What is interrupt ?
An interrupt is an external or internal event that interrupts the microcontroller to inform it that a device needs its service.
Why we need interrupt?
-A single microcontroller can serve several devices by two ways
- Interrupt-Whenever any device needs its service, the device notifies the microcontroller by sending it an interrupt signal. Upon receiving an interrupt signal, the microcontroller interrupts whatever it is doing and serves the device. The program which is associated with the interrupt is called the interrupt service routine (ISR) or interrupt handler
- Polling- The microcontroller continuously monitors the status of a given device. When the conditions met, it performs the service. After that, it moves on to monitor the next device until every one is serviced
Advantage of interrupt
– The polling method is not efficient, since it wastes much of the microcontroller’s time by polling devices that do not need service. The advantage of interrupts is that the microcontroller can serve many devices, Each devices can get the attention of the microcontroller based on the assigned priority . For the polling method, it is not possible to assign priority since it checks all devices in a round-robin fashion.
How does interrupt works?
- Whenever any device needs service of microcontroller, the device notifies the microcontroller by sending it an interrupt signal.
- Upon receiving an interrupt signal, the microcontroller interrupts whatever it is doing and saves the address of the next instruction (PC) on the stack pointer (SP).
- It jumps to a fixed location in memory, called the interrupt vector table, that holds the address of the ISR(interrupt service routine). Each interrupt has its own ISR. The microcontroller gets the address of the ISR from the interrupt vector table and jumps to it
- It starts to execute the interrupt service subroutine until it reaches the last instruction of the subroutine which is RETI (return from interrupt).RETI not used in C coding.
- Upon executing the RETI instruction, the microcontroller returns to the place where it was interrupted and First, it gets the program counter (PC) address from the stack pointer by popping the top two bytes of the stack into the PC.
- Then it starts to execute from that address and continue what it executing before.
- This whole process is shown graphically in above pics.
Interrupt vector table
Interrupt vector table shows priority of different interrupts. Upon Reset, all interrupts are disabled (masked), meaning that none will be responded to by the microcontroller if they are activated. There are 21 total interrupts in ATmega32 microcontroller.
- Applications – To provide services to the devices efficiently.
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