Alternate Mode

An important benefit of USB Type-C is its ability to eliminate the need for nearly every cable in consumer devices (High-Definition Multimedia Interface [HDMI], DisplayPort/Thunderbolt, power barrels, USB Type-A/B). To do this, USB Type-C needed additional functionality beyond USB 3.0, which led the USB Implementers Forum to define Alternate Mode. Alternate Mode enables the repurposing of USB Type-C pins (TX/RX pairs and SBU) for a different function [1]. Up to this point, video has been the primary focus for Alternate Mode with DisplayPort and Thunderbolt being the main two Alternate Modes for implementing video across a USB Type-C cable.

It is possible to transfer 4K video over USB Type-C cables, but not without Alternate Mode. Note that the USB Implementers Forum requires that they both approve and certify any Alternate Mode. Figure 12 highlights two new blocks required to support Alternate Mode.

The first new block is the Alternate Mode PHY. For example, with DisplayPort, you need a DisplayPort source (from the graphics processing unit). The second is the Alternate Mode mux. A USB Type-C USB 3.0 system requires a SuperSpeed mux in order to support different cable orientations. Alternate Mode needs the ability to support switching in the Alternate Mode PHY while still supporting different cable orientations.

Alternate Mode and USB 3.0 multiplexing is typically integrated into a single active or passive USB Type-C Alternate Mode mux. Two other important blocks required for Alternate Mode are the USB PD PHY and USB PD manager. It is possible to support USB PD and Alternate Mode simultaneously (imagine a monitor that takes in HDMI video but also charges a laptop when connected).

Even if USB PD power levels are not required, you must include a USB PD PHY and USB PD manager to support Alternate Mode because it is negotiated the same way as USB PD – a vendor-defined message over the CC line.

Without a USB PD PHY and USB PD manager, the system could not advertise and settle on an Alternate Mode.

One final note on Alternate Mode is how to handle an incompatible connection. Imagine that the user connects their USB Type-C laptop into a USB Type-C monitor. This laptop has two USB Type-C ports: one port supports USB PD with DisplayPort as an Alternate Mode at USB 3.1 speeds, while the second port supports only USB 2.0 over USB Type-C. In this case, it is likely that the monitor requires Alternate Mode to function; for example, DisplayPort for video. If the user connects the monitor to the USB 2.0 Type-C port, the monitor will not work.

If Alternate Mode negotiation fails, there are two options:

• Support USB functionality without Alternate Mode
• Provide a USB billboard message over the D+/D– lines to communicate information that identifies the device. Once a monitor sees that the USB 2.0 USB Type-C port cannot support DisplayPort, it will provide a billboard to communicate to the operating system (OS) that it requires DisplayPort to function. At this time, the OS could notify the user to use the other full-featured USB Type-C port on the laptop to support DisplayPort [1].

A full-featured system that supports Alternate Mode, USB PD and USB 3.1 can be quite powerful, but also complex. TI has solutions specifically addressing this need. For example, TI USB PD controllers integrate the USB PD manager and PHY, the high-voltage power path and CC logic, and can control an external SuperSpeed/Alternate Mode mux.