PM0223 datasheet(31/110 Pages) STMICROELECTRONICS

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PM0223

The STM32L0 Cortex-M0+ Processor

Exception entry

Exception entry occurs when there is a pending exception with sufficient priority and either:

•

The processor is in Thread mode.

•

The new exception is of higher priority than the exception being handled, in which case

the new exception preempts the exception being handled.

When one exception preempts another, the exceptions are nested.

Sufficient priority means the exception has greater priority than any limit set by the mask

pending but is not handled by the processor.

When the processor takes an exception, unless the exception is a tail-chained or a late-

arriving exception, the processor pushes information onto the current stack. This operation

is referred to as stacking and the structure of eight data words is referred to as a stack

frame. The stack frame contains the following information:

Figure 8. Stack frame

Immediately after stacking, the stack pointer indicates the lowest address in the stack

frame. The stack frame is aligned to a double-word address.

The stack frame includes the return address. This is the address of the next instruction in

the interrupted program. This value is restored to the PC at exception return so that the

interrupted program resumes.

The processor performs a vector fetch that reads the exception handler start address from

the vector table. When stacking is complete, the processor starts executing the exception

handler. At the same time, the processor writes an EXC_RETURN value to the LR. This

indicates which stack pointer corresponds to the stack frame and what operation mode the

processor was in before the entry occurred.

If no higher priority exception occurs during exception entry, the processor starts executing

the exception handler and automatically changes the status of the corresponding pending

interrupt to active.

Late-arriving This mechanism speeds up preemption. If a higher priority exception occurs

during state saving for a previous exception, the processor switches to

handle the higher priority exception and initiates the vector fetch for that

exception. State saving is not affected by late arrival because the state saved

would be the same for both exceptions. On return from the exception handler

of the late-arriving exception, the normal tail-chaining rules apply.

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