ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

The System Level Programmers’ Model
The WaitForEvent() pseudocode procedure optionally suspends execution until a WFE wake-up event or
reset occurs, or until some earlier time if the implementation chooses. It is IMPLEMENTATION DEFINED
whether restarting execution after the period of suspension causes a ClearEventRegister() to occur.
The SendEvent() pseudocode procedure sets the Event Register of every processor in the multiprocessor
system.
B1.6.9 Wait For Interrupt
Previous versions of the ARM architecture have included a Wait For Interrupt concept, and Wait For
Interrupt is a required feature of the architecture from ARMv6. In ARMv7, Wait For Interrupt is supported
only through an instruction, WFI, that is provided in the ARM and Thumb instruction sets. For more
information, see WFI on page A8-810.
Note
In ARMv7 the CP15 c7 encoding previously used for WFI is redefined as a NOP, see CP15 c7, No
Operation (NOP) on page B3-138 and CP15 c7, Miscellaneous functions on page B4-72.
When a processor issues a WFI instruction it can suspend execution and enter a low-power state. It can remain
in that state until the processor detects a reset or one of the following WFI wake-up events:
an IRQ interrupt, regardless of the value of the CPSR.I bit
an FIQ interrupt, regardless of the value of the CPSR.F bit
an asynchronous abort, regardless of the value of the CPSR.A bit
a debug event, when invasive debug is enabled and the debug event is permitted.
When the hardware detects a WFI wake-up event, or earlier if the implementation chooses, the WFI
instruction completes.
WFI wake-up events cannot be masked by the mask bits in the CPSR.
Note
Because debug entry is one of the WFI wake-up events, ARM strongly recommends that Wait For
Interrupt is used as part of an idle loop rather than waiting for a single specific interrupt event to occur
and then moving forward. This ensures the intervention of debug while waiting does not significantly
change the function of the program being debugged.
In some previous implementations of Wait For Interrupt, the idle loop is followed by exit functions
that must be executed before the interrupt is taken. The operation of Wait For Interrupt remains
consistent with this model, and therefore differs from the operation of Wait For Event.
Some implementations of Wait For Interrupt drain down any pending memory activity before
suspending execution. This increases the power saving, by increasing the area over which clocks can
be stopped. This operation is not required by the ARM architecture, and code must not rely on Wait
For Interrupt operating in this way.
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