user60412: First of all: my board used to work fine.
Now my Nano 33 IoT turns ON (green light) when connected to USB on Mac but:

doesn't enter in upload mode after pressing twice on board button.
doesn't shows up in any serial port: nor /dev/cu.usbmodem14*01, neither /dev/tty.usbmodem14*01

For reasons too long to explain, I want to connect via ANY pins to the SAMD21 and force the burning of a new bootloader.
Is that possible in any way? Any idea is welcome!
Tx

user37622: Use the SWD interface and a programmer to burn the bootloader to SAMD.

source of the picture https://medium.com/@manuel.bl/arduino-in-circuit-debugging-with-platformio-9f699da57ddc
I had luck with Adafruit DAP library using an Arduino M0 to burn a bootloader to MKR Zero. 

60412: <p>First of all: my board used to work fine.</p>

<p>Now my Nano 33 IoT turns ON (green light) when connected to USB on Mac but:</p>

<ol>
<li>doesn't enter in upload mode after pressing twice on board button.</li>
<li>doesn't shows up in any serial port: nor <code>/dev/cu.usbmodem14*01</code>, neither <code>/dev/tty.usbmodem14*01</code></li>
</ol>

<p>For reasons too long to explain, I want to connect via ANY pins to the SAMD21 and force the burning of a new bootloader.</p>

<p>Is that possible in any way? Any idea is welcome!</p>

<p>Tx</p>
 37622: <p>Use the SWD interface and a programmer to burn the bootloader to SAMD.</p>

<p><a href="https://i.stack.imgur.com/ZFNM6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZFNM6.png" alt="enter image description here"></a></p>

<p>source of the picture <a href="https://medium.com/@manuel.bl/arduino-in-circuit-debugging-with-platformio-9f699da57ddc" rel="nofollow noreferrer">https://medium.com/@manuel.bl/arduino-in-circuit-debugging-with-platformio-9f699da57ddc</a></p>

<p>I had luck with <a href="https://learn.adafruit.com/programming-an-m0-using-an-arduino" rel="nofollow noreferrer">Adafruit DAP library</a> using an Arduino M0 to burn a bootloader to MKR Zero. </p>
