USB Softcore for FPGA : Extra resistor on FPGA transmitter pin
First of all USB doesn't have a "TX pin" and an "RX pin" &mdash; both D+ and D- are used bidirectionally.

The reason for the series resistors (one on each pin) is that the output impedance of the FPGA pin drivers is significantly lower than the nominal impedance of the USB cable. A series resistor addresses this mismatch, eliminating signal reflections at that end of the connection.

This particular softcore uses two pairs of unidirectional pins to connect to the USB D+ and D- signals, rather than a single bidirectional pair. The termination resistors go between the Tx pair and the Rx pair, as follows:







simulate this circuit &ndash; Schematic created using CircuitLab

It isn't clear from the README.md file what additional logic is required for the rx_se0, tx_se0 and tx_en pins.