Sharing USB 3.0 links in embedded applications

June 09, 2017

gaya-June 09, 2017

A USB 3.0 port can be shared by splitting the link into a SuperSpeed port and standard USB 2.0 port. Currently Shared Link is a proprietary feature of the Cypress’ HX3 USB 3.0 hub controllers.  This article shows how to implement USB 3.0 link sharing in embedded applications such docking stations, notebooks, TVs, and set top boxes, among others.

The HX3 is a family of USB 3.0 hub controllers compliant with the USB 3.0 specification revision 1.0. The controller supports SuperSpeed (SS), Hi-Speed (HS), Full-Speed (FS), and Low-Speed (LS) operation on all the ports. It has integrated termination, pull-up, and pull-down resistors, and supports configuration options through pin-straps to reduce the overall BOM of the system. HX3 includes Cypress-proprietary Shared Link feature, which provides 8 downstream (DS) ports from a 4-port USB 3.0 hub.

Shared Link enables the USB 3.0 DS port of a hub to be split into an embedded SuperSpeed port and a standard USB 2.0 port. Shared Link enables a maximum of eight DS ports from a four-port USB 3.0 hub. Standard USB 3.0 port has 8 signal lines: two lines (D+, D-) for USB 2.0 communication, four lines (SSTX+, SSTX-, SSRX+, SSRX-) for SuperSpeed communication and two power lines for VBUS and GND as shown in Figure 1 below.

Figure 1. Standard USB3.0 Port (Source: Cypress)

The VBUS enable signal (DSx_PWREN) controls the delivery of VBUS signal to the attached USB 3.0 device. DSx_PWREN signal together with the overcurrent signal (DSx_OVRCURR) implements the overcurrent protection circuitry of the removable USB 3.0 port. When an overcurrent condition occurs in the port, the DSx_PWREN connected to the output enable pin of power switch turns off the port power.

USB 2.0 signal lines are required for backward compatibility. When a SuperSpeed device (USB 3.0 device) is plugged into a USB 3.0 port, communication happens only over the SuperSpeed lines and the USB 2.0 lines of that particular port are idle. Similarly, when a Hi-Speed device (USB 2.0 device) is plugged into USB 3.0 port, SuperSpeed lines are idle. Hence, in a USB 3.0 port, either SuperSpeed lines or USB 2.0 lines are operational at any given point of time depending on the device (SuperSpeed Device or USB2.0 device) connected.

Shared Link feature enables the USB 3.0 DS port to be split into 2 independent ports, one embedded SuperSpeed port and a standard USB 2.0 port, thus utilizing the redundant lines effectively. For example, if one of the DS ports is connected to an embedded SuperSpeed device, such as a USB 3.0 camera, HX3 enables the system designer to use the USB 2.0 signals of that specific port to connect to a standard USB 2.0 port. Figure 2 shows how Shared Link port can be implemented in a system.

In a Shared Link DS port, as the SuperSpeed port is embedded, backward compatibility to USB 2.0 is not required as the SuperSpeed device is permanently connected to the SuperSpeed port internally via a physical PCB trace. A Shared Link enabled system should not be connected to a USB 2.0 host or a USB 2.0 Hub. Since the Shared Link SuperSpeed DS ports cannot support USB 2.0 functionality (as expected by the USB 2.0 Host or Hub), the embedded devices connected to the Shared Link SuperSpeed port will fail.  

Figure 2. Example: Shared Link Port of Notebook PC Motherboard (Source: Cypress)

In a generic USB 3.0 port, the connected USB 3.0 device falls back to USB 2.0 speed when SuperSpeed communication fails for any reason. In a Shared Link port, this is not possible as the USB 2.0 lines and SuperSpeed lines are connected to two separate devices. Shared Link overcomes this limitation by implementing a separate VBUS enable pin (DSx_VBUSEN_SS) for the embedded SuperSpeed port (Shared Link port) in addition to the Power enable pin (DSx_PWREN) for the USB 2.0 port.  This helps to independently control the embedded SuperSpeed port power. HX3 identifies SuperSpeed communication failures and toggles its DSx_VBUSEN_SS pin connected to the VBUS detect pin of embedded SuperSpeed device. The DS embedded SuperSpeed device will start enumeration again considering this VBUS toggle as unplug disconnect and connect event. This is the unique implementation and specific to HX3’s Shared Link feature. Figure 3 shows the implementation of a Shared Link Port.

Figure 3. Shared Link Port (Source: Cypress)

Conventional Docking Station
Today’s portable devices are designed to be compact with minimum number of peripheral support. Ports such as serial, HDMI, Ethernet, etc. are usually excluded. To extend peripheral support, docking stations are designed to support additional ports such as USB, Serial, VGA, Ethernet etc. Figure 4 shows the block diagram of a conventional Laptop Docking station.

As shown in Figure 4, a conventional USB 3.0 docking station requires 6 to 8 USB ports (including embedded ports). They are mostly designed with cascade of two 4-port hub controller ICs. USB 3.0 hub is a mandatory requirement in docking stations for supporting high bandwidth peripherals such as Gigabit Ethernet and HDMI in addition to providing exposed USB 3.0 ports. Adding USB 3.0 Hub for connecting slower peripherals such as mouse, keyboard, serial etc is not cost effective. Therefore, conventional docking station needs to have both USB 3.0 Hubs and USB 2.0 Hubs. Two hubs in a system design increase the PCB area, power requirement, routing complexity and the number of passive components, increasing BOM costs significantly.

Figure 4. Conventional docking station design (Source: Cypress)

Shared Link Docking Station
On a 4-port HX3 using Shared Link, we can get up to 8 ports, four embedded SuperSpeed ports and four standard USB 2.0 ports. Figure 5 below shows an example of how Shared Link can be used to implement a cost effective Notebook PC docking station design. Cypress’ Shared Link feature provides a cost effective optimum solution to the customers compared to the Figure 4 Conventional docking station design docking station implementation.

Figure 5. Docking Station with a Shared Link enabled USB3.0 Hub (Source: Cypress)

As shown in Figure 5, downstream ports DS3 and DS4 are standard USB 3.0 ports and DS1 and DS2 are Shared Link ports. SuperSpeed embedded ports of Shared Link ports DS1 and DS2 are dedicated to high speed communication ports such as HDMI and Ethernet. Standard USB 2.0 Port available in the DS1 Shared Link port is used for adding RS232 port to the docking station. The exposed USB 2.0 Standard port available in DS2 extends connectivity to removable devices such as keyboard, mouse, mass storage devices etc.

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