PM0223 datasheet(32/110 Pages) STMICROELECTRONICS

32 / 110 page

The STM32L0 Cortex-M0+ Processor

PM0223

32/110

DocID025763 Rev 1

If another higher priority exception occurs during exception entry, the processor starts exe-

cuting the exception handler for this exception and does not change the pending status of

the earlier exception. This is the late arrival case.

Exception return

Exception return occurs when the processor is in Handler mode and execution of one of the

following instructions attempts to set the PC to an EXC_RETURN value:

•

A POP instruction that loads the PC.

•

B PBX instruction using any register.

The processor saves an EXC_RETURN value to the LR on exception entry. The exception

mechanism relies on this value to detect when the processor has completed an exception

handler. Bits[31:4] of an EXC_RETURN value are 0xFFFFFFF. When the processor loads a

value matching this pattern to the PC it detects that the operation is a not a normal branch

operation and, instead, that the exception is complete. As a result, it starts the exception

return sequence. Bits[3:0] of the EXC_RETURN value indicate the required return stack and

processor mode, as Table 14 on page 32 shows.

Table 14. Exception return behavior

EXC_RETURN

Description

0xFFFFFF1

Return to Handler mode.

Exception return gets state from the main stack.

Execution uses MSP after return.

0xFFFFFF9

Return to Thread mode.

Exception return gets state from MSP.

Execution uses MSP after return.

0xFFFFFFD

Return to Thread mode.

Exception return gets state from PSP.

Execution uses PSP after return.

All other values

Reserved.

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