USB from 1.1 to 3.2 and Beyond
First launched in 1996, the universal serial bus (USB) unified the roles of multiple different types of connections and is ubiquitous in computing and consumer tech products. Its arrival made connecting multiple peripherals, such as keyboard, mouse, printer, camera, external drive, or others, to a computer easy and convenient. Peripherals were no longer defined by their interfaces and users no longer needed to deal with multiple cable types to connect devices they wanted to use.
USB 1.1 allowed a maximum data rate of 12Mbps. USB 2.0 raised the bar to 480Mbps to handle a wide range of roles including streaming video and rapidly transferring data from external devices to a PC hard drive. By supplying up to 2.5W at 5V DC through designated VBUS and ground pins, the USB interface also enabled users to power small devices, such as external drives, or to charge laptops and mobile phones without additional power supply connections. In 2007, the smartphone industry mandated the USB charging interface for handsets to allow charging from a standard USB Type-A outlet and to avoid the electrical-waste burden due to discarded dedicated chargers.
Today’s consumer trends demand even more interconnect bandwidth for embedded systems in smart products, such as streaming HD and 4K ultra-HD video systems that need to cast the content to increasingly large screen sizes and exchanging data with high-speed multi-gigabit storage drives. New standards such as HDMI at 6Gbps, DisplayPort at 8.1Gbps, and Thunderbolt at 20Gbps emerged to handle the increased demands.
To retain USB’s universal crown, the USB Implementer’s Forum (USB-IF) first introduced the USB 3.2 specification, which identifies three transfer rates: USB 3.2 Gen1 (5Gbps), USB 3.2 Gen2 (10Gbps) and USB 3.2 Gen2x2 (20Gbps leveraging dual-lane physical interface). These are marketed to consumers as SuperSpeed USB 5Gbps, SuperSpeed USB 10Gbps, and SuperSpeed USB 20Gbps.
Most recently, USB4 has been specified with support for 20Gbps (USB4 20Gbps) and 40Gbps (USB4 40Gbps) transfer rates. Backwards compatible with USB 3.2, USB 2.0, and Thunderbolt 3, USB4 introduces changes including a connection-oriented tunneling architecture that allows multiple protocols to be combined on the same physical interface and share the overall speed and performance of the USB4 fabric.
Upgrading the Physical Connection
To support the new dual-lane high-speed specifications while at the same time allowing backwards compatibility with legacy USB 2.0 equipment, a new physical interface is required. The USB Type-C (USB-C) interface not only incorporates more connections for two sets of differential data channels and a USB 2.0 bus operating in parallel but also adds features to support the USB Power Delivery (USB PD) specification. These features include two sets of power and ground pins and a communication channel over which connected devices can negotiate their power-consumption demands and power-supply capabilities ranging from legacy USB 2.0 5V to the latest 20V/5A specification. Additional side band use (SBU) is also included to allow for future performance enhancements and new features.
click for larger image
Figure 1. USB-C Connector Pins (Source: Diodes Inc.)
USB-C simplifies connecting devices from the user’s standpoint. The connector is non-polarized, allowing the cable to be inserted either way up; hence, the USB-C connector now has 24 pins to cater for the large number of power and data connections required to support USB 3.2, USB4, and USB Power Delivery (PD), and to allow backwards-compatibility with USB 2.0, as shown in Figure 1.
In addition, the interface is bidirectional, allowing the cables to have the same connector at each end and permitting connected devices to act as host or device or as power consumer or supplier.
With this extra flexibility and demand for additional pins, the USB-C interface is considerably more complex than its predecessors. Connected devices may be classified as downstream facing port (DFP or source), upstream facing port (UFP, or Sink), or dual role port (DRP) able to both source and sink data and power. Logic is required to handle configuration control in each case. It is also necessary to detect the plug-in orientation of the cable and correctly switch signals, such as USB 3.2 and DisplayPort to the USB-C connector. In addition, multiplexing of USB 2.0 signals, power switching, and charge control, and, of course, provisions for signal integrity and transient-voltage protection are required.
