USB4

USB4 (Universal Serial Bus 4), sometimes referred to as USB 4.0, is the most recent technical specification of the USB (Universal Serial Bus) data communication standard. The USB Implementers Forum announced USB4 in 2019.

USB4 enables multiple devices to share dynamically a single high-speed data link. USB4 devices must support a data communication bit rate of 20 gigabits (Gbit/s). Versions of the standard optionally support bit rates of 40 Gbit/s (USB4 version 1.0), 80 Gbit/s (USB4 version 2.0), and 120 Gbit/s. In contrast to prior USB standards, USB4 mandates the exclusive use of the USB-C connector and the USB Power Delivery specification.

USB4 architecture builds on the USB 3.2 specification. It also incorporates elements of the Thunderbolt 3 protocol; however, interoperability with Thunderbolt 3 products is mandatory only on selected USB4 device types.

History
USB4 was announced in March 2019. The USB4 specification version 1.0, released 29 August 2019, uses "Universal Serial Bus 4" and specifically "USB4", that is, the short name branding is deliberate without a separating space, which is different from prior versions. 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 sources write "USB 4", claiming "to reflect the way readers search".

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, Intel, Microsoft, Renesas Electronics, STMicroelectronics, and Texas Instruments.

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.

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

On 1 September 2022, the USB Promoter Group announced the pending release of the USB4 Version 2.0 specification, and the specification was subsequently released on 18 October 2022.

Data transfer modes
USB4 by itself does not 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. With the USB4 1.0 specification, when the host and device do not support optional PCIe tunneling, the non-display bandwidth is limited to mandatory USB 3.2 10 Gbit/s, with optional support for USB 3.2 20 Gbit/s. The USB4 2.0 specification named this USB3 Gen X tunneling and introduced optional support for a new USB3 Gen T tunneling that extends the USB3 protocol to be able to use the maximum available bandwidth.

USB4 V2.0 specifies tunneling of:
 * USB 3.2
 * DisplayPort 2.1
 * PCI Express

USB4 also includes support of a DisplayPort alternate mode. That means, DisplayPort signalling (including audio/video signals) can be sent either via USB4 tunneling or using a USB-C alternate mode. DisplayPort 2.1 specification supports a video resolution up to 8K at 60 Hz with HDR10 color depth, and data rate up to 80 Gbit/s which is the same amount available to USB data, but just unidirectional.

Legacy USB (1–2) is always supported using the dedicated wires in the USB-C connector.

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.

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 Gbit/s (for Gen 2) and 20.625 Gbit/s (for Gen 3), as required by Thunderbolt specifications (these are called legacy speeds and rounded speeds ). After removal of 64b/66b encoding, those also become round, 20.625/66 × 64 = 20.000 Gbit/s.

Power delivery
USB4 requires USB Power Delivery (USB PD). A USB4 connection needs to negotiate a USB PD contract before being established. A USB4 source must at least provide 7.5 W (5 V, 1.5 A) per port. A USB4 sink must require less than 250 mA (default), 1.5 A, or 3 A @ 5 V of power (depending on USB-C resistor configuration) before USB PD negotiation. With USB PD, up to 240 W of power is possible with 'Extended power range' (5 A at 48 V). For 'Standard Power range' up to 100 W is possible (5 A at 20 V).

Thunderbolt 3 compatibility
The USB4 specification states that a design goal is to "Retain compatibility with existing ecosystem of USB and Thunderbolt products." Compatibility with Thunderbolt 3 is required for USB4 hubs; it is optional for USB4 hosts and USB4 peripheral devices. Compatible products need to implement 40 Gbit/s mode, at least 15 W of supplied power, and the different clock; implementers need to sign the license agreement and register a Vendor ID with Intel.

Pinout


USB4 has 24 pins in a symmetrical USB type C shell. USB4 has 12 A pins on the top and 12 B pins on the bottom.

USB4 has two lanes of differential SuperSpeed pairs. Lane one uses TX1+, TX1−, RX1+, RX1− and lane two uses TX2+, TX2−, RX2+, RX2−. USB4 transfers data at 20 Gbit/s per lane. USB4 also keeps the differential D+ and D− for USB 2.0 transfer.

The CC configuration channels have the roles of creating a relationship between attached ports, detecting plug orientation due to the reversible USB type C shell, discovering the VBUS power supply pins, determining the lane ordering of the SuperSpeed lanes, and finally the USB protocol makes the CC configuration channel responsible for entering USB4 operation.

Software support
USB4 is supported by:


 * Linux kernel 5.6, released on 29 March 2020
 * macOS Big Sur (11.0), released on 12 November 2020
 * Windows 11, released on 5 October 2021

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 were Intel's Tiger Lake processors, with more devices appearing around the end of 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 Type-C power (PD) controllers supporting USB4, CCG6DF as dual port and CCG6SF as single-port.

In November 2020, Apple unveiled MacBook Air (M1, 2020), MacBook Pro (13-inch, M1, 2020), and Mac mini (M1, 2020) featuring two USB4 ports.

AMD also stated that Zen 3+ (Rembrandt) processors will support USB4 and released products do have this feature after a chipset driver update. However, AMD has only announced support for USB 3.2 Gen 2x2 in Zen 4 processors that were released in September 2022. Intel supports Thunderbolt 3 and USB-C with the mobile 9th generation processors in 2019.