This is an example of a simple project which receives characters over the FT231X USB serial port and echoes them back down the serial port. The characters' bits are also shown on the LEDs.
To build the ULX3S bitstream, do a make ulx3s.bit . If you don't have
a 45F FPGA chip, change the nextpnr-ecp5 --45k line in the Makefile
to something more suitable. If you have an older board, you will need to
find one that matches your board. Have a look in
https://github.com/emard/ulx3s/tree/master/doc/constraints .
The serial port runs at 115,200 bps. You will also need to disable hardware flow control. With minicom, ctrl-A O, Serial port setup, F, Exit.