Subject : Embedded Systems Design

Topic : Fundamentals of Embedded Systems

Difficulty level : High

i) ISR (Interrupt Service Routine):

• There is a subroutine written for servicing ISR.
• The location of ISR in the IVT (Interrupt Vector Table). And IVT in is the memory.
• In microprocessor/ microcontroller, there are fixed predefined interrupt vector addresses for each interrupt source.

ii) Interrupt Priorities:

• When multiple interrupt requests come at the same time or one request is being serves and another request comes, then in such cases the need for priorities arises.
• Non-maskable interrupts and software interrupts have high priority.
• General purpose interrupts have low priority.
• Vectored interrupts have high priority whereas non-vectored interrupts have low priority.

iii) Interrupt Nesting:

• When a high priority interrupt interrupts a low priority ISR, then it is known as interrupt nesting.
• If a high priority ISR is executing and a low priority interrupt comes in, then the higher priority ISR is finished first and then the lower priority ISR is executed.
• Interrupt nesting is useful and effective in real time embedded systems where devices may require urgent service from processor.

iv) Interrupt Latency:

The time gap between the occurrence of interrupt and beginning of execution of corresponding ISR is called interrupt latency. Following methods are used to reduce interrupt latency-

• Short interrupt routines – Waiting time for same or lower priority interrupt is reduced.
• Disabling interrupt for short period of time.
• Usage of nested interrupt ladder – allows further interrupt to occur while servicing exiting.
• Prioritize interrupts – Give time to lower interrupts until higher priority interrupt occurs. Therefore, higher priority interrupt provides lower or average interrupt latency.