The rapid role exchange in USB Type-C PD will promote a new user experience

We live in an era of constant change, and people generally feel that it is progress rather than chaos. Emerging technologies may indeed be disruptive, including 5G and artificial intelligence, so it is easy to understand why they have received a lot of attention. When they develop independently, they may be regarded as marginal. However, for those closer developments, it is clear that they will enable next-generation application areas such as autonomous driving and a smarter Internet of Things (IoT). At the same time, other major changes are taking place in familiar areas, including more mature technologies. Including USB.

We live in an era of constant change, and people generally feel that it is progress rather than chaos. Emerging technologies may indeed be disruptive, including 5G and artificial intelligence, so it is easy to understand why they have received a lot of attention. When they develop independently, they may be regarded as marginal. However, for those closer developments, it is clear that they will enable next-generation application areas such as autonomous driving and a smarter Internet of Things (IoT). At the same time, other major changes are taking place in familiar areas, including more mature technologies. Including USB.

The Type-C interface represents a significant advancement in USB because it provides much more features than the previous generation interface. To support this vision, the USB Implementers Forum (USB-IF) is responsible for overseeing the development of new functions and features that may fundamentally change the way we connect. When USB was first introduced as a technology to support and expand PC functions, it had a huge impact in the consumer field and other fields. While becoming the default method for connecting peripherals and computers, it also provides developers with a convenient debugging interface. Soon it became as universal as its name suggests. When considering the way we use wired communication, the USB Type-C interface brings a disruptive factor comparable to 5G. One of the important factors is that the device can be powered and powered at the same time, and switch between these two modes without any user intervention, which is unique to wired buses. It is this ability that will help promote the introduction of a new device, namely the independent USB docking station (Dock).

The evolution of the dock

USB hubs are not a new concept. The host has always been able to count more devices than actually supported by physical ports, so a hub with multiple USB ports has become an easy way for the host to expand the number of physical USB ports. In this case, the control always belongs to the host device, usually a PC, as does the power supply.

Under the USB Type-C specification, the definition of the host and the device is more flexible, and the dual role port (DRP) mode is adopted to support the device to switch between the downstream port (DFP) and the upstream port (UFP). The power supply is also more flexible, allowing the device to dynamically provide and receive power. This goes beyond the concept of USB On-The-Go (OTG), although it is fair to say that USB OTG has played a big role in extending the USB ecosystem from the desktop to our pockets.

One of the main features of USB Type-C is power transmission (PD). It is this protocol that allows any device to become a power source or be powered without relying on data flow.

For example, an externally powered hub can charge a laptop while connecting to a projector or Display. In this case, the laptop is a power receiver and a data source. In short, the hub has become the center of the network. Although this has many benefits, it also brings some burdens.

Compared with all devices are hubs, people expect a new kind of hub USB Dock; it supports multiple USB Type-C ports and old USB ports. In order to negotiate and decide between these ports in terms of power transmission, the Dock will need to support features specifically developed for this new model.

Quick role swap

In addition to DRP, a key technology supporting the new Type-C mode is Fast Role Switching (FRS), which effectively ensures that devices that require power can always get power. In fact, in the standard sequence of events, this will happen whenever the power supply is detached from the docking station. FRS is a method to accelerate the sequence of power role exchanges so that data will not be interrupted (and therefore the user experience will not be interrupted). In fact, if the power supply is removed from the docking station, the peripherals attached to it should not be affected.

The process is controlled and initialized by using the CC pin on the Type-C connector. If the power supply is removed from the docking station, it will send a signal to the device it is currently powering by driving the CC pin to ground potential. At this time, FRS signaling covers any activity on the CC port, and messages queued for transmission will be deleted. When the receiving device responds to the FR_Swap message, the handshake protocol is completed; the flowchart in Figure 1 illustrates the FRS process.

The rapid role exchange in USB Type-C PD will promote a new user experience

0: The hub is powered by TA and supplies power to the device

1: The wall adapter is disconnected

2: The hub receiving power device detects the disconnection through VBUS_SINK_DISC, and pulls the GPO2 output low

3: The hub initializes the power supply device to detect low transmission on the GPI2 input and stops power supply

4: The hub initializes the power supply device with ITCPCSendFRSwap to send the FR_Swap signal to the initialization receiving power device of the device

5: Initialization of the device The receiving power device receives the FR_Swap signal, stops receiving power and starts supplying power to VBUS

Fast Role Swap Sequence: Fast Role Swap Sequence

Wall Adapter: Wall Adapter

Hub: Hub

Device: Device

Figure 1: Flow chart of rapid role exchange

This specification requires implementation in 150 µs or less, so some additional techniques are required. Generally, a hub that supports FRS will have multiple Type-C ports available, and each port has its own associated Type-C port controller (including power delivery (PD)). In order to coordinate all ports, PD controllers need to work together. This can be achieved by using a Type-C port manager (TCPM) or a PD controller designed for PD 3.0 with dual-role ports and FRS support, such as ON semiconductor FUSB307B.

Implementing FRS in a durable way requires consideration of many scenarios, such as what happens if the main power supply of a docking station is suddenly removed. This will affect all devices on the docking station, including TCPM.

The rapid role exchange in USB Type-C PD will promote a new user experience

Figure 2: Example of a notebook application using FUSB307B

Rapid role exchange is a supporting technology that will promote the development of USB Dock to provide a new user experience. The USB ecosystem is developing rapidly, and the adoption of USB Type-C in a wider range of devices will promote this technological change. By adopting PD controllers that support dual-role ports and rapid role switching, original equipment manufacturers (OEMs) will better take advantage of this new opportunity.

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Author: Yoyokuo