What is the purpose of the RREF resistor in HX2VL™? Can you use a resistor with a value other than 649 Ω?
The RREF resistor is used to calibrate the high-speed termination to control the drive level of the high-speed (HS) signals. If you use a resistor with a value other than 649 Ω, the HS signal quality will be degraded over the range of temperature. The voltage level that will be seen on RREF pin is 400mV ± 10% (same as high speed differential signaling voltage on USB lines).
Question: How to force HX2VL into ‘Full-Speed only’ mode?
Answer: HX2VL can be forced to operate in ‘Full-speed only’ mode by removing the RREF resistor, thereby disabling the High-speed PHY itself.