Add logic level RS232 to the PDI/UPDI header, with an FTDI breakout
A simple 2 layer board with 3x (U)PDI headers, and an FTDI cable header. A debug serial port is really handy, and STM debuggers have one built in.
J1 (U)PDI cable to target MCU, 2.54mm pitch
J2 (U)PDI cable to target MCU, 1.27mm pitch
J3 (U)PDI header for debugger/programmer
J4 FTDI cable header for TTL level RS232 and USB power
J5 Easy access to FTDI_VCC (5V or 3.3V depending on the cable)
J6 Connects FTDI_DTR to MCU's reset line via open drain