ATmega328PB has a second UART port that is sharing pins with ICSP/SPI.

While this can be handy for debugging you may need to swap ICSP and UART dongles. Or you can solder the UART wires to the ICSP connector but then you will get ICSP traffic on UART and vice versa.

So I've decided to create this simple breakout PCB that connects between your device and the ICSP/UART dongles and features switches to enable/disable the shared pins. I've also included one SMD LED with resistor. It is not needed for the main function but it will signal if the device is powered. And it is a great way to try SMD soldering. And I've added also a reset button as there was a free space on the PCB...

Here you can see the finished board plugged into a board with ATmega328PB MCU. You can see both UART and ICSP programs running on the screenshot (of course they can not communicate both in the same moment):

If you look closely, you can see that I had to scratch a ground plane as I had error in my gerbers - I forgot to update ground zone fills in KiCad before exporting the gerbers... :

The gerbers in this project are of course fixed.

I also decided to be a little bit fancy and try white soldermask that PCBWAY offers and it looks great...