The document explains the methods of I/O operation in Atmega328P, focusing on synchronous and asynchronous operations, the role of polling and interrupts, and how to enable and disable interrupts. It details the structure of interrupt vectors and various control registers, describing settings for external interrupts and pin change interrupts. Additionally, it provides examples of Arduino language support for handling interrupts effectively.

1. Interrupts in Atmega328P

2. Methods of I/O Operation
 Synchronous I/O operation.
•This is possible when the I/O device operates at the same speed as
that of MCU.
•In this case the I/O operation with the device is performed by the MCU
assuming that the device is always ready.
Asynchronous I/O Operation.
In this case, generally the I/O device is very slow when compared to the
MCU. Then, it can perform I/O operation by the following methods:
• By polling.
• By using interrupts.

3. I/O Operation by Polling
Polling results in waste of MCU time and power and does not permit
masking of devices since all devices have to be checked.

4. Polling Example

5. I/O Operation by Interrupts
• The MCU keeps on doing some useful operation, or it remains in a low-
power state after enabling interrupts.
• When I/O device is ready, it interrupts the MCU.
• The MCU completes execution of the current instruction, saves the return
address on the stack, and then jumps to the Interrupt Service Routine
(ISR) to serve the device.
• After this, the MCU returns back to the interrupted program by loading the
return address from the stack to the Program Counter.
• Interrupts can be enabled or disabled dynamically and when multiple
interrupts come simultaneously priorities can be assigned.

6. Interrupt Example

7. Default Interrupt Vectors in Atmega328P

8. Interrupt Vectors in Atmega328P
Interrupt vector is a definite address in the program memory where
a JMP instruction should be written to jump to the ISR
corresponding to the interrupt.
Each interrupt vector occupies two program memory words in
order to provide space for JMP instruction.
In Atmega328P, the RESET Vector is affected by BOOTRST fuse
and the Interrupt Vector start address is affected by IVSEL bit in
the MCUCR.

9. Enabling and Disabling Interrupts in Atmega328P
• In SREG if I = 1, then all interrupts are enabled.
• We can make I = 1 by using SEI instruction in ALP and sei() function in
C program.
• In SREG if I = 0, then all interrupts are disabled.
• We can make I = 0 by using CLI instruction in ALP and cli() function in
C program.
• Upon reset I = 0 so all interrupts are disabled.

10. Bits of (EICRA) External Interrupt Control Register A
Control Nature of INT1 and INT0
Bits 7, 6, 5, and 4 are not used and are always read as 0.

11. Bits of EIMSK(External Interrupt Mask Register)
Mask/Unmask INT1 and INT0
• Masking/unmasking means disabling/enabling respectively.
• When INT1 = 1 INT1 is unmasked if I = 1 in SREG.
• When INT1 = 0 INT1 is masked even if I = 1 in SREG.
• When INT0 = 1 INT0 is unmasked if I = 1 in SREG.
• When INT0 = 0 INT0 is masked even if I = 1 in SREG.
• Activity on the pins of INT1/INT0 will cause an interrupt
• request even if the corresponding pins are configured as output
pins.
• Bits 2 to 7 are unused and will always be read as 0.

12. Bits of (EIFR) External Interrupt Flag Register
Show Status of INT1 and INT0
• Bits 2 to 7 are unused and will always be read as 0.
• When INT1 interrupt is triggered, INTF1 becomes 1. This flag is cleared
when ISR is executed.
• When INT0 interrupt is triggered, INTF0 becomes 1. This flag is cleared
when ISR is executed.
• INTF1/INTF0 flags can be cleared by writing a logical 0 to the respective
bit.
• INTF1/INTF0 flags are always cleared when the respective interrupts are
configured as a level interrupt.
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13. Interrupts
Type 1 – Event is remembered when interrupt is disabled
If interrupt is not enabled, flag is set
When interrupt is enabled again, interrupt takes place, and flag is reset
Type 2 – Event is not remembered when interrupt is disabled
Signal level causes interrupt
If level occurs when interrupt is enabled, interrupt takes place
If interrupt is not enabled, and level goes away before the interrupt is enabled,
nothing happens

