Partially discussed in Using SWO trace output from Atmel-ICE and STM32, libopencm3 and SWV/SWO but I’ve not yet seen information about it with PIO. The serial monitor is designed to connect to serial devices (/dev/ttyX / COMX devices). OpenOCD seems to be able to write it into a logfile (ref)… Do you know of a cross-platform tool for viewing SWO output?