13.2 Overview

An interrupt request signals a change of state inside a peripheral, and can be used to alter program execution. Peripherals can have one or more interrupts, and all are individually enabled and configured.

When an interrupt is enabled and configured, it will generate an interrupt request when the interrupt condition occurs.

The CPU Interrupt Controller (CPUINT) handles and prioritizes interrupt requests. When an interrupt is enabled and the interrupt condition occurs, the CPUINT will receive the interrupt request. Based on the interrupt's priority level and the priority level of any ongoing interrupts, the interrupt request is either acknowledged or kept pending until it has priority. When an interrupt request is acknowledged by the CPUINT, the program counter is set to point to the interrupt vector. The interrupt vector is normally a jump to the interrupt handler (i.e., the software routine that handles the interrupt). After returning from the interrupt handler, program execution continues from where it was before the interrupt occurred. One instruction is always executed before any pending interrupt is served.

The CPUINT Status register (CPUINT.STATUS) contains state information that ensures that the CPUINT returns to the correct interrupt level when the RETI (interrupt return) instruction is executed at the end of an interrupt handler. Returning from an interrupt will return the CPUINT to the state it had before entering the interrupt. CPUINT.STATUS is not saved automatically upon an interrupt request.

By default, all peripherals are priority level 0. It is possible to set one single interrupt vector to the higher priority level 1. Interrupts are prioritized according to their priority level and their interrupt vector address. Priority level 1 interrupts will interrupt level 0 interrupt handlers. Among priority level 0 interrupts, the priority is determined from the interrupt vector address, where the lowest interrupt vector address has the highest interrupt priority.

Optionally, a round-robin scheduling scheme can be enabled for priority level 0 interrupts. This ensures that all interrupts are serviced within a certain amount of time.

Interrupt generation must be globally enabled by writing a '1' to the Global Interrupt Enable bit (I) in the CPU Status register (CPU.SREG). This bit is not cleared when an interrupt is acknowledged.