Enumerates as USB1.1 serial port seen as /dev/ttyACM0 on linux. Connect to serial port with some teriminal emulator like

connecting with "cu":

cu -l ttyACM0

exit:

<Enter> ~ .

or (on some keyboard)

<Enter> AltGr-1 .

connecting with "screen":

screen /dev/ttyACM0

exit:

ctrl-a \

or (on some keyboard)

ctrl-a AltGr q

Type anything and press RETURN or ENTER. The core should respond with everything you typed but in the reverse order.

Application side is the interface as seen from soft core CPU or test example state machine that reverses typed chars.

This core application side "speaks" complete serial bytes with READY/VALID signaling reaching up to 11 Mbit/s. It is not classic RX/TX RS232 at 115.2 kbit/s, however such interface should be made too as full usb-serial cable replacement.

It has clock domain crossing so application side serial buffer should accept any clock synchronous with the application (or soft-core CPU). Tested with 7.5, 12, 48, 60, 100 and 200 MHz

PHY side (hardware side) clock should run at 48 MHz or 60 MHz.

Total 6 FPGA pins are used with "27-ohm" interface. (see "usb" sheet of ULX3S schematics):

2 differential input pins
2 single-ended bidirectional pins
2 single-ended pullup/pulldown control pins (optional)

To save 2 pins, instead of pullup/pulldown control pins, a fixed pullup resistor of 1.5k between D+ and 3.3V can be used. To save 2 additional pins, differential input may be omitted and replaced by single-ended D+ input only, so the minimal hardware is:

2 single-ended bidirectional pins

Differential mode should provide wider hardware coverage as it's electrically more compatible. USB standard requires tolerance to +5V on any pin, therefore 27-ohm resistors and 3.6V Zener diodes protect FPGA from +5V. Core can work without 3.6V Zener diodes if normal USB devices with properly wired connectors are used.

When it doesn't enumerate, try to re-plug or choose another USB port. This situation should occur quite rarely I hope.

Soft-core UTMI works slightly more reliable (less line errors) when clocked at 48 MHz rather than 60 MHz (more line errors). Frequency of line errors also depends on cable quality and USB voltage.

USB-serial link occasionaly loses data in chunks of 64 bytes in direction when PC sends data to USB.