USB4

From Wikipedia, the free encyclopedia

USB4
Type USB
Production history
Designer USB Promoter Group
Designed 29 August 2019; 4 years ago (2019-08-29)
Superseded USB 3.2
Daisy chain No
Audio signal DisplayPort
Video signal DisplayPort
Connector USB-C
Electrical
Max. voltage 48 V (PD 3.1)
Max. current 5 A (PD)
Data
Data signal Yes
Bitrate 80 Gbit/s (10 GB/s)
USB4 Gen3×2 cable (40 Gbps) with 100 W Power Delivery

USB4 (official style), sometimes referred to as USB 4.0, is a technical specification that the USB Implementers Forum (USB-IF) released on 29 August 2019.[1] The USB4 architecture can share a single, high-speed link with multiple hardware endpoints dynamically, best serving each transfer by data type and application.

In contrast to prior USB protocol standards, USB4 mandates the exclusive use of the Type-C connector and USB Power Delivery (USB-PD) specification. USB4 products must support 20 Gbit/s throughput and can support 40 Gbit/s throughput, but because of tunneling, even nominal 20 Gbit/s can result in higher effective data rates in USB4, compared to USB 3.2, when sending mixed data. In contrast to USB 3.2, it allows tunneling of DisplayPort and PCI Express.

USB4 is said to be based[marketing claim?] on the Thunderbolt 3 protocol specification, which Intel had contributed to the USB-IF.[2] Although Thunderbolt 3 is stated by Intel to be merely USB4 compatible, its successor Thunderbolt 4 is variously claimed to be USB4 compliant in Intel's technology comparison expando table[3] and other marketing material.[4] Regardless, the USB4 specification text itself states "...the USB 3.2 Specification, remains the fundamental architecture for USB data transfer on a USB4 Fabric."[5]

Support of interoperability with Thunderbolt 3 products is optional for USB4 hosts and USB4 peripheral devices; but is mandatory for USB4 hubs on all of their downstream facing ports (DFP), and for USB4-based docks, on their upstream facing port (UFP) in addition to all of their downstream facing ports.[6]

On the other hand, "USB4 specification compliance" is required in Thunderbolt 4 according to a speculative 2020 Intel marketing press release.[7][8] However, Intel's present FAQ "What Is Thunderbolt 4?" makes no mention at all of USB4.[9] Similarly, Intel's present FAQ "What’s the Difference Between Thunderbolt™ 4 and USB-C?" makes a lesser claim "Thunderbolt™ 4 ...is compatible with USB4" as opposed to requiring USB4 compliance.[10]

The USB4 specification was updated on 18 October 2022 by the USB Implementers Forum, adding a new 80 Gbit/s bi-directional mode and 120 Gbit/s asymmetric mode.[11][12]

History[edit]

USB4 was announced in March 2019.[13][14] 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 deliberately without a separating space versus the prior versions. Several news reports before the release of that version use the terminology "USB 4.0" and "USB 4".[15][16] Even after publication of rev. 1.0, some sources write "USB 4", claiming "to reflect the way readers search".[17]

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.[18][19]

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.[20]

On 29 April 2020, DisplayPort Alt Mode version 2.0 was released, supporting DisplayPort 2.0 over USB4.[21]

Data transfer modes[edit]

USB4 by itself does not introduce any new generic data transfer mechanism or device classes like USB 3.x. It adds a way to tunnel USB 3.2 and other protocols like DisplayPort and optionally PCIe, yet retains earlier USB 3.2 native protocol as well.[5][22] 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[reference needed] [disputed ] to mandatory USB 3.2 10 Gbit/s, with optional support for USB 3.2 20 Gbit/s. The USB4 2.0 specification names version 1.0 USB3 Tunneling as USB3 Gen X tunneling (eXisting in Ver 1.0) and introduces optional support for a new USB3 Gen T tunneling. Gen T extends the USB 3.2 protocol that Gen X employed to now be able to use the maximum available USB4 bandwidth.

USB4 V2.0 specifies tunneling of:

USB4 also requires support of DisplayPort Alternate Mode. That means, DP can be sent via USB4 tunneling or by DP Alternate Mode.[23] USB4 supports DisplayPort 2.0 over its alternative mode. DisplayPort 2.0 can support 8K resolution at 60 Hz with HDR10 color and can use up to 80 Gbit/s which is the same amount available to USB data, but just unidirectional.[24]

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

Native USB 3.2 Enhanced SuperSpeed is supported in all three categories Host, Hub/Dock, Peripheral Device.[5][22]

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.

