CYUSB3314 REF_SS and REF_USB2

[oystein]

Hi

Looking in AN91378 for the HX3 Hub transceiver on page it says RREF_SS: This pin should be connected to a precision resistor (200 Ω ±1%) for SS PHY termination impedance calibration (as shown in Figure 10). Can anyone explain how this resistor is related to 90 ohm input impedance of the USB3 superspeed interface, or if its related at all?

Then it also says RREF_USB2: This pin should be connected to a precision resistor (6.04 kΩ ±1%) to generate a current reference for USB 2.0 PHY (as shown in Figure 10). I never seen any designs where we need to generate ref currents for a USB2 interface. Whats the rationale behind this reference?

BR

Øystein @ sevendof

[Ajeethkumar_P · Accepted solution]

Hi,

Thanks for posting in Infineon Developer Community!

RREF_SS and RREF_USB2 are used to calibrate the Super-speed and high-speed termination impedance respectively and control the drive level of SS and HS signals. These resistors only control the drive level and will not add up to SS PHY impedance.

These two precision resistors are used for the internal circuit of the HX3 Silicon. Even a very small change in the value of these resistors would cause the device to fail the signal quality compliance tests. Characterization has proved that 1% is the maximum allowable limit.

External High precision resistors are used in the design where long-term stability and low-temperature coefficients are required. Using the recommended value of the precision resistors, USB signals cannot be degraded on aging or over the temperature range.

Thanks,
Ajeeth