ULP Advisor > Rule 11.1 Use lower bits for loop program control flow

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

The MSP430 microcontroller CPU architecture provides a constant generator that can generate constants for the most commonly used bits: -1, 0, 1, 2, 4, and 8 (1, 2, 4, 8 are the lower 4 bits in a byte). These constants are directly available for instructions that require access/compare against the aforementioned values. On the other hand, access/compare with other values requires additional cycles for the instruction. Hence, it is best to leverage these constants in the application code whenever possible. The benefit becomes more visible when the access/comparison is repeated in a loop structure. One of the most common use-cases is comparing a program flow controlling variable against certain flags. It would be therefore more power-effective to use the lower 4-bits as the program flow controlling flags.

Risks, Severity

Using a regular bit/value instead of the lower 4 bits for program flow control can add additional clock cycle for each comparison instruction. In a loop structure, the additional cycles executed can add up depending on the number of iterations the program has to go through.

Why it is happening

The code contains at least one loop program flow controlling flag that is not one of the 4 lower bits (1, 2, 4, 8 or BIT0, BIT1, BIT2, BIT3)

Remedy

If possible, change the loop program flow controlling flag(s) to be one of the 4 lower bits.

Code Example

 #define FLAG_1     1
 #define FLAG_2     2
 #define FLAG_3     4
 #define FLAG_4     8

void main(void) 
{ 
  unsigned int i, variable=FLAG_1;
  while ( variable & FLAG_1)
  {
     /* Execute application code */
  }
} 

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

More Resources

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