A device, such as a notebook PC or tablet, can contain circuitry, as shown in Figure 2, to provide a fully functioning USB-C interface capable of handling USB 3.2 and multimedia data as well as USB PD functionality.
click for larger image
Figure 2. USB-C Interface Supporting USB 3.2 Multimedia, and USB PD (Source: Diodes Inc.)
Bidirectional matrix switches such as the Diodes PI3USB31532, shown in Figure 2, deliver a fully integrated solution capable of multiplexing USB 3.2 Gen2 (single-lane, 10Gbps SuperSpeed+) and/or up to four channels of DisplayPort 1.4 signals as well as auxiliary channels through the USB-C connector. The switch is designed with low insertion loss and a broad -3dB bandwidth of 8.3GHz to ensure signal fidelity at up to 10Gbps.
In addition to supporting the above PI5USB31532 function, an active mux such as the 6-channel 4-lane PI3DPX1205A1 can be used. This mux incorporates a ReDriver function to drive longer distances. Features including receive-side linear equalization and output settings for flat gain and equalization ensure double the signal integrity of comparable CMOS ReDrivers.
USB Power Delivery function is performed via the PD controller, which allows power delivery up to 100W through the USB Type-C connector as well as enabling alternative modes of multimedia data, such as DP or Thunderbolt, through the USB Type-C interface.
A device such as the PI5USB2546A integrates charging port control and a 2.4A power switch as well as the switching for USB 2.0 D+ and D‐ data lines. The part supports the USB Battery Charging 1.2 specification, including charging downstream port (CDP) and dedicated charging port (DCP) modes and can be used in wall-charging adapters as well as host and hub devices.
click for larger image
Figure 3. Implementing USB-C in Smartphones (Source: Diodes Inc.)
Figure 3 shows a USB-C port implementation suitable for a smartphone. This circuit uses the example of a Diodes PI5USB31213A, which incorporates the USB Type-C configuration channel controller function along with USB 3.2 Gen2 10Gbps multiplexing function to enable the proper data to the non-polarized USB Type-C connector. The device handles automatic configuration of host mode, device mode, or dual-role port based on the voltage levels detected on the CC pin. It also provides connector orientation detect as well as negotiating the charging current through the USB Type-C interface. Alternatively, a device such as the PI3EQX10312 could be used. This contains all of the functions included in PI5USB31213A with the only change being the inclusion of a ReDriver to enable driving longer trace distances.
click for larger image
Figure 4. USB-C Dock (Source: Diodes Inc.)
As a final example, Figure 4 illustrates a universal docking station that connects to an upstream host via a single USB Type‐C port and provides DisplayPort, HDMI, VGA, and multiple USB 3.2 output ports for downstream devices, such as a monitor and external storage. It also provides a Gigabit Ethernet LAN port. Here, a device such as the PI3USB31532 USB Type‐C crossbar switch or PI3DPX1205A1 USB 3.2 Gen 2 / DisplayPort 1.4 active crossbar can be used to handle USB 3.2 and DisplayPort switching. The power switch shown in the diagram enables the dock to deliver power to the host computer through the VBUS pins. Output from the DP switch (for example PI3WVR31310A) either goes directly to the DP connector or through the HDMI or VGA converter to HDMI and VGA connectors respectively.
Equipment designers must confront the complexities of the USB-C port to take full advantage of the latest USB power and data capabilities, including power delivery at up to 100W, USB 3.2 and USB4 data rates, and multi-protocol support. A variety of integrated solutions are available to handle data switching, power switching, charging control, and cable-orientation detection, which simplify design and ease product certification as well as save board space and bill-of-materials costs.
|Kay Annamalai is Senior Marketing Director at Diodes and specializes in high-speed signal integrity products. A 30-year semiconductor industry veteran, Kay holds several patents and has published in conferences and technical journals as well as contributing to the establishment of several physical layer standards. He holds a BSc in electronics and communications engineering from Madras University and an MS in electrical and computer engineering from UC Santa Barbara.|