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USB 3.0 ports can achieve link sharing by separating the Ultra-High Speed port and the standard USB 2.0 port . Currently, link sharing is a unique feature of the Cypress HX3 USB 3.0 hub controller. This article will introduce how to implement USB 3.0 link sharing in embedded applications such as docking stations, laptops, TVs, and set-top boxes.
The HX3 controller is part of Cypress's USB 3.0 hub controller family and complies with the USB 3.0 specification version 1.0 . All ports of this controller support Ultra-High Speed (SS), High Speed (HS), Full Speed (FS), and Low Speed (LS) operation. It integrates termination resistors, pull-up resistors, and pull-down resistors and supports pin-strap pin settings to reduce the overall system material cost. The HX3 controller features Cypress's proprietary shared link capability, enabling it to provide eight downstream (DS) ports from a four-port USB 3.0 hub.
The shared link feature allows the hub's USB 3.0 DS ports to be divided into embedded high-speed ports and standard USB 2.0 ports. The shared link can support up to eight DS ports from a four-port USB 3.0 hub. A standard USB 3.0 port has eight signal lines: two for USB 2.0 communication (D+, D-), four for high-speed communication (SSTX+, SSTX-, SSRX+, SSRX-), and two for power (VBUS and GND). See Figure 1.
Figure 1 Standard USB 3.0 port (Source : Cypress)
The VBUS enable signal (DSx_PWREN) controls the transmission of VBUS signals to connected USB 3.0 devices . Together with the overcurrent signal (DSx_OVRCURR), the DSx_PWREN signal implements overcurrent protection for the pluggable USB 3.0 port . When an overcurrent occurs at the port, the DSx_PWREN signal, connected to the port power switch output enable pin, can shut down the port power.
Backward compatibility requires the use of USB 2.0 signal lines. When an Ultra High Speed device (USB 3.0 device) is plugged into a USB 3.0 port, only the Ultra High Speed line can communicate; at this time, the USB 2.0 line on that particular port is idle. Similarly, when a High Speed device (USB 2.0 device) is plugged into a USB 3.0 port, the Ultra High Speed line is idle. Therefore, in a USB 3.0 port, depending on the connected device (Ultra High Speed device or USB 2.0 device), at any given point in time, only one of the Ultra High Speed line or USB 2.0 line is active.
The shared link feature allows the USB 3.0 DS port to be split into two independent ports: an embedded high-speed port and a standard USB 2.0 port, thus effectively utilizing redundant lines. For example, if an embedded high-speed device (such as a USB 3.0 camera) is connected to one of the DS ports, the HX3 controller allows system designers to use the USB 2.0 signal of that specific port to connect to the standard USB 2.0 port. Figure 2 illustrates how the shared link port works in a system.
In a shared-link DS port, because an ultra-high-speed port is already embedded, ultra-high-speed devices can be permanently linked to the ultra-high-speed port via board wiring, and therefore are not backward compatible with USB 2.0. Systems with shared-link functionality enabled should not be connected to USB 2.0 hosts or USB 2.0 hubs, because the shared-link ultra-high-speed DS port does not support USB 2.0 functionality (e.g., USB 2.0 hosts or hubs), and ultra-high-speed embedded devices connected to the shared link will not function properly.
Figure 2 Example : Shared link port on a laptop motherboard (Source: Cypress)
In a standard USB 3.0 port, when UltraSpeed communication fails, the connected USB 3.0 device will revert to USB 2.0 speeds. However, this is not possible in a shared link port because the USB 2.0 line and the UltraSpeed line connect two separate devices. To overcome this limitation, in addition to the power enable control pin (DSx_PWREN) of the USB 2.0 port, the shared link also implements a separate VBUS enable control pin (DSx_VBUSEN_SS) for the embedded UltraSpeed port (link-shared port). This allows for independent power control of the embedded UltraSpeed port. When the HX3 controller detects an UltraSpeed communication failure, it toggles the DSx_VBUSEN_SS signal on the VBUS detection pin of the DSx_VBUSEN_SS pin connected to the embedded UltraSpeed device. The embedded UltraSpeed device will then re-enumerate the VBUS toggle, interpreting it as either a disconnection or connection event. This is the unique shared link functionality of the HX3 controller. Figure 3 illustrates the implementation of the shared link port.
