Atmel XMEGA B, User Manual

Page: 118 / 415

118 XMEGA B [DATASHEET]
8291B–AVR–01/2013
11. Interrupts and Programmable Multilevel Interrupt Controller
11.1 Features

Short and predictable interrupt response time

Separate interrupt configuration and vector address for each interrupt

Programmable multilevel interrupt controller

Interrupt prioritizing according to level and vector address

Three selectable interrupt levels for all interrupts: low, medium and high

Selectable, round-robin priority scheme within low-level interrupts

Non-maskable interrupts for critical functions

Interrupt vectors optionally placed in the application section or the boot loader section
11.2 Overview
Interrupts signal a change of state in peripherals, and this 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 is present. The programmable multilevel interrupt
controller (PMIC) controls the handling and prioritizing of interrupt requests. When an interrupt request is acknowledged
by the PMIC, the program counter is set to point to the interrupt vector, and the interrupt handler can be executed.
All peripherals can select between three different priority levels for their interrupts: low, medium, and high. Interrupts are
prioritized according to their level and their interrupt vector address. Medium-level interrupts will interrupt low-level
interrupt handlers. High-level interrupts will interrupt both medium- and low-level interrupt handlers. Within each level, the
interrupt priority is decided from the interrupt vector address, where the lowest interrupt vector address has the highest
interrupt priority. Low-level interrupts have an optional round-robin scheduling scheme to ensure that all interrupts are
serviced within a certain amount of time.
Non-maskable interrupts (NMI) are also supported, and can be used for system critical functions.
11.3 Operation
Interrupts must be globally enabled for any interrupts to be generated. This is done by setting the global interrupt enable
( I ) bit in the CPU status register. The I bit will not be cleared when an interrupt is acknowledged. Each interrupt level
must also be enabled before interrupts with the corresponding level can be generated.
When an interrupt is enabled and the interrupt condition is present, the PMIC will receive the interrupt request. Based on
the interrupt level and interrupt priority of any ongoing interrupts, the interrupt is either acknowledged or kept pending
until it has priority. When the interrupt request is acknowledged, the program counter is updated to point to the interrupt
vector. The interrupt vector is normally a jump to the interrupt handler; 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 PMIC status register contains state information that ensures that the PMIC 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 PMIC to the state it had before entering the interrupt. The status register (SREG) is not saved automatically upon an
interrupt request. The RET (subroutine return) instruction cannot be used when returning from the interrupt handler
routine, as this will not return the PMIC to its correct state.