Transfer mode support by endpoints[12]
Mode Host Hub Peripheral device
Legacy USB (1–2) (max. 480 Mbit/s) Yes Yes Yes
USB4 Gen 2 (10 or 20 Gbit/s) Yes Yes Yes
USB4 Gen 3 (20 or 40 Gbit/s) Optional Yes Optional
USB4 Gen 4 (80 or 120 Gbit/s) Optional Optional Optional
Tunneled USB 3.2 Gen 2×1 (10 Gbit/s) Yes Yes Optional
Tunneled USB 3.2 Gen 2×2 (20 Gbit/s) Optional Optional Optional
Tunneled USB3 Gen T (10–80 Gbit/s) Optional Optional Optional
Tunneled DisplayPort Yes Yes Optional
Tunneled PCI Express Optional Yes Optional
Host-to-Host communications Yes Yes
DisplayPort Alternate Mode Yes Yes Optional
Thunderbolt Alternate Mode Optional Yes Optional
USB-C Alternate Modes Optional Optional Optional
Comparison of transfer modes
Mode Name Old Name(s) Encoding Multiple Lanes Lane Rate
(Gbit/s)
Nominal Rate USB-IF Marketing Name[25][26] Logo
(Gbit/s) (GB/s)
USB 2.0 (High-Speed) Does not appear NRZI w/ bit stuffing Single 0.480 0.480 0.060 Hi-Speed USB
USB 3.2 Gen 1×1 USB 3.0 (SuperSpeed),
USB 3.1 Gen 1
8b/10b Single 5 5 0.625 USB 5Gbps
USB 3.2 Gen 1×2 Does not appear Dual 5 10 1.2
USB 3.2 Gen 2×1 USB 3.1 Gen 2 128b/132b Single 10 10 1.2 USB 10Gbps
USB 3.2 Gen 2×2[a] Does not appear Dual 10 20 2.4
USB4 Gen 2×1[a] 64b/66b[b] Single 10 10 1.2
USB4 Gen 2×2 Dual 10 20 2.4 USB 20Gbps
USB4 Gen 3×1 128b/132b[b] Single 20 20 2.4
USB4 Gen 3×2 Dual 20 40 4.8 USB 40Gbps
USB4 Gen 4[c] PAM-3[27] Symmetric 40 80 9.6 USB 80Gbps
Asymmetric 40 120 14.4
  1. ^ a b USB4 Gen 2×1 is different from USB 3.2 Gen 2×2. They only signify the same speed (10 Gbit/s), but are coded differently on the electrical layer.
  2. ^ a b USB4 can use optional Reed–Solomon forward error correction (RS FEC). In this mode, 12 × 16 B (128 bit) symbols are assembled together with 2 B (12 bit + 4 bit reserved) synchronisation bits indicating the respective symbol types and 4 B of RS FEC to allow to correct up to 1 B of errors anywhere in the total 198 B block.
  3. ^ Gen 4 always has dual lanes.

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[28]). After removal of 64b/66b encoding, those also become round, 20.625/66*64 = 20.000 Gbit/s.

Power delivery[edit]

Unlike its predecessors, USB4 does not directly specify its power parameters. It instead defers to USB Power Delivery (USB PD) both for this purpose and also to identify and then to engage USB4 mode of operation between upstream-downstream facing port link partners.[29] (else they may fallback to USB 3.2 or use other modes).

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[clarification needed], but regardless, USB4 devices are restricted to only one USB 3.2 unit load 150 / 250 mA before configuration.[30][31] 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[edit]

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 docks/hubs; it is optional for USB4 hosts and USB4 peripheral devices.[32] Thunderbolt 3 compatible docks/hubs "shall incorporate an internal host controller to support USB3 functionality", which means it will appear and serve as a USB3 (aka USB 3.2) root port to downstream link devices. The USB4 specification defines the abbreviations "USB3" to mean "USB 3.2 Specification", "TBT3" to mean "Thunderbolt™ 3", and "TBT3-Compatible" to mean "Able to interoperate with TBT3 products." TBT3-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.[33]

Pinout[edit]

Type-C receptacle pinout (end-on view)

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.[34]

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.[35]

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.[36]

Type-C receptacle A pin layout
Pin Name Description
A1 GND Ground return
A2 SSTXp1 ("TX1+") SuperSpeed differential pair #1, TX, positive
A3 SSTXn1 ("TX1-") SuperSpeed differential pair #1, TX, negative
A4 VBUS Bus power
A5 CC1 Configuration channel
A6 Dp1 USB 2.0 differential pair, position 1, positive
A7 Dn1 USB 2.0 differential pair, position 1, negative
A8 SBU1 Sideband use (SBU)
A9 VBUS Bus power
A10 SSRXn2 ("RX2-") SuperSpeed differential pair #4, RX, negative
A11 SSRXp2 ("RX2+") SuperSpeed differential pair #4, RX, positive
A12 GND Ground return
Type-C receptacle B pin layout
Pin Name Description
B12 GND Ground return
B11 SSRXp1 SuperSpeed differential pair #2, RX, positive
B10 SSRXn1 SuperSpeed differential pair #2, RX, negative
B9 VBUS Bus power
B8 SBU2 Sideband use (SBU)
B7 Dn2 USB 2.0 differential pair, position 2, negative[a]
B6 Dp2 USB 2.0 differential pair, position 2, positive[a]
B5 CC2 Configuration channel
B4 VBUS Bus power
B3 SSTXn2 SuperSpeed differential pair #3, TX, negative
B2 SSTXp2 SuperSpeed differential pair #3, TX, positive
B1 GND Ground return
  1. ^ a b There is only a single non-SuperSpeed differential pair in the cable. This pin is not connected in the plug/cable.

