PM0056 datasheet(41/156 Pages) STMICROELECTRONICS

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PM0056

The Cortex-M3 processor

2.4.2

Fault escalation and hard faults

All faults exceptions except for hard fault have configurable exception priority, see System

handler priority registers (SHPRx) on page 138. Software can disable execution of the

handlers for these faults, see System handler control and state register (SCB_SHCSR) on

page 140.

Usually, the exception priority, together with the values of the exception mask registers,

determines whether the processor enters the fault handler, and whether a fault handler can

preempt another fault handler. as described in Section 2.3: Exception model on page 33.

In some situations, a fault with configurable priority is treated as a hard fault. This is called

priority escalation, and the fault is described as escalated to hard fault. Escalation to hard

fault occurs when:

•

A fault handler causes the same kind of fault as the one it is servicing. This escalation

to hard fault occurs because a fault handler cannot preempt itself because it must have

the same priority as the current priority level.

•

A fault handler causes a fault with the same or lower priority as the fault it is servicing.

This is because the handler for the new fault cannot preempt the currently executing

fault handler.

•

An exception handler causes a fault for which the priority is the same as or lower than

the currently executing exception.

•

A fault occurs and the handler for that fault is not enabled.

If a bus fault occurs during a stack push when entering a bus fault handler, the bus fault

does not escalate to a hard fault. This means that if a corrupted stack causes a fault, the

fault handler executes even though the stack push for the handler failed. The fault handler

operates but the stack contents are corrupted.

Only Reset and NMI can preempt the fixed priority hard fault. A hard fault can preempt any

exception other than Reset, NMI, or another hard fault.

Bus error:

Bus fault

During exception stacking

STKERR

Configurable fault status register

(SCB_CFSR) on page 142

During exception unstacking

UNSTKERR

During instruction prefetch

IBUSERR

Precise data bus error

PRECISERR

Imprecise data bus error

IMPRECISERR

Attempt to access a coprocessor

Usage fault

NOCP

Configurable fault status register

(SCB_CFSR) on page 142

Undefined instruction

UNDEFINSTR

Attempt to enter an invalid

instruction set state(1)

INVSTATE

Invalid EXC_RETURN value

INVPC

Illegal unaligned load or store

UNALIGNED

Divide By 0

DIVBYZERO

1. Attempting to use an instruction set other than the Thumb instruction set.

Table 18. Faults (continued)

Fault

Handler

Bit name

Fault status register

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