Figure 3 Shared link port (Source : Cypress)
Traditional docking station
Modern portable devices feature a compact design and support only a limited number of peripherals, typically excluding ports such as serial interfaces, HDMI, and Ethernet. To enable devices to connect to more peripherals, docking stations are designed to integrate additional ports such as USB, serial, VGA, and Ethernet. Figure 4 shows a block diagram of a traditional laptop docking station.
As shown in Figure 4, a traditional USB 3.0 docking station requires 6-8 USB ports (including embedded ports). The design utilizes cascaded integrated circuits with two four-port hub controllers. To support high-bandwidth peripherals such as Gigabit Ethernet and HDMI, the docking station must include both external USB 3.0 ports and a USB 3.0 hub. Adding a USB 3.0 hub only for connecting slower peripherals like mice, keyboards, and serial ports would be very inefficient in terms of cost. Therefore, traditional docking stations typically include both USB 3.0 and USB 2.0 hubs. However, using two hubs increases printed circuit board area, power requirements, wiring complexity, and the number of passive components, significantly increasing overall material costs.
Figure 4. Traditional docking station design (Source: Cypress)
Shared Link Expansion Dock
On the four-port HX3 controller configured with shared link functionality, we can use up to eight ports: four embedded high-speed ports and four standard USB 2.0 ports. Figure 5 illustrates how shared link enables a low-cost notebook docking station design. Compared to the traditional docking station design shown in Figure 4, Cypress's shared link functionality provides customers with an optimal cost-effective solution.
Figure 5 shows a docking station configured with a shared link USB 3.0 hub (Source: Cypress).
As shown in Figure 5, downstream ports DS3 and DS4 are standard USB 3.0 ports, while DS1 and DS2 are shared link ports. The ultra-high-speed embedded ports on the shared link ports DS1 and DS2 are dedicated to high-speed communication ports, such as HDMI and Ethernet ports. Using the standard USB 2.0 port on the DS1 shared link port , an RS232 port can be added to the docking station . External USB 2.0 standard ports on DS2 can also be used to connect pluggable devices such as keyboards, mice, and external hard drives.
Other embedded applications
With the rapid growth of USB 3.0 applications over the past few years, USB 3.0 host ports have become standard on all new PCs and laptops. Furthermore, the increasing demand for real-time high-definition video has led to the adoption of the USB 3.0 standard in consumer electronics devices. Shared link functionality can be used in most consumer devices, including CPU motherboards, docking stations, monitors, set-top boxes, gaming devices, and medical devices.
The USB 3.0 standard supports 5-Gbps SuperSpeed (SS) operation, which is 10 times faster than the USB 2.0 standard. Therefore, USB 3.0 is not only suitable for connecting high-bandwidth peripherals such as HD cameras, but can also be used as a system bus to support intra-system connections between embedded devices, as shown in Figure 6.
Shared links in embedded applications allow more devices to be connected to the USB host, reducing material costs, PCB complexity, and power consumption. Furthermore, because the DS port is located at the edge of the PCB, deploying short cabling is difficult in ultra-high-speed and high-speed embedded applications. The HX3 controller features flexible, programmable USB 3.0 and USB 2.0 PHYs, enabling cabling up to 11 inches compared to the typical 6-inch interface.
Portable computing devices
In portable devices such as laptops, tablets, and smartphones, USB 3.0 is typically used as an internal system bus. As shown in Figure 6, a CPU integrated circuit usually supports a single USB 3.0 host. In this application, the upstream port of the hub is permanently connected to the embedded USB 3.0 host via physical printed circuit board wiring . The hub's DS port can be connected to an integrated circuit that supports embedded USB 3.0 functionality, or used as a general-purpose USB 3.0 external port.
Figure 7 illustrates how Ultrabook laptop designs can implement shared links to reduce material costs and design complexity.
Figure 7. Ultrabook laptops supporting shared links (Source: Cypress)
USB 3.0 docking station application
USB 3.0 docking stations can be divided into general-purpose and dedicated types. The main difference between general-purpose and dedicated docking stations lies in their upstream connections. General-purpose docking stations typically support standard USB 3.0 upstream ports, while dedicated docking stations can support custom ports on certain laptops.
