Intel SA-1100 manual Power Manager, Run Mode, Entering Idle Mode

System Control Module

9.5Power Manager

The SA-1100 contains power management logic that controls the transition between three different modes of operation: run, idle, and sleep. These modes are used to reduce processor power consumption at times when some functions are not needed, or when the system’s power supply is low or out of regulation. Each of the respective modes is associated with a reduced level of power consumption. Idle mode is entered via software. Sleep mode is entered either via software or by asserting one of two input pins that indicate a power supply fault. Idle mode is exited through an interrupt. Sleep mode is exited through a preprogrammed wake-up condition. Both modes may be exited in extreme cases via hardware reset. If none of the power management modes is active and the SA-1100 is out of reset, then it is said to be in run mode.

9.5.1Run Mode

Run mode is the normal operating mode of the SA-1100: all power supplies are enabled, all clocks are running, and every on-chip resource is functional. This is the normal state of operation for the processor while it is executing code. Under usual conditions, the processor enters run mode after successful power-up and reset of the part.

9.5.2Idle Mode

Idle mode allows a software application to stop the CPU when not in use, while continuing to monitor interrupt service requests both on or off-chip. When an interrupt occurs, the CPU is reactivated. During idle mode, the SCM, PM, and MPCM are each fully operational.

In idle mode, the CPU clock is stopped. Since the SA-1100 is static, all CPU state information is saved. This allows the part to be switched back to run mode, starting operation exactly where it left off. During idle mode, all other on-chip resources are active, including: all system unit modules (real-time clock, operating system timer, interrupt controller, general-purpose I/O, and power manager); all peripheral unit modules (DMA controller, LCD controller, serial controller 0-4); and all memory controller resources. The PLL also remains in lock so that the part can be brought out of idle mode quickly when an interrupt occurs.

9.5.2.1Entering Idle Mode

Idle mode is entered while in run mode by executing a three instruction sequence consisting of the privileged on-chip coprocessor 15 instruction ‘disable clock switching’, a load from a noncacheable memory location (C=B=0), and the privileged on-chip coprocessor 15 instruction ‘wait for interrupt’. This sequence must reside in the first three words of an instruction cache line, which requires that the linker align the idle mode instruction sequence on an eight word boundary. Idle mode is entered by following the exact code sequence:

AREA ½ Idle$$Code ½, CODE, READONLY, ALIGN=5

		;Aligned to 8 word boundary
		;p15 = coprocessor 15
		;r0 = register 0 (contents not used)
		;c15 = test, clk, and idle cntl register
		;c2 = CRm = 0b0010
mcr	p15, 0, r0, c15, c2, 2	;2 = OPC_2 = 0b010
ldr	r0, <r1>	;<r1> points to non-cachable mem loc
mcr	p15, 0, r0, c15, c8, 2	;c8 = CRm = 0b1000

9-26 SA-1100 Developer’s Manual