Hi vincenzo giudicissi,
Please check your KL03 hardware at first, you should make sure your board hardware design is correct.
1. reset_b pin have 4.7K to 10K pullup resistor, and 0.1uf capacitor to ground.
2. NMI_b pin have 4.7K to 10K pullup resistor
3.SWD_DIO have 10K pullup
Besides, before you download the code, did you choose the correct debug interface? Which debugger you are using?
Do you have JLINK debugger? You can use the JLINK , and JLINK_commander interface to check, whether it can identify the cortex M0+ core through SWD interface.
If can't find the core, it should be the hardware problem, if Jlink commander can find the core, then hardware have no problem, you should check the software configuration.
Please check it at first, any question update information, please contact me!
Have a great day,
Jingjing
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