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When the above devices get connected, a drop in the CC line for a DFP and a rise in the CC line for a UFP confirms a connection. The rise and drop is to a predefined voltage range as defined by USB Type-C specification.

Based on the CC line voltage range, a UFP will identify the type of DFP connected and its power capabilities. Also, a DFP can identify if a UFP is connected directly or through a USB Type-C cable with VCONN requirement.

fig 4
Fig. 4: USB PD DFP-UFP connection

USB Type-C host (DFP) to USB Type-C device (UFP) with PD support. A USB Type-C device with PD support is an advanced USB Type-C device that provides or consumes up to 20V/5A power. It can also enter modal operations to reuse USB Type-C signals for additional functionalities. Fig. 4 illustrates USB Type-C device with PD support connection.

PD communication using PD controller starts after a successful USB Type-C connection. At the start of the process, a PD source starts sending its capabilities over the CC line using BMC encoding. The sink decodes capabilities of the source and requests the power it requires. Based on the request, the source may accept the request and change its VBUS to output the requested power. The source then sends a source-ready packet to the sink indicating acceptance.

When the port partners agree on power, state is called contract. This is the basic state requirement for any further protocol or mode negotiation.

Things you can do with USB Type-C
A USB Type-C cable plays a key role in the USB Type-C ecosystem and can dictate the behaviour of the ecosystem. In the USB Type-C environment, cable products are called electronically-marked cables that can be built with intelligence for robust and better user experience.

The flip side to this is that users need to be educated about the cables’ capability. This section explores key things that a USB Type-C cable can do, which the legacy USB cables can not.

Can talk. Unlike legacy USB cables, USB Type-C cables can be electronically marked and these respond to commands from a host device. In a USB Type-C ecosystem, a host sends series of discover commands to know about the cable and decide further action on how to set up the ecosystem. Thus, a cable plays a key role in deciding the USB Type-C ecosystem.

Can decide the power a source can provide. During the discover process, a host device receives identity of a Type-C cable that contains details like manufacturer name, type of cable, USB capabilities, VBUS current handling capability and so on. This information about VBUS current capability restricts a power provider its power capability advertisement. This means, even though a power provider is capable of providing 5A to a consumer and the cable in its identity communicates its limits as 1.5A, current capability is restricted to 1.5A.
Can be active or passive. A USB Type-C cable can be categorised as electronically-marked or non-electronically-marked cables, where the latter is dumb and does not contain any electronics inside.

Electronically-marked cables can be further classified as passive cables that are USB Type-C cables that do not support data bus signal-conditioning circuits but can respond to discover commands, and active cables that are Type-C cables that support data bus signal-conditioning circuits along with discover commands.

This classification of the cables enables bringing more interesting functionalities along with some complications to the end user.

Can enter modes. Type-C connectors allow functionalities like Thunderbolt or HDMI over existing USB signal lines. This is achieved through CC protocol. Such alternate modes are not restricted to a host or a device but can also be extended to a cable. For example, these alternate modes can help an active enable signal conditioning or at times perform some other proprietary action before entering alternate mode between a host and a device.

USB Type-C hub
Another major change the USB Type-C technology brings in is on the USB hub front, which is now a multi-function device that supports functionalities like Display Port or MHL, along with USB in a USB hub solution.

fig 5
Fig. 5: Legacy USB hub

Let us now explore what a legacy hub solution looks like and how it can be modified to a Type-C solution. Fig. 5 illustrates a simple legacy hub controller with four downstream ports and an upstream port, all supporting USB functionality.

In simple terms, a USB Type-C hub solution is just a tweak of the above legacy solution with Type-C controllers managing ports and signals as illustrated in Fig. 2.

fig 6
Fig. 6: USB Type-C USB hub solution

Fig. 6 is a break up of a USB Type hub, which has the following features:
• Three USB-A 3.0
• Mini display port
• Two USB-C

From Fig. 6, it is easy to understand how an existing legacy USB hub design can be tweaked to a USB Type-C hub. It is important to note that in this new hub model a USB Type-C controller manages the signal lines of the USB Type-C connector after successful CC protocol negotiation.

Some key points to remember in a USB Type-C hub solution are:
• Only an upstream port of a hub can support alternate mode, that is, functionality other than USB.
• When operating in display port mode, a USB Type-C hub may not support USB 3.0 functionality and will operate in USB 2.0 mode.


Rajaram Regupathy works with GoArks Software Systems, developing USB Type-C and USB solutions. He has product patents in embedded domain and is a senior ACM member. He has published books on Linux USB, Android USB and USB Type-C

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