I'm using a STM32L031 with Eclipse for my application. When in debug mode everything works fine. But when I stop debug mode or power off and on again, the STM32L031 stops working.

I had expected that after downloading the code into the chip, it should work whenever I power it on.

And since I'm not using an evaluation board, there is no reset button so I cannot reset it.

How to solve this problem? Or is there a way to reset the chip without requiring the Reset button hardware?

  • \$\begingroup\$ What is it doing when you 'stop debug mode'? I guess there are some breakpoints still in the code which didn't get cleaned up before you reset. \$\endgroup\$ Commented Sep 14, 2016 at 11:58
  • \$\begingroup\$ Have you wired the NRST pin in a way which could prevent normal startup? Do you use some sort of semi-hosting which might prevent normal running (because of hard coded breakpoints)? Is the debugger still connected when you do the power cycle? \$\endgroup\$
    – Arsenal
    Commented Sep 14, 2016 at 13:18
  • \$\begingroup\$ If you probably have an SMT resistor on the NRST line, to which you can solder a small wire to pull low for a test reset. Or (at least if your supply is current limited) you can just touch something grounded (or via a low value resistor) to it. \$\endgroup\$ Commented Oct 14, 2016 at 18:56
  • \$\begingroup\$ You ought to be able to re-attach the debugger to a running target - this will help to investigate where it is stuck. \$\endgroup\$ Commented Oct 14, 2016 at 19:43
  • 1
    \$\begingroup\$ Supposing you HAVE a pullup resistor on the NRST line just short NRST to ground. If you don't - add a 10kOhm one. Also, how is your BOOT0 wired? \$\endgroup\$
    – jaskij
    Commented Dec 14, 2016 at 12:21

2 Answers 2


One thing that can stop it from running is if you are using semi-hosting. In your project properties, try to remove the following two symbols:

enter image description here

I have multiple projects, and only some of them require the removal of semi-hosting. I haven't yet figured out why it is necessary for some and not others...

Oh, and if powercycling the board doesn't work, then a reset button wouldn't, either.

  • 1
    \$\begingroup\$ Semihosting comes into play if you're using functions that use standard I/O, like printf(). If you don't call those functions, your application is no different with and without semihosting. \$\endgroup\$
    – user39382
    Commented Oct 15, 2016 at 5:51
  • \$\begingroup\$ @duskwuff No experience with this controller in particular, but AFAIK "hosted application" in C means your code starts with main, as opposed to "unhosted" applications which start at something like __program_start. Incidentally, if you run unhosted, you have to initialize STDOUT yourself before you can use printf, but it may be not the only difference. \$\endgroup\$ Commented Jun 27, 2017 at 13:26
  • \$\begingroup\$ @DmitryGrigoryev True, but semihosting is a separate thing entirely. Enabling/disabling semihosting doesn't affect how applications start up. \$\endgroup\$
    – user39382
    Commented Jun 27, 2017 at 16:30

When using the debugger, the processor is forced to execute from a certain region.
On power-on reset, the bootloader determines where it should start. Some pins or flash location provide the bootloader with instructions.

stm32l031 boot options


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