Pin Change Interrupt - PortC on Arduino Mega 2560
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You can double-up on pins if both devices signal in the same direction (HIGH or LOW) or their signals be conditioned to do so. When the interrupt fires, your interrupt routine first polls the devices to determine which one fired and then dispatches to the code for that one. If a race-condition is possible, you should remember the dual-firing (store it in a bool or as two one-bit flags) and dispatch to one after the other.
Update:
What do you mean with 'double-up on pins'?
Connect two devices' "ready" outputs to the same interrupt pin. The interrupt routine will have to determine which device is asking for service and act accordingly.
If you read data sheet of AVR chip, you might come across PCINT in addition to INT pins.
INT refers to the dedicated hardware interrupt pins described in Arduino Reference. The INT pin is linked to a dedicated interrupt vector so you always know what pin caused the interrupt when in the ISR.
PCINT refers to Pin-Change Interrupt that can be generated by almost(not always though) any of the I/O pins. PCINT has more overhead in determining what pin caused the interrupt as a group of pins(pins on the same GPIO port) share the same PCINT vector so you need to determine which pin caused the interrupt within the ISR before acting on it. Furthermore, PCINT can wake from sleep mode power down on change, while INT can only wake on low level. Depends on your application, PCINT is very good if you want to look at several pin status at once, for example, when using a rotary encoders.
This Article provides a good description and example on using PCINT, there is a library PinChangeInterrupt by NicoHood for using Pin Change Interrupt.
Specifically for ATmega2560, there are 24 PCINT-capable pins.
I'm trying to experiment with the RAMPS 1.4 board with a RepRapDiscount Full Graphic Smart Controller connected to it. I've gotten as far as displaying basic text on the screen and polling the pushbuttons/rotary encoder. I'm starting to hit a wall when trying to poll the rotary encoder via interrupt.
The encoder is connected to pins 31 and 33 on the Mega. These pins translate to PC6 (A14) and PC4 (A12) on the AtMega2560. Looking at multiple sources on external interrupts/pin change interrupts, it looks like the only pins supported for interrupts on the Mega are PCINT 0-23 and INT 0-7.
So now to the actual question. Can I trigger an interrupt on pins 31/33 changing in any way? I can make due with an interrupt triggering when either pin is changed. If I can't get that working, what are my alternatives? A timer interrupt that polls the pins seems like it might kinda sorta work, but it also seems expensive and likely to miss changes in the encoder state. Any help is appreciated!