Hi Jean-Michel, With the 5V supply you are still powering up the st-link Please look at the STM Nucleo User Manual UM1724 page 20 section 5.33 you have to use a 3V3 supply and remove SB12 and SB2 Hope this helps

Hello Martin, thank you for your answer. Unfortunately we're forced to use the +5V supply, we're using an SD card shield that needs at least 3V5. The st-link is activited with the E5V pin, which is linked to the +3V3_ST_LINK. This is why we put the JP5 jumper off (links between E5V and . But We tried what you suggested and the result is the same. There must something with the st-link that prevents the processor from working well. Thank you a lot for your answer though.

Hi Jean-Michel, All JP5 does is toi swap input 5V from USB to External supply the on board 3V3 regulator is activated either way and so the st-link is activated. However, have you removed CN3? and/or removed SB15 which is the SWO line for jtag programming? This may cause the STLINK to activate and effect PB3 on the ST processor. Please check the schematics in the previous document refered too. Kind Regards

Hi Jean-Micheal, Please check https://developer.mbed.org/questions/5362/Using-external-33V-Power-supply/ I think your problem is also compounded by Oscillator selection check page 22 of UM1724 document. You either have to use above solution in html link or provide your own XTAL (8MHz) since this MCO clock is provided by the STLINK.With out a defined (either by settings or hardware) then this will give you problems with your Serial timings and hence problems communicating with your devices. Kind Regards

Hi Martin, the CN3 was removed but not the SB15 jumper, that may be the source of the problem. And the MCO clock provided by the STLINK also, it's a serial communication problem probably due to frequencies. I guess we will have to add our own XTAL. We will check on these two parts. Thanks a lot Martin, that really hepled us.