PM0223 datasheet(26/110 Pages) STMICROELECTRONICS

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The STM32L0 Cortex-M0+ Processor

PM0223

26/110

DocID025763 Rev 1

2.3

Exception model

This section describes the exception model.

2.3.1

Exception states

Each exception is in one of the following states:

2.3.2

Exception types

The exception types are:

Inactive

The exception is not active and not pending.

Pending

The exception is waiting to be serviced by the processor.

An interrupt request from a peripheral or from software can change

the state of the corresponding interrupt to pending.

Active

An exception that is being serviced by the processor but has not com-

pleted.

Note: An exception handler can interrupt the execution of another exception

handler. In this case both exceptions are in the active state.

Active and pendingThe exception is being serviced by the processor and there is a pend-

ing exception from the same source.

Reset

Reset is invoked on power up or a warm reset. The exception model

treats reset as a special form of exception. When reset is asserted,

the operation of the processor stops, potentially at any point in an

instruction. When reset is deasserted, execution restarts from the

address provided by the reset entry in the vector table. Execution

restarts as privileged execution in Thread mode.

NMI

A NonMaskable Interrupt (NMI) can be signalled by a peripheral or

triggered by software. This is the highest priority exception other than

reset. It is permanently enabled and has a fixed priority of -2. NMIs

cannot be:

•

Masked or prevented from activation by any other exception.

•

Preempted by any exception other than Reset.

HardFault

A HardFault is an exception that occurs because of an error during

normal or exception processing. HardFaults have a fixed priority of -1,

meaning they have higher priority than any exception with

configurable priority.

SVCall

A Supervisor Call (SVC) is an exception that is triggered by the SVC

instruction. In an OS environment, applications can use SVC

instructions to access OS kernel functions and device drivers.

PendSV

PendSV is an interrupt-driven request for system-level service. In an

OS environment, use PendSV for context switching when no other

exception is active.

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