Question

I am working on some firmware for an embedded device that uses a 16 bit PIC operating at 40 MIPS and programming in C. The system will control the position of two stepper motors and maintain the step position of each motor at all times. The max position of each motor is around 125000 steps so I cannot use a 16bit integer to keep track of the position. I must use a 32 bit unsigned integer (DWORD). The motor moves at 1000 steps per second and I have designed the firmware so that steps are processed in a Timer ISR. The timer ISR does the following:

1) compare the current position of one motor to the target position, if they are the same set the isMoving flag false and return. If they are different set the isMoving flag true.

2) If the target position is larger than the current position, move one step forward, then increment the current position.

3) If the target position is smaller than the current position, move one step backward, then decrement the current position.

Here is the code:

void _ISR _NOPSV _T4Interrupt(void)
{
    static char StepperIndex1 = 'A';    

    if(Device1.statusStr.CurrentPosition == Device1.statusStr.TargetPosition)
    {
        Device1.statusStr.IsMoving = 0;
        // Do Nothing
    }   
    else if (Device1.statusStr.CurrentPosition > Device1.statusStr.TargetPosition)
    {
        switch (StepperIndex1)      // MOVE OUT
        {
            case 'A':
                SetMotor1PosB();
                StepperIndex1 = 'B';
                break;
            case 'B':
                SetMotor1PosC();
                StepperIndex1 = 'C';
                break;
            case 'C':
                SetMotor1PosD();
                StepperIndex1 = 'D';
                break;
            case 'D':
                default:
                SetMotor1PosA();
                StepperIndex1 = 'A';
                break;      
        }
        Device1.statusStr.CurrentPosition--;    
        Device1.statusStr.IsMoving = 1;
    }   
    else
    {
        switch (StepperIndex1)      // MOVE IN 
        {
            case 'A':
                SetMotor1PosD();
                StepperIndex1 = 'D';
                break;
            case 'B':
                SetMotor1PosA();
                StepperIndex1 = 'A';
                break;
            case 'C':
                SetMotor1PosB();
                StepperIndex1 = 'B';
                break;
            case 'D':
                default:
                SetMotor1PosC();
                StepperIndex1 = 'C';
                break;      
        }
        Device1.statusStr.CurrentPosition++;
        Device1.statusStr.IsMoving = 1;
    }   
    _T4IF = 0;          // Clear the Timer 4 Interrupt Flag.
}

The target position is set in the main program loop when move requests are received. The SetMotorPos lines are just macros to turn on/off specific port pins.

My question is: Is there any way to improve the efficiency of this code? The code functions fine as is if the positions are 16bit integers but as 32bit integers there is too much processing. This device must communicate with a PC without hesitation and as written there is a noticeable performance hit. I really only need 18 bit math but I don’t know of an easy way of doing that! Any constructive input/suggestions would be most appreciated.

Answer 1

Warning: all numbers are made up…

Supposing that the above ISR has about 200 (likely, fewer) instructions of compiled code and those include the instructions to save/restore the CPU registers before and after the ISR, each taking 5 clock cycles (likely, 1 to 3) and you call 2 of them 1000 times a second each, we end up with 2*1000*200*5 = 2 millions of clock cycles per second or 2 MIPS.

Do you actually consume the rest 38 MIPS elsewhere?

The only thing that may be important here and I can’t see it, is what’s done inside of the SetMotor*Pos*() functions. Do they do any complex calculations? Do they perform some slow communication with the motors, e.g. wait for them to respond to the commands sent to them?

At any rate, it’s doubtful that such simple code would be noticeably slower when working with 32-bit integers than with 16-bit.

If your code is slow, find out where time is spent and how much, profile it. Generate a square pulse signal in the ISR (going to 1 when the ISR starts, going to 0 when the ISR is about to return) and measure its duration with an oscilloscope. Or do whatever is easier to find it out. Measure the time spent in all parts of the program, then optimize where really necessary, not where you have previously thought it would be.