ULP Advisor > Rule 1.1 Ensure LPM Usage

Rule1.1 Use Low Power Modes

ULP Advisor - Rule Table ULP 1.1 Ensure LPM usage ULP 2.1 Leverage timer module for delay loops ULP 3.1 Use ISRs instead of flag polling ULP 4.1 Terminate unused GPIOs ULP 5.1 Avoid processing-intensive operations: modulo, divide. ULP 5.2 Avoid processing-intensive operations: floating point ULP 5.3 Avoid processing-intensive operations: (s)printf() ULP 6.1 Avoid multiplication on devices without hardware multiplier ULP 6.2 Use MATHLIB for complex math operations ULP 6.3 Use Low Energy Accelerator (LEA) software library ULP 7.1 Use local instead of global variables where possible ULP 8.1 Use 'static' & 'const' modifiers for local variables ULP 9.1 Use pass by reference for large variables ULP 10.1 Minimize function calls from within ISRs ULP 11.1 Use lower bits for loop program control flow ULP 11.2 Use lower bits for port bit-banging ULP 12.1 Use DMA for large memcpy() calls ULP 12.1b Use DMA for potentially large memcpy() calls ULP 12.2 Use DMA for repetitive transfer ULP 13.1 Count down in loops ULP 14.1 Use unsigned variables for indexing ULP 15.1 Use bit-masks instead of bit-fields Let us know what you think! Feedback, suggestions & comments are welcome @ ULPAdvisorFeedback@list.ti.com

What it means

Low power mode (LPM) usage is highly recommended. One of the necessary steps to achieve low power consumption in an MSP430 application is to minimize the time spent active mode and maximize the time in low power modes. Peripherals can be configured to operate without CPU intervention and the CPU only needs to wake up to perform critical tasks and quickly return to low power mode.

Risks, Severity

An application that always stays in active mode is not energy efficient and could significantly reduce lifetime for battery-powered applications.

Why it is happening

This remark is issued when no LPM-entering instruction can be found in any code file in the project.

Remedy

• Use low power modes in your application when applicable, i.e. while waiting for certain peripheral tasks to complete, waiting for an interrupt, spinning in a while (1) loop, etc.
  
• Add LPM-entering instructions in the code where applicable such as:

__bis_SR_register( LPM0_bits);

Code Example

//******************************************************************************
//  MSP430G2xx3 Demo - Basic Clock, LPM3 Using WDT ISR, 32kHz ACLK
//
//  Description: This program operates MSP430 normally in LPM3, pulsing P1.0
//  at 4 second intervals. WDT ISR used to wake-up system. All I/O configured
//  as low outputs to eliminate floating inputs. Current consumption does
//  increase when LED is powered on P1.0. Demo for measuring LPM3 current.
//  ACLK = LFXT1/4 = 32768/4, MCLK = SMCLK = default DCO ~ 800kHz
//  //* External watch crystal installed on XIN XOUT is required for ACLK *//	
//
//
//           MSP430G2xx3
//         ---------------
//     /|\|            XIN|-
//      | |               | 32kHz
//      --|RST        XOUT|-
//        |               |
//        |           P1.0|-->LED
//
//  D. Dang
//  Texas Instruments Inc.
//  December 2010
//   Built with CCS Version 4.2.0 and IAR Embedded Workbench Version: 5.10
//******************************************************************************


#include <msp430g2553.h>

void main(void)
{
BCSCTL1 |= DIVA_2;                        // ACLK/4
WDTCTL = WDT_ADLY_1000;                   // WDT 1s/4 interval timer
IE1 |= WDTIE;                             // Enable WDT interrupt
P1DIR = 0xFF;                             // All P1.x outputs
P1OUT = 0;                                // All P1.x reset
P2DIR = 0xFF;                             // All P2.x outputs
P2OUT = 0;                                // All P2.x reset

while(1)
{
int i;
P1OUT |= 0x01;                            // Set P1.0 LED on
for (i = 5000; i>0; i--);              // Delay
P1OUT &= ~0x01;                       // Reset P1.0 LED off
__bis_SR_register(LPM3_bits + GIE);       // Enter LPM3
}  
}

#pragma vector=WDT_VECTOR
__interrupt void watchdog_timer (void)
{
 __bic_SR_register_on_exit(LPM3_bits);                 // Clear LPM3 bits from 0(SR)  
}

See the rest of the code examples for all MSP430 devices here!

More Resources

Want to squeeze a few more nanoAmps out of your application? Leverage the e2e (Engineer-to-Engineer) online community to get all of your ULP questions answered! Or, if you are an Ultra-Low Power pro, give back to the community with your expertise.

Go to MSP430's e2e online forum! 

If you are posting on the forums in relation to this rule, try using the tag "ULP_1.1"

For technical support please post your questions at http://e2e.ti.com. Please post only comments about the article Compiler/diagnostic messages/MSP430/10371 here.