I uploaded the blink example code to nRF52840, but then the microcontroller doesn’t connect to my computer anymore. We can see the blink example working, but my computer doesn’t recognize it in any COM Port even though I double pressed the reset button.
It depends on how your code is written, but this is how it works, it is a little frustrating for sure, till you get used to it, but just be patient
you may need to use a “double click” motion on the reset button
Also some limited success by reducing serial com speed in your sketch to 19200 or below
Q2: My board is not showing up as a serial device on Arduino IDE
You can first try to reset the board by clicking the “Reset Button” once. If that does not work, rapidly click it twice to enter bootloader mode.
Hi there,
Some times it takes a few tries to get it to go into boot mode with a sketch that thin.
the cadence for the press is like morse code, NOT pressing it too quickly like DOT,DOT more like,
plug in USB and press it like Daash, Daaash, and be intentional about it. and do it more than 3 times if need be, The window will pop open and you’ll see the drive contents… Now your there. 
Go drag the RGB test Uf2 file in this thread it is in a ZIP file and drop it on the open file explorer drive.(Bootloader mode)
Read that thread too, it will help you. Sometimes the older example codes Nurf-UP(technical term) the Bootloader or code space or if you wrote garbage code to it , so this fixes that in a snap. and it just works again , even without using the boot loader mode now.
Note, the IDE may see it as a different com port so Know that too. YMMV
HTH
GL 
PJ
Hi @luis_batista,
I’ve had exactly the same issue, literally - my xiao stopped being recognized on ports after uploading the bink example. Have you managed to fix it afterwards? Or at least prevent that problem for other boards in the future?
Thanks in advance,
Mike
Hi there,
And Welcome here.
So it’s an easy fix and simple mistake. Let us know what kind of development situation you have going on.
HTH
GL PJ
Where did you load the blink sketch from? WHich BSP was used to compile on the Board you chose? Answer some basic Q’s we can get you squared away.
Hi,
I built a Zephyr blinky sample and flash to Xiao nrf52840 (.uf2), then nothing happen, no led and my PC did not recognize xiao port (usually COM17) in device driver. My xiao bootloader mode is normal, when I double clicked it show COM16 and allow me to flash .uf2 to xiao, however nothing worked after flashed (no COM17, no led).
Can you tell me what happen and solution?
Hi there,
You overwrote the bootloader, Your Zephyr flash was only the App.
and it will only run from the flash if connected to a debugger.
If you hold the reset button down while you plug it in, Does it make the windows New Device found sound/Gong. If so just drag and drop a UF2 test file from the drag and drop testing thread on here.
If not then you need to reWrite the WHOLE flash.
HTH
GL PJ
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Hi @PJ_Glasso In my case it is nRF52840 Sense. What I did exactly was
- short RST (pin 1) and GND pin
- Xiao entered a bootloader (a folder opened)
- I uploaded my sketch
- It told me the sketch upload was successful but than xiao disconnected completely and isnt recognized on the usb anymore
Not when I short the RST and GND again, nothing happens and I cant get the xiao to be recognized. Any advice? Did i do something wrong?
I am programming it from the arduino IDE.
Thanks so much for any help here!