Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

12-2

ARM DDI0198D

The power management facilities provided by the ARM926EJ-S processor are:

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The ARM926EJ-S processor can be put into a low-power state by the wait for interrupt 
instruction:

MCR p15,0,<Rd>,c7,c0,4

This instruction switches the ARM926EJ-S processor into a low-power state until either 
an interrupt (IRQ or FIQ) or a debug request occurs. The debug request can either be an 
external debug request EDBGRQ or a debug request made by the debugger by writing 
to the DBGRQ bit of the ARM9EJ-S debug control register using scan chain 2.

In wait for interrupt mode, all internal ARM926EJ-S clocks can be stopped. The switch 
into the low-power state is delayed until all write buffers have been drained, and the 
ARM926EJ-S memory system is in a quiescent state. 

The switch into low-power state is indicated by the assertion of the STANDBYWFI 
signal. If STANDBYWFI is asserted then it is guaranteed that all of ARM926EJ-S 
external interfaces (AHB, TCM, and external coprocessor) are in an idle state. The 
STANDBYWFI signal is intended to be used to shut down clocks to other parts of the 
system, such as external coprocessors, that do not have to be clocked if the 
ARM926EJ-S processor is idle.

The STANDBYWFI signal is deasserted in the second cycle following an interrupt or 
a debug request. It is guaranteed that no form of access on any external interface is 
started until the cycle after STANDBYWFI is deasserted. Figure 12-1 shows the 
deassertion of the STANDBYWFI signal after an IRQ interrupt.

CLK

STANDBYWFI

nIRQ