switching thresholds of the input pin, i get lots of Interrupts (Rising and falling edge).
BUT now its getting freaky: if i turn off the running FMC access to an external RAM, instantly everything works as expected - only one single interrupt per transition.
Test 2:
same as above, but the FMC-access only bursts data every few seconds for approx. 250ms. Result: only, during FMC-access i get this accidential interrupts. (just to be sure: i checked the signal with the scope again: the waveform looks perfectly clean, without any noise, glitches, or something else)
I don't know if this can be an mcu-internal emc-issue or something like that, but fact is, if i enable the data transfer to the external RAM, the EXTI-Peripheral (or the schmitt trigger inside) gets extremely disturbed.
Of course, in this use case with buttons, a simple workaround is possible, but i also need a solution for other, more complex applications with combinations of FMC and EXTI (e.g. FPGA Communications, ...)