14. When Will External Interrupts be Triggered?
• When the INT0 or INT1 interrupts are enabled and are configured as low
level triggered (Type 2), the interrupts will trigger as long as…
• The pin is held low.
• The low level is held until the completion of the currently executing
instruction.
• The level is held long enough for the MCU to completely wake-up
(assuming it was asleep).
– Low level interrupt on INT0 and INT1 are detected asynchronously (no
clock required). The I/O clock is halted in all sleep modes except idle mode.
Therefore low level interrupts can be used for waking the part from all
sleep modes.
• Among other applications, low level interrupts may be used to implement a
handshake protocol.

15. When the INT0 or INT1 interrupts are enabled and are configured as
edge or logic change (toggle) triggered, (Type 11) the interrupts will
trigger as long as…
• The I/O clock is present.
– This implies that these interrupts cannot be used for waking up the
part from sleep modes other than idle mode.
• The pulse lasts longer than one I/O clock period. Shorter pulses are
not guaranteed to generate an interrupt.

16. • In addition to our two (2) external interrupts, twenty-three (23) pins
can be programmed to trigger an interrupt if there pin changes state.
• These 23 pins are in turn divided into three (3) interrupt groups (PCI
2:0) corresponding to the three GPIO Ports B, C, and D
• Each of the groups are assigned to one pin change interrupt flag
(PCIF) bit (2:0).
• A pin change interrupt flag will be set, if the interrupt is enabled (see
How to Enable a Pin Change Interrupt), and any pin assigned to the
group changes state (toggles).

17. How to Enable a PIN Change Interrupt
In addition to our two (2) external interrupts, twenty-three (23) pins PCINT 23:16,
14:0 can be programmed to trigger an interrupt if there pin changes state.
These 23 pins are divided into three (3) interrupt groups (PCI 2:0) of eight (8),
seven (7) and (8).
Consequently to enable and individual pin change interrupt 3 interrupt mask bits
must be set to one (1).
The SREG global interrupt enable bit I
The pin change interrupt enable bit (PCIE 2:0) group the pin is assigned. Specifically,
a pin change interrupt PCI2 will trigger if any enabled PCINT23..16 pin toggles.
A pin change interrupt PCI1 will trigger if any enabled PCINT14..8 pin toggles.
A pin change interrupt PCI0 will trigger if any enabled PCINT7..0 pin toggles.

18. The individual pin change interrupt enable mask bit assigned to the pin (PCINT 23:0) is set. These mask bits are located
in the three pin change mask registers assigned to each group.

19. Arduino Language Support for External Interrupts
attachInterrupt(interrupt, function, mode)
interrupt: 0 or 1
function: interrupt function to call
mode: LOW, CHANGE, RISING, FALLING
detachInterrupt(interrupt)
interrupts( ) – Enable interrupts : sei( )
noInterrupts( ) – Disable interrupts : cli( )

20. int pin = 2; //define interrupt pin to 2
volatile int state = LOW; // To make sure variables shared between an ISR
//the main program are updated correctly,declare them as volatile.
void setup()
{
pinMode(13, OUTPUT); //set pin 13 as output
attachInterrupt(digitalPinToInterrupt(pin), blink, CHANGE); //interrupt at
pin 2 blink ISR when pin to change the value
}
void loop()
{
digitalWrite(13, state); //pin 13 equal the state value
}
void blink()
{
//ISR function
state = !state; //toggle the state when the interrupt occurs
}

22. const int ledPin = 13;
const int buttonPin = 2;
int ledToggle = LOW;
int previousState = HIGH;
int buttonState;
void setup()
{
pinMode(ledPin, OUTPUT);
pinMode(buttonPin, INPUT_PULLUP);
}
void loop()
{
buttonState = digitalRead(buttonPin);
if((buttonState == LOW) && (previousState == HIGH))
{
ledToggle = !ledToggle;
digitalWrite(ledPin, ledToggle);
}
previousState = buttonState;
delay(300);
}