![]() The lower the interrupt vector, the higher the priority of the interrupt, as seen on the table below. Pins in the same port will fire the same interrupt signal if any of them has. This is import to note if you have multiple interrupts since certain interrupts will take priority over one another and can interrupt lower priority interrupts. Arduino PCINT (Pin Change Interrupts) fires each time a pin state is changed. You may notice that the pins that INT5:0 are attached to are not the same used for attachInterrupt(). The external interrupts and the pins they are wired to: ![]() Each of the PCI interrupts have eight pins they are attached to, PCINT23:0. These three interrupts are set whenever the pins they monitor are toggled, which would be the same function as the CHANGE setting for the six other external interrupts. These three extra mystery interrupts are the Pin Change Interrupts, referred to as PCI2:0. This isnt the number of the pin itself - its actually a reference to where in. There are eleven external interrupts for the ATmega2560 chip, but the Arduino only allows you to use nine of them and only six of them, INT5:0, can be accessed through attachInterrupt(). The interrupt vector, which determines what pin can generate an interrupt. It's also a good exercise to figure out what these functions are doing behind the scenes. While convenient, it's important to note that there's always going to be a bit more overhead when using these functions instead of setting the registers yourself. Last post I covered how to set External Interrupts using the provided attachInterrupt() function.
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