1. Surjo Dutta and David Friedman
Interrupts
Surjo Dutta and David Friedman

2. What are interrupts?
A signal which indicates to the microprocessor to pause one task, and perform another task
Generally, there are two sources of interrupts
Hardware/other devices in communication with the microprocessor
Software triggers

3. Interrupts on the Arduino Mega
The Arduino Mega has specific ‘interrupt pins’ which are directly read by hardware.
Interrupt pins (Digital): 2,3,18,19,20,21
There are several ‘modes’ specifying interrupt conditions
LOW: triggers when pin is low
HIGH: triggers when pin is high
CHANGE: trigger when pin changes at all
RISING: trigger when pin goes from low -> high
FALLING: trigger when pin goes from high -> low
Can run a special function called an interrupt service routine (ISR)
Can only run one ISR at a time, and queues incoming interrupts in a First Come First Serve queue, ie, no way to prioritize interrupts
But reset gets priority over other interrupts

4. Interrupt service routines (ISR)
A function that executes when an interrupt is triggered
An ISR cannot have any parameters and cannot return anything
Associate an ISR with an interrupt pin with the ‘attachInterrupt’ function
Triggered
Not triggered
ISR
Main code
Interrupt condition

5. How do you use them?
attachInterrupt (digitalPinToInterrupt(intPin), ISR, INT_MODE);
intPin: Digital pin to use as interrupt pin
ISR: The interrupt handler, takes no arguments and returns nothing
INT_MODE: LOW, HIGH, CHANGE, RISING, FALLING
Delay() depends on interrupts and hence can’t be used inside an ISR
Keypad-LCD device was also a rudimentary form of an interrupt
Remember to explain INPUT_PULLUP if required

6. Software Interrupts
Arduinos do not natively support software interrupts
External libraries like TimerThree.h can be used
They use one of the three built in hardware timers which were initially meant to set PWM periods
Now also used for timer overflow interrupt handling, and other features
Enables the microcontroller to be interrupted at even time intervals
Useful for two to four actions to be performed at different, indivisible frequencies

7. Sources https://www.teachmemicro.com/arduino-interrupt-tutorial/