Question

I have a while loop implemented in C for an MSP430 processor that currently looks like this:

register unsigned int sw_loop_count = 0U;
...

while (TACCL0 & CCIE)
{
    ++sw_loop_count;
}

...
#pragma vector=TIMERA0_VECTOR
__interrupt void Timer_A(void)
{
    // Disable the timer interrupt flag and enable.
    TACCTL0 &= ~CCIFG;
    TACCTL0 &= ~CCIE;
}

I’m using this loop for calibration purposes, the context of which I don’t think matters too much for my question. I’ve calculated that each iteration of the loop, including the check TACCL0 & CCIE takes 11 clock cycles. For purposes of granularity, I would really like to get this number as low as possible, and programmatically if possible. I might be being a complete moron, but I can’t think of a way of reducing the cycle count for the loop, so any advice would be appreciated. I need the sw_loop_count value, one way or another.

Answer 1

Hmm, after I put a comment I realized that there may be something you can do 😉 In your while() condition you are checking two values. From the looks of it both those values must be defined as volatile so that they are ready from memory every single time they are used…

Can you reduce those two into a single one? Have your interrupt handler do the necessary comparison and set a single flag that you will be checking in your loop.

Or you can get really fancy and do it another way around, like that:

// signed and global (or you can pass it's address into your interrupt's routine)
volatile signed int sw_loop_count = 0;

Then there’s your “measurement” loop:

while(++sw_loop_count) {}

and in your interrupt routine:

if(TACCL0 & CCIE)
{
  real_count = sw_loop_count; // save the value for future use before we destroy it
  sw_loop_count = -1; // this will turn into 0 in that while's pre-increment, ending the loop
}

OTOH… introducing the volatile may get so much hit from memory access that it may in fact slow down the while() loop. It really does all depend on your actual architecture (down to what type of memory controller and cache controllers there are) and I still maintain that you may be better off running it through an assembler mode and looking at what the compiler is doing.