Microcontroller Programming - Program not exiting for loop

Question

I have a simple PIC16F18877 circuit setup on my breadboard and I've successfully gotten an LED to blink within an infinite while loop. I attempted to put the same code inside a for loop that should only execute 5 times, but the LED keeps blinking.

My Code (MPLAB with XC8 Compiler):

#include <xc.h>

#define _XTAL_FREQ 8000000

int main()
{
    TRISD1 = 0;

    for (int i = 0; i < 5; i++)
    {
        RD1 = 1;
        __delay_ms(500);
        RD1 = 0;
        __delay_ms(500);
    }

    return 0;
}

tofro · Accepted Answer · 2017-04-23 17:35:33Z

5

Where do you expect the CPU to jump on return from main? Or rather, what do you expect it to do when you don't tell it what to do? On a desktop computer, the program would normally return to the OS - On an embedded system, there is none.

Most probably, the return from main returns to the startup code and, eventually (either "by accident" or deliberately) to the reset vector, starting your program from the beginning.

If you want the MCU to "stop" actually, "do nothing" you need to force it into an infinite loop instead of return. This is, however, not a common approach on an MCU.

answered Apr 23, 2017 at 17:35

tofro

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4 Comments

Unimportant Over a year ago

Not probably, certainly: "After execution reaches the end of main(), code added by the compiler jumps back to the Reset vector. The device then executes the runtime startup code and the main() function again." - Source: ww1.microchip.com/downloads/en/DeviceDoc/50002173A.pdf.

tofro Over a year ago

@Unimportant what's really certain these days at all?

Unimportant Over a year ago

Death and taxes, like they always have.

Jake Rieger Over a year ago

I tried adding a while loop after the for loop and something strange occurred. It appeared to be executing the for and while loop simultaneously. The light would blink link normal and then blink really fast, almost like a stutter and then blink normally, etc.. but it never stopped blinking

imqqmi · Accepted Answer · 2017-05-01 08:43:41Z

I tried adding a while loop after the for loop and something strange occurred. It appeared to be executing the for and while loop simultaneously. The light would blink link normal and then blink really fast, almost like a stutter and then blink normally, etc.. but it never stopped blinking

Check the watchdog timer. If it's set, the mcu will just reset after a set amount of clock cycles and run the code again and again. You can use CLRWDT() to reset the watchdog timer or just turn the WDT off. I highly recommend to go through these steps in order to be sure that the mcu does as expected:

Check the PIC configuration bits, are they setup properly? See the documentation in the microchip program folder /docs/chips
Make sure the oscillator is setup correctly.
Read the datasheet and make sure the ports are set correctly, especially the analogue ports using the analogue selection registers.

(my reputation isn't high enough to comment, sorry about that.)

old_timer · Accepted Answer · 2017-04-23 21:40:13Z

0

you are not on an operating system here, can you show us the disassembly to show the call to main and what it returns to? Or what if you put an infinite loop before main ends (while(1) continue;) do you get 5 blinks then?

answered Apr 23, 2017 at 21:40

old_timer

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