USB4 Alt-Mode Testing: DPAUX and USB-PD

Figure 1: The interfaces that make up the USB-C connector and their relationships.

The USB Type-C® (USB-C) connector supports not only USB data transfers at speeds of up to 20 Gb/s but also can be reconfigured to support a wide variety of other interfaces through the USB4 provision for alternative (Alt) modes. The Alt-Mode protocols supported include Thunderbolt™, Mobile High-Definition Link (MHL), Peripheral Component Interconnect Express (PCIe®), High-Definition Multimedia interface (HDMI®) and DisplayPort™ (DP).

The diagram of the USB-C receptacle in Figure 1 shows where it potentially switches from normal operation into Alt-Mode.

Testing DisplayPort 2.0 vs. USB4 Over USB Type-C Connectors

Figure 1: The pin out of the USB Type-C connector.

DisplayPort™ 2.0 (DP 2.0) is a high-resolution video interface and USB4® is a high-speed data interface; what they have in common is the USB Type-C® (USB-C) connector. While DP 2.0 can also be deployed on the standard DisplayPort as well as mini-DisplayPort connectors, it is the USB-C connector that really excites electronics manufacturers because now they can use a single connector for high-speed data, high-resolution video and even power distribution. 

Figure 1 shows the pin assignments for a USB-C connector, which is a mechanically reversible connector that includes four, high-speed differential data lines: TX1, TX2, RX1 and RX2.

In USB4 operations, the four data lines TX1, TX2, RX1 and RX2 form a dual-lane, duplex signal path, supporting 10 and 20 Gb/s transfers on each line. When operating in USB4 Alt mode, up-to-four of these buses can be reassigned to become four DP 2.0 video lanes, which operate at 10, 13.5 or 20 Gb/s.

Testing for both interfaces over the USB-C connector is similar, but there are some notable differences.