Interrupts

Question 1

What is an interrupt?

Answer 1

• an asynchronous signal to the processor indicating the need for attention (normally by hardware) or
• a synchronous event in software indicating the processor the need for a change in execution

Question 2

ISR and IRQ?

Answer 2

ISR - interrupt service routine

IRQ - interrupt request (act of interrupting)

Question 3

RETI used when?

Answer 3

ISR execution completes -> Returning from interrupt

–> restores registers and sets I-Bit in SREG

Question 4

What is an interrupt source?

Answer 4

• signal starting an interrupt
• external = rising/fall ing edge
• internal = timer fired , ADC completion, I2C, SPI

Question 5

What is an interrupt flag?

Answer 5

• each hardware interrupt source has an associate Interrupt flag which is indicating that an intterupt occured

Question 6

What is the interrupt mask?

Answer 6

• Identical bit set to interrupt flag
• setting a bit here will trigger ISR (unmasking) if flag is set (enabing certain interrupt)
• when bit is cleared (masked), ISR is not executed, even if flag register signals an interrupt

Question 7

What is the interrupt vector table?

Answer 7

• location in memory with adresses of ISRs

Question 8

ISR Steps?

Answer 8

• relevant registers are pushed on stack (saving processors context)
• acknowleding the interrupt
• restoring the processor context

Question 9

Example of an ISR definition (signal/interrupt). Explain difference=

Answer 9

• void ISRname (void) _ attribute _((signal, used));
• ‘used’ so that the compiler does not say function not used
• “ISRname” is predefined by the Machine
• interrupt (re-) enables interrupts on entry

Question 10

What is a race conditions?

Answer 10

• Problems arising when using ISRs

- Situation where outcome varies depending on precise order in which instructions of main code and ISR are executed

Question 11

What is a critical section?

Answer 11

Code that must be executed without interruptions

Question 12

Example of an atomic access block?

Answer 12

Non-nested:
cli();
do_stuff();
sei();

For nested interrupts:
uint8_t sreg = SREG;
cli();
do_stuff();
SREG = sreg;

Question 13

Definition: Time triggered system

Answer 13

• all activities are carried out at a certain point in time
• no interrupts except by timer
• schedule computed off-line (algorithms)
• deterministic behavior at run time
• interaction with environment through polling (con, critical aspect)