SLYY228 November 2024
A USB4 host must be able to support legacy USB products as well as DisplayPort Alternate Mode products such as USB-C docking stations. In order to support both types of products, the host needs to multiplex both USB4 sideband (SBTX and SBTX) and DisplayPort sideband (AUXP and AUXP) over the USB-C SBU1 and SBU2 pins. Figure 36 shows an example of how to multiplex USB4 and DisplayPort sideband signals over SBU1 and SBU2.
Figure 36 shows the auxiliary biasing required by the DisplayPort specification and the necessary SBTX pullup resistor and SBRX pulldown resistor required by USB4 specification. Table 13 lists these resistors for reference. The level shifter on SBRX and SBTX may be necessary if the USB4 router or retimer doesn’t support the 3.3V LVCMOS levels present at the USB-C receptacle. Some or all of the components in Figure 36 may be integrated in a retimer or a discrete sideband multiplexer.
| Parameter | Minimum (kΩ) | Maximum (kΩ) |
|---|---|---|
| SBTX pullup resistor | 7 | 10.5 |
| SBRX pulldown resistor | 700 | 1050 |
| DisplayPort source AUXP pulldown resistor | 10 | 105 |
| DisplayPort source AUXN pullup resistor | 10 | 105 |
| DisplayPort sink AUXP pullup resistor | 800 | 1,200 |
| DisplayPort sink AUXN pulldown resistor | 800 | 1,200 |