Heads Up on USB 3.2 and USB Type-C



Before you put USB 3.2 through its paces, here’s what you need to know.

Introduction
This article describes applications that benefit from USB 3.2’s increased bandwidth, explains the latest USB 3.2 nomenclature and specification, discusses USB 3.2 implementation, and clarifies how Type-C™ connectors and cables affect embedded systems.

USB 3.2 Applications
Many applications have an insatiable “need for speed.” For example, mass storage devices based on spinning Hard Disk Drives (HDD) work well with USB 3.0 connections. However, USB 3.0 is a bottleneck for flash-based Solid State Disks (SSD). USB 3.2-based mass storage devices, connected at 20Gbps, offer more than four times the actual throughput of USB 3.0 and match the latest SSD capabilities.

Compression is normally not an option in industrial vision systems. In these systems image capture, processing, and taking appropriate actions, like removing an item from a highspeed conveyor belt, is time sensitive. However, USB 3.2 enables time-sensitive systems to support higher resolutions or frame rates. Also, USB 3.2 supports one lane pair for USB and one or two lanes for DisplayPort, enabling instant camera input and display output using a single cable connected to the embedded system.

In contrast, automotive systems do not normally support USB 3.1 Gen2 connections due to cable length and proprietary automotive connectors. However, automotive applications can take advantage of USB 3.2 Gen1x2 connections that double throughput to 10Gbps compared to USB 3.1 Gen1.

Firmware engineers and software developers can utilize the increased bandwidth of USB 3.2 to replace dedicated Trace and Debug ports. USB 3.2 allows the use of an existing Type-C connector, standard USB cables, and PCs/laptops to capture high-bandwidth trace and debug data.

Defining USB 3.2
The USB 3.2 specification replaces the USB 3.1 specification and introduces a new nomenclature. USB 3.2 defines the following connection speeds:

  • General Nomenclature: Gen XxY – (Speed x Lanes)
  • Enhanced SuperSpeed Gen 1×1 – (5G)
  • Enhanced SuperSpeed Gen 2×1 – (10G)
  • Enhanced SuperSpeed Gen 1×2 – (5G*2 =10G)
  • Enhanced SuperSpeed Gen 2×2 – (10G*2 =20G)

Both USB 3.2 Gen2x1 and Gen1x2 provide a 10Gbps raw data rate. However, due to the more efficient line encoding for Gen2, throughput for Gen2x1 is approximately 1.2 times higher than for Gen1x2. Both 10Gbps connection speeds are needed and support different use cases.

USB 3.2 Implementation
USB 3.2 takes advantage of the four differential SuperSpeed/SuperSpeedPlus pairs present in the USB Type-C connector, unlike USB 3.1 and USB 3.0, which used one or the other TX/RX lane pair, depending on Type-C connector orientation (Figure 1).

Figure 1: USB Type-C receptacle with four differential pairs/lanes (Source: USB Type-C Specification Figure 2-2)

A USB 3.2 implementation achieves a 20Gbps raw data rate by lane striping and lane bonding (e.g., splitting and combining data) with two USB 3.1 (10G) lanes. USB 3.2 also supports 10Gbps by striping and bonding two USB 3.0 (5G) lanes. USB 3.2 supports USB Type-C features like Alternate Modes, Power Delivery, and Digital Audio.

Figure 2 illustrates USB 3.2 lane striping and lane bonding. In USB 3.2 Gen Xx2 mode, the Host and Device controller run TX paths at twice the speed of a single-lane USB 3.1 or USB 3.0 connection. Payload data is split (striped) across two TX/RX lanes in the PHY and cable and combined (bonded) in the Host and Device controller RX paths.

Figure 2: USB 3.2 lane striping and lane bonding

USB Type-C Lane Usage for USB 3.2 and DisplayPort
USB Type-C supports power, audio, video, and data on the same cable. Type-C connector lane usage that supports USB and/or DisplayPort is shown in Figure 3.

Figure 3: USB 3.2 and DisplayPort (DP) Alt Mode Lane usage on Type-C connector

The required switching between USB TX or USB RX, DP TX, and ‘Not Used’ pins (Figure 3) for each lane and each use case is best handled by a digital switch that is integrated in the PHY to preserve signal integrity. In USB and USB/DisplayPort PHYs, switching can be handled by the Type-C Assist (TCA) function (Figure 2).

USB 3.2 Software Stacks
Just as the USB 3.1 programming model did not change from USB 3.0, the programming model for USB 3.2 Host and Device controllers does not change to support x2 connections. USB 3.0, USB 3.1, and USB 3.2 xHCI compliant Host controllers all use the same xHCI Host software stack.

Synopsys’ USB Device Controller uses the same Device software stack for USB 3.0, USB 3.1, and USB 3.2. However, 20Gbps throughput can reveal operating system and/or CPU and memory bottlenecks that were not present at 5Gbps or 10Gbps. Also, Device class drivers and/or Device functions like mass storage, networking, and video might need to be optimized to take advantage of the new 20Gbps connection speed.

USB 3.2 and USB Type-C Cables and Connectors
USB Type-C is the new standard USB connector. The USB-IF is emphasizing the transition to Type-C by moving the USB cable and connector chapter to a separate document and renaming the standard-A, standard-B, and mini/micro connectors as legacy USB connectors.

USB Type-C is a small, robust connector suitable for PCs, laptops, tablets, phones etc. Type-C offers orientation agnostic insertion and easy removal suitable for busy consumers. The USB-IF Cable and Connector workgroup defined single- and dual-screw locking connectors for systems where the Type-C connector must be protected against accidental removal or vibration, which can cause intermittent loss of connectivity.

Figure 4: USB Type-C locking connectors (Source: USB Type-C Locking Connector Specification Figures 3-2 and 3-3)

All passive USB Type-C cables can be used for USB 3.2 GenXx2 connections since four SuperSpeed/SuperSpeedPlus differential pairs are mandatory per the USB Type-C specification. A passive cable designed for Gen2 (10G) is limited to approx. 1m length and can support the new 20G connection speed. Two- to three-meter passive cables designed for Gen1 (5G) can support the new 10G connection speed.

Active cables are used to extend USB Type-C cable length beyond 1m for Gen2 and up to 5m for Gen1. Existing active cables might have chosen to not support four differential pairs, since this is not required for USB 3.0 or USB 3.1. USB-IF (USB) and VESA (DisplayPort) are defining active cable specifications to ensure that future active cables will work seamlessly with USB 3.2 connections, including DisplayPort Alternate Mode.

Conclusion
USB 3.2 will benefit customers due to its increased bandwidth and support for the USB Type-C connectors and cables, but designers need to be aware of the challenges of implementing this high-speed standard. Synopsys is actively developing USB 3.2 IP for use in high-performance SoCs, using knowledge that is based on thousands of successful customer designs. See more in Synopsys’ USB 3.2 demonstration showing a USB 3.2 Host and Device communicating at USB 3.2 speeds over a standard USB Type-C cable.


Morten Christiansen is the technical marketing manager for Synopsys’ DesignWare USB and DisplayPort IP. Prior to joining Synopsys, Christiansen was a principal system designer at ST-Ericsson and Ericsson, designing mobile phone and modem chipsets for 19 years. He was also member of technical Staff at ST-Ericsson. Christiansen has contributed to more than 20 USB standards, including USB 3.1, Battery Charging, HSIC and SSIC, as well as communication standards including WMC, EEM, NCM and MBIM, which are used in billions of USB products. In addition to the non-patented USB standard contributions, Christiansen holds six international patents for other USB-related inventions. He holds a Master of Science degree from The Norwegian Institute of Technology 1983.

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