Because the shared link UltraSpeed port lacks a USB 2.0 port, embedded UltraSpeed devices connected to this port will not function when the docking station is connected to a USB 2.0 host. To avoid unintentional connection to a USB 2.0 host, using a shared link on a generic docking station is not recommended.
Schematic diagram
Figure 8 shows how to connect a shared link signal to a DSUSB2.0 device and an embedded ultra-high-speed device.
Figure 8. USB data cable connection for shared link port (Source: Cypress)
Figure 8 shows 8 USB communication pins.
• Four pins (SSTX+, SSTX-, SSRX+, and SSRX-) are used for ultra-high-speed communication, and one VBUS pin controls the power switch.
• Two pins (D+ and D-) are used for USB 2.0 communication and one VBUS pin controls the power switch.
The HX3 controller's four ultra-high-speed signals are connected to the ultra-high-speed pins of the embedded ultra-high-speed device, while the USB 2.0 pins connected to the embedded ultra-high-speed device remain disconnected. The USB 2.0 port connection for the shared link port is the same as a standard USB 2.0 port.
The shared link port in the Hx3 controller is equipped with the following relevant pins:
0. USB2.0 standard port pins
• D+ and D- cables for USB 2.0 data transfer
• Responsible for controlling the DSx_PWREN signal (as shown in Figure 9).
1. DSX_OVRCURR is used as an overcurrent indicator for the Hx3 controller (not shown in Figure 8).
Embedded ultra-high speed pins
• SSRX+, SSRX-, SSTX+, and SSTX- pins for ultra-high-speed data transmission.
VBUS is responsible for controlling the DSx_VBUSEN_SS signal (as shown in Figure 9).
As shown in Figure 8, the USB 2.0 data lines (D+ and D-) connect to the pluggable USB 2.0 port connector pins, while the Ultra-High Speed (UHS) lines connect to the UHS lines of the embedded device. According to the USB specification, each removable DS port must have a minimum capacitance of 120μF on the VBUS pin to maintain a stable voltage under maximum load conditions. This is why a large 150μF capacitor is needed connected to the VBUS_DS2 line, while the VBUS pin of the embedded UHS port does not require a large capacitor.
The USB connector shields (SHD1 and SHD2) should be grounded in parallel via an RC circuit to reduce electromagnetic interference, as shown in Figure 8.
Figure 9. VBSU control of the DS port in the shared link (Source: Cypress)
Shared link mode requires separate VBUS control for pluggable USB 2.0 devices and embedded high-speed devices. Figure 9 shows how VBUS control is implemented.
To ensure that the embedded high-speed device does not revert to USB 2.0 speed operation, an external power switch is required. This switch, controlled by the HX3, generates a DSx_VBUSEN_SL output signal. This signal controls the VBUS of the embedded device.
DSx_PWREN is another output signal generated by the HX3 controller, capable of controlling the VBUS of pluggable USB 2.0 devices . For example, DSx_PWREN can shut down the port power in the event of an overcurrent. The DSx_OVRCURR pin (not shown in Figure 9) is used to indicate the overcurrent state of the removable port. This pin is not required for the embedded high-speed port, as it is permanently connected internally within the embedded high-speed port.
The CY4613 is a Cypress shared link development kit based on the CYUSB3326 component. Click here to download the complete schematic of the shared link hub.
Note: Developers can disable link sharing if needed. The default configuration enables link sharing, requiring changes to the EEPROM configuration parameters. The modified configuration can be programmed using the Blasterplus configuration utility. Cypress provides designers with a Windows GUI tool for the Blasterplus configuration utility, allowing configuration downloads to the EEPROM via a PC's USB port. Please refer to the KBA91657 manual to download the HX3 controller firmware. For more information on the HX3 controller's features, please refer to the HX3 BlasterPlus User Guide.
Shared links can reduce the number of components and the area of printed circuit boards, thereby reducing material costs. For example, Table 1 compares the components used in a shared-link docking station design and a conventional docking station design using two hub controllers. Compared to the conventional docking station design, the shared-link-based docking station design can save 28 components. The space required to install the components is also reduced accordingly.
Table 1 Comparison of Shared Link Dock and Traditional Dock (Source: Cypress)
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