USB4


USB4 is a USB system specified in the USB4 specification which was released in version 1.0 on 29 August 2019 by USB Implementers Forum.
In contrast to prior USB protocol standards, USB4 requires USB-C connectors and for power delivery it requires support of USB PD. In contrast to USB 3.2, it allows tunneling of DisplayPort and PCI Express. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application. USB4 products must support 20 Gbit/s throughput and can support 40 Gbit/s throughput, but due to tunneling even nominal 20 Gbit/s can result in higher effective data rates in USB4, compared to USB 3.2, when sending mixed data.
The USB4 specification is based on the Thunderbolt 3 protocol specification. Support of interoperability with Thunderbolt 3 products is optional for USB4 hosts and USB4 peripheral devices and required for USB4 hubs on its downward facing ports and for USB4-based docks on its downward and upward facing ports.

Overview

Name

The USB4 specification version 1.0, released 29 August 2019, uses "Universal Serial Bus 4" and "USB4". Several news reports before the release of that version use the terminology "USB 4.0" and "USB 4". Even after publication of rev 1.0 some knowingly write "USB 4", claiming "to reflect the way readers search".

Specifications

USB4 Specification

History

It was first announced officially in March 2019.

Contributors

At time of publication of version 1.0 promoter companies having employees that participated in the USB4 Specification technical work group were: Apple Inc., Hewlett-Packard Inc., Intel Corporation, Microsoft Corporation, Renesas Corporation, STMicroelectronics, and Texas Instruments.

Design goals

Goals stated in the USB4 specification are increasing bandwidth, helping to converge the USB-C connector ecosystem and "minimize end-user confusion". Some of the key areas to achieve this are using a single USB-C connector type, while retaining compatibility with existing USB and Thunderbolt products.

Data transfer modes

USB4 by itself doesn't provide any generic data transfer mechanism or device classes like USB 3.x, but serves mostly as a way to tunnel other protocols like USB 3.2, DisplayPort and optionally PCIe.
While it does provide a native Host-to-Host protocol, as the name implies it is only available between two connected hosts. It is used to implement Host IP Networking.
Therefore, when the host and device do not support optional PCIe tunneling, the maximum non-display bandwidth is limited to USB 3.2 20 Gbps, while only USB 3.2 10 Gbps is mandatory.
USB4 allows tunneling:
USB4 also requires support of DisplayPort Alternate Mode. That means, DP can be sent via USB4 tunneling or by DP Alternate Mode.
USB 2.0 is always supported using the dedicated wires in the USB-C connector.
Support of data transfer modes
Some transfer modes are supported by all USB4 devices, support for others is optional. The requirements for supported modes depend on the type of device.
ModeHostHubPeripheral device
USB 2.0
USB4 20 Gbit/s Transport
USB4 40 Gbit/s Transport
Tunneled USB 3.2
Tunneled USB 3.2
Tunneled Displayport
Tunneled PCI Express
Host-to-Host communications
DisplayPort Alternate Mode
Thunderbolt Alternate Mode
USB-C Alternate Modes
USB data transfer modes
USB4 Gen 2 is different from USB 3.2 Gen 2. They only signify the same speed, ie 10Gbps, but they are coded differently on the electrical layer.
Although USB4 is required to support dual-lane modes, it uses single-lane operations during initialization of a dual-lane link; single-lane link can also be used as a fallback mode in case of a lane bonding error.
In Thunderbolt compatibility mode, the lanes are driven slightly faster at 10.3125 Gbps and 20.625 Gbps, as required by Thunderbolt specifications.

Power delivery

A USB4 connection requires a USB Power Delivery contract before being established. This is not a problem, as USB4 requires USB PD to negotiate USB4 mode in the first place. A USB4 source must at least provide 7.5W per port. A USB4 sink must require less than 250mA, 1.5A or 3A @5V of power before USB PD negotiation. With USB PD up to 100W of power are possible.

Thunderbolt 3 compatibility

The USB4 specification states that a design goal is to "Retain compatibility with existing ecosystem of USB and Thunderbolt products." But compatibility with Thunderbolt 3 is only optional for USB4 hosts and USB4 peripheral devices.

Alternate Mode partner specifications

DisplayPort Alt Mode 2.0

On 29 April 2020, DisplayPort Alt Mode version 2.0 was released, supporting DisplayPort version 2.0 over USB4.

Software support

Linux 5.6, released on 29 March 2020, supports USB4.

Hardware support

During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products. The first products compatible with USB4 are expected to be Intel's Tiger Lake series and AMD's Zen 3 series of CPUs, due for release in late 2020.
Devices supporting USB4 are expected to appear in 2020.
Brad Saunders, CEO of the USB Promoter Group anticipates that most PCs with USB4 will support Thunderbolt 3 but for phones the manufacturers are less likely to implement Thunderbolt 3 support.
On 3 March 2020, Cypress Semiconductor announced new controllers supporting USB4, CCG6DF as dual port and CCG6SF as single-port.