I have come across some Andruino Sensors like Motion, Light and all recommend to be connected to pin 2 or 3 on Andruino Uno board which are interrupt pins. This is especially true if you are looking for HIGH or LOW values. Why is that to be connected to interrupt pins?
One example would be is I used CC3000 Wifi Breakout and Motion sensor. Both were in fight to occupy the interrupt pin 2. I have seen the Motion sesnor not working correctly after getting it's signal hooked to Pin 4[It has pull up resistor enabled]
What should I do if i need to hook up more sensor which uses the limited interrupt pins and how should i handle the above scenario?
2 Answers 2
The interrupt pins are used to notify the MCU that a device needs attention. The INTx pins are chosen first because they are easy to program for; each gets a dedicated interrupt vector and hence ISR.
There is another type of external interrupt though, the Pin Change interrupt. This can be invoked via any of the PCINTxx pins on the device; on the '328 any pin can act as an interrupt.
The main difference between INT and PCINT is that PCINTs are grouped into banks of 8, which means that once the ISR has been invoked the appropriate pins need to be checked for their level, which furthermore means that it is possible to misread an interrupt if a bank is shared by multiple devices and the interrupt pulse itself is very short.
Further details can be read in the "External Interrupts" and "Interrupts" sections of the MCU datasheet.
Ignacio provides a great answer on the differences between the two types of interrupts. But to add to that, it isn't a universal requirement to even use an interrupt.
Polling is also a possibility, though it has been compared to periodically picking up the phone to see if anyone is trying to talk to you.
Advantages of using an interrupt can include faster response, background response while a program is primarily doing something else, and often an associated ability wake up the processor from low-power sleep mode used in battery powered gadgets.
But polling can have a role, for example when the program runs in a tight loop and can only take action in response to input at certain times anyway - an interrupt service routine that merely sets a flag to tell the main program loop that something has happened may or may not be much benefit.
Often the best solution is a hybrid approach - an interrupt service routine records events - especially ones which might vanish or be repeated before the main program loop can poll for them. Then when the main program loop reaches a logical decision point, it checks the flags or event queue to see if the interrupt service routine has recorded anything to which it should respond. Serial receive is typically handled this way in the Arduino framework.