Datasheet

Principles for Low-Power Applications

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2.3.1 Entering and Exiting Low-Power Modes

An enabled interrupt event wakes the MSP430 from any of the low-power operating modes. The program

flow is:

• Enter interrupt service routine:

– The PC and SR are stored on the stack

– The CPUOFF, SCG1, and OSCOFF bits are automatically reset

• Options for returning from the interrupt service routine:

– The original SR is popped from the stack, restoring the previous operating mode.

– The SR bits stored on the stack can be modified within the interrupt service routine returning to a

different operating mode when the RETI instruction is executed.

; Enter LPM0 Example

BIS #GIE+CPUOFF,SR ; Enter LPM0

; ... ; Program stops here

;

; Exit LPM0 Interrupt Service Routine

BIC #CPUOFF,0(SP) ; Exit LPM0 on RETI

RETI

; Enter LPM3 Example

BIS #GIE+CPUOFF+SCG1+SCG0,SR ; Enter LPM3

; ... ; Program stops here

;

; Exit LPM3 Interrupt Service Routine

BIC #CPUOFF+SCG1+SCG0,0(SP) ; Exit LPM3 on RETI

RETI

2.4 Principles for Low-Power Applications

Often, the most important factor for reducing power consumption is using the MSP430 clock system to

maximize the time in LPM3. LPM3 power consumption is less than 2 µA typical with both a real-time clock

function and all interrupts active. A 32-kHz watch crystal is used for the ACLK and the CPU is clocked

from the DCO (normally off) which has a 1-µs wake-up.

• Use interrupts to wake the processor and control program flow.

• Peripherals should be switched on only when needed.

• Use low-power integrated peripheral modules in place of software driven functions. For example

Timer_A and Timer_B can automatically generate PWM and capture external timing, with no CPU

resources.

• Calculated branching and fast table look-ups should be used in place of flag polling and long software

calculations.

• Avoid frequent subroutine and function calls due to overhead.

• For longer software routines, single-cycle CPU registers should be used.

System Resets, Interrupts, and Operating Modes SLAU144J–December 2004–Revised July 2013

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