Raspberry Pi Pico: can read from FTDI dongle but can't write
Switching to a different set of pins GP0 and GP1 for UART made it work, now I can communicate in both directions again.
So it seems like at some point port GP4 (UART1 TX) god damaged somehow. But not fully, because I can still send data to itself on Pico by shorting ports GP4 and GP5. Also, I can see a pulse on the oscilloscope when sending.
So it seems like port GP4 is still working, but somehow it's less powerful now or something.
Maybe a sequence of connecting FTDI dongle matters? Maybe when Pico is powered on, but FTDI USB is not connected yet it's somehow drawing power from Pico ports and it caused damage to the port?
Not sure, but if it happened once I suspect it might be a matter of time before it happens again to the new port.