L10: Interrupts

Interrupts: 

• Allow the program to respond to events when they occur.
• Allow program to ignore events until they occur.
• External events
• UART ready with/for next character.
• signal change on pin.
• number of edges arrived on pin.
• Internal events
• Power failure
• arithmetic exception like dividing by zero.
• Timer "tick" (e.g., output compare match)
• ADC conversion complete

Uses for Interrupts: 

• To detect pin changes (like: rotary encoders, button presses)
• Watchdog timer (like: if nothing happens after 8 seconds interrupt me)
• Timer interrupts used for comparing/overflow timers
• SPI data transfers
• I2C data transfers
• USART data transfers
• ADC conversions
• EEPROM ready for use
• Flash memory ready

In this tutorial we are going to be discussing more about the external and pin change interrupts than the internal interrupts.

External Arduino Interrupts: 

• Using Arduino IDE
• Only 2 interrupts available INT0, INT1 (digital pins 2 and 3) 
• can name interrupts arbitrarily
• able to use all 4 modes: 
• LOW(ISR keeps on triggering as long the pin is in logic low state)
• CHANGE(ISR triggers when there is a falling/rising edge)
• RISING(ISR triggers when there is a rising edge)
• FALLING(ISR triggers when there falling edge)
• Using C, Assembly language
• able to use the 2 external, or 24 pin-change interrupts
• ISR names are predefined
• the pin change interrupts can only CHANGE mode
• can work with timers, counters, comm port, ..........

Example: Interrupts placement

• void setup(){
• attachInterrupt(0, Dr_ZAP_Int0, CHANGE);//on pin INT0 and hence first arg is 0
• attachInterrupt(1, Dr_ZAP_Int1, RISING);//on pin INT1 and hence first arg is 1
• }
• void Dr_ZAP_Int0(){
•     //do what you need to do
• }
• void Dr_ZAP_Int1(){
•     //do very important stuff
• }

Example: BLINK (every time there is a change on INT0 pin(digital pin-2), toggles the pin13 state.

int pin = 13;
volatile int state = LOW;
void setup(){
pinMode(pin, OUTPUT);
attachInterrupt(0, blink, CHANGE);
}
void loop(){
digitalWrite(pin, state);
}
void blink(){
state = !state;
}

The Interrupt Sequence: Interrupts are complex as compared to the other peripheral. Hence, it is a better practice to follow a fixed sequence while handling interrupts in Arduino Atmega328.

• void setup(){
• //instructions
• }
• void Dr_ZAP_Interrupts0(){
• //instructions for interrupt-0
• //no special return instruction needed.
• }
• void Dr_ZAP_Interrupt1(){
• //instructions for interrupt 1
• }
• void loop(){
• //main loop instructions
• //do stuff here that can be interrupted
• }
• float compute(int var){
• return (float)var*3.1415927;
• }

Arduino IDE interrupts:

• attachInterrupt() function

• detachInterrupt() function is used to detach the attached interrupt. Let say there is some attached interrupt at INT0(digital pin-2) and you want to disable it, then use the detachInterrupt() function with argument-0 (0: for INT0 and 1: for INT1).
• interrupts(): enables the interrupts to occur, meaning enables the global interrupt flag.
• noInterrupts(): disables all the interrupts to occur, except the reset interrupt

Atmega328 built-in interrupt Vectors:

Arduino IDE predefined names for the above interrupts:

Interrupt Service routine: also called as Interrupt handler

• Interrupt handler must be invisible to program.
• except through side-effect, e.g. via flags or variables
• changes program timing
• can't rely on dead-reckoning
• con not stomp on program state, e.g. registers
• save and restore any registers used(including SREG)
• must be short and sweet
• Global interrupt enable
• Global interrupt enable bit is present in SREG
• Allows all interrupts to be disabled with one bit 
• sei(): set the bit, same as interrupt() function
• cli(): clear the bit, same as noInterrupt() function
• Interrupt priority is determined by order in table shown above
• Lower addresses have higher priority

External Interrupts:

• monitors changes in signals on pins
• what causes an interrupt can be configured by setting the control resgisters appropriately.
• If there is a pulse coming on external input pins then pulses must be slower than I/O clock rate.
• INT0(PORTD digital pin2) and INT1(PORTD digital pin3) are the two external interrupts
• PCINT[23:0] pin change interrupts, there will be an external interrupts if any of these pin toggles. Remember that PCINT[15] is not available here, and PCINT[14] is reset pin.
• PCINT[7:0] at PORTB[7:0]
• PCINT[13:8] at PORTC[6:0]
• PCINT[23:16] at PORTD[7:0]

EICRA: External Interrupt Control Register A

ISC0 and ISC1 are for INT0 and INT1 respectively.

EIMSK: External Interrupt Mask Register

EIFR: External Interrupt Flag Register

EIMSK's INT0 and INT1 bits are used to enable and disable the interrupt on INT0(D2 pin) and INT1(D3 pin).

Status register SREG:

Enable/Disable Interrupts:

It is possible to use pin change interrupts on "all" pins of the arduino using Pin Change Interrupt Requests. The example below uses some macros from the pins_arduino.h library.

The interrupt can be enabled for each pin individually (analog and digital!), but there are only 3 interrupt vectors, so 6-8 pins share one service routine:

• ISR (PCINT0_vect) pin change interrupt for D8 to D13
• ISR (PCINT1_vect) pin change interrupt for A0 to A5
• ISR (PCINT2_vect) pin change interrupt for D0 to D7
  
  

The interrupt fires on a change of the pin state (rising and falling).