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8331B–AVR–03/12
Atmel AVR XMEGA AU
12. Interrupts and Programmable Multilevel Interrupt Controller
12.1
Features
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Short and predictable interrupt response time
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Separate interrupt configuration and vector address for each interrupt
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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
12.2
Overview
Interrupts signal a change of state in peripherals, and this can be used to alter program execu-
tion. 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 acknowl-
edged 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 inter-
rupt 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.
12.3
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 enter-
ing the interrupt. The status register (SREG) is not saved automatically upon an interrupt