Functional Description
ARM DDI 0500D
Copyright © 2013-2014 ARM. All rights reserved.
2-19
ID021414
Non-Confidential
Standby state
The following sections describe the methods of entering standby state:
•
Core Wait for Interrupt
.
•
Core Wait for Event
on page 2-20
.
•
L2 Wait for Interrupt
on page 2-20
.
Core Wait for Interrupt
Wait for Interrupt is a feature of the ARMv8-A architecture that puts the core in a low-power
state by disabling most of the clocks in the core while keeping the core powered up. Apart from
a small dynamic power overhead on the logic to enable the core to wake up from WFI
low-power state, this reduces the power drawn to static leakage current only.
Software indicates that the core can enter the WFI low-power state by executing the
WFI
instruction.
When the core is executing the
WFI
instruction, the core waits for all instructions in the core to
retire before entering the idle or low power state. The
WFI
instruction ensures that all explicit
memory accesses, that occurred before the
WFI
instruction in program order, have retired. For
example, the
WFI
instruction ensures that the following instructions received the required data or
responses from the L2 memory system:
•
Load instructions.
•
Cache and TLB maintenance operations.
•
Store exclusive instructions.
In addition, the
WFI
instruction ensures that store instructions have updated the cache or have
been issued to the SCU.
While the core is in WFI low-power state, the clocks in the core are temporarily enabled without
causing the core to exit WFI low-power state, when any of the following events are detected:
•
A snoop request that must be serviced by the core L1 Data cache.
•
A cache or TLB maintenance operation that must be serviced by the core L1 Instruction
cache, data cache, or TLB.
•
An APB access to the debug or trace registers residing in the core power domain.
Exit from WFI low-power state occurs when the core detects a reset or one of the WFI wake up
events as described in the
ARM
®
Architecture Reference Manual ARMv8, for ARMv8-A
architecture profile
.
On entry into WFI low-power state,
STANDBYWFI
for that core is asserted. Assertion of
STANDBYWFI
guarantees that the core is in idle and low power state.
STANDBYWFI
continues to assert even if the clocks in the core are temporarily enabled because of an L2 snoop
request, cache, or TLB maintenance operation or an APB access.
Note
STANDBYWFI
does not indicate completion of L2 memory system transactions initiated by
the processor. All Cortex-A53 processor implementations contain an L2 memory system. This
includes implementations without an L2 cache.