ULP Advisor > Rule 11.2 Use lower bits for port bit-banging

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 bit-banging control/data via one or more port pins repeatedly to control or transfer data externally. If the hardware design allows for flexible placement of the port pins, it is recommended to place the most commonly used pins on the lower 4 bits of the port to leverage the constant generator.

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 containing one or more port pin bit-banging operation that does not use one of the pins of the 4 lower bits (1, 2, 4, 8 or BIT0, BIT1, BIT2, BIT3)

Remedy

If hardware design can be flexible & replacement of device pin-out is possible, change the bit-banging pins to be the lower 4 bits of the port; change the software code accordingly.

Code Example

void main(void) 
{ 
  unsigned int i, variable=FLAG_1;
  P1DIR |= BIT2;                   // Use P1.2 for bit-banging, change direction to output
  P1DIR &= ~BIT2;                  // Reset pin P1.2
  for (i=0;i < 1000;i++)
  {
     /* Execute application code */
     P1DIR ^= BIT2;                // Toggle port pin P1.2, leverage BIT2 constant generator
  }
} 

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

More Resources

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