Software support[edit]

USB4 is supported by:

Hardware support[edit]

On 3 March 2020, Cypress Semiconductor announced new Type-C power (PD) controllers supporting USB4, CCG6DF as dual port and CCG6SF as single-port.[40]

In November 2020, Apple unveiled MacBook Air (M1, 2020), MacBook Pro (13-inch, M1, 2020), and Mac mini (M1, 2020) featuring two "Thunderbolt 4" or "Thunderbolt / USB 4" ports. A list of Apple devices featuring "Thunderbolt 4" or "Thunderbolt / USB 4" ports include the following.[41] Note that those two types of ports are listed separately. Neither port moniker denotes authentic USB-IF logos for USB4, nor is the reason for distinction between those two (nor is their difference with USB4) explained in the cited[41] Apple website. As of Nov 2023, USB-IF online Product Search[42] for USB4 certified products produces zero listings for Apple products, using filters Date: Jan 2019-present and Features: USB4.

  • MacBook Air (M2, 2022)
  • MacBook Pro (13-inch, M2, 2022)
  • iMac (24-inch, M1, 2021)
  • MacBook Pro (13-inch, M1, 2020)
  • MacBook Air (M1, 2020)
  • Mac mini (M1, 2020)

AMD also stated that Zen 3+ (Rembrandt) processors will support USB4[43] and released products do have this feature after a chipset driver update.[44] However, AMD has only announced support for USB 3.2 Gen 2×2 in Zen 4 processors that were released in September 2022.[45][46]

  • Dell Precision 5540

During CES 2020, USB-IF and Intel stated their intention[reference needed] to allow[in what way?] USB4 products that support all the optional functionality[ThunderBolt 3] as Thunderbolt 4 products. Brad Saunders, a representative of Intel[pg 3] and Chairman of the USB Promoter Group, is quoted in online articles to say he anticipates that most PCs with USB4 will support Thunderbolt 3, even though it's not mandatory[47] in the USB4 specification, but for phones the manufacturers are less likely to implement Thunderbolt 3 support.[17] The first[reference needed] products "compatible" with USB4 were Intel's Tiger Lake processors[embedded SKUs], with more devices appearing around the end of 2020.[48][49] Meanwhile, Intel supports[partly] Thunderbolt 3 and USB-C with the mobile 9th generation processors in 2019, but still has not generally extended support to Desktop CPU SKUs to date (Nov 2023). Only the latest Intel JHL8440 series Thunderbolt controllers[50] make any suggestion of USB4, denoted in online datasheets as hybrid "Thunderbolt™/USB4". Their full USB4 compliance is questionable as they only advertise support for native USB up to USB 3.1 10Gbps, not USB 3.2 Gen 2x2 20Gbps. As of Nov 2023, USB-IF online Product Search[42] for USB4 certified products produces only a single Intel listing (namely a Meteor Lake[51][52] USB component), using filters Date: Jan 2019-present and Features: USB4.


References[edit]

  1. ^ "USB Promoter Group USB4 Specification". USB implementers forum. 2019-08-29. Archived from the original on 2021-02-13. Retrieved 2020-04-28.
  2. ^ Bright, Peter (2019-03-04). "Thunderbolt 3 becomes USB4, as Intel's interconnect goes royalty-free". Ars Technica. Archived from the original on 2021-02-13. Retrieved 2019-03-04.
  3. ^ The Differences Between Thunderbolt™ 3 and Thunderbolt™ 4 Technology (View all)
  4. ^ "Thunderbolt™ 3 vs. Thunderbolt 4 Technology". Intel. Retrieved 2023-05-29.
  5. ^ a b c USB4® Specification v2.0

    2 Architectural Overview
    Enhanced SuperSpeed USB, as defined in the USB 3.2 Specification, remains the fundamental architecture for USB data transfer on a USB4 Fabric.
  6. ^ see 2.1.5 (page 15) in https://www.usb.org/sites/default/files/USB4%20Specification.zip
  7. ^ [Introducing Thunderbolt 4: Universal Cable Connectivity for Everyone]
  8. ^ Introducing Thunderbolt 4: Universal Cable Connectivity for Everyone Archived 22 November 2021 at the Wayback Machine. Intel newsroom. July 8, 2020
  9. ^ What Is Thunderbolt 4?
  10. ^ Thunderbolt™ Key Takeaways
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  20. ^ USB4 Spec. p.1
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  22. ^ a b USB4® Specification v2.0

    2.1 USB4 System Description
    Figure 2-1 illustrates the dual bus architecture of USB 3.2 as augmented by USB4. As architected, backward compatibility is supported with minimum interoperability starting at USB 2.0, working up through USB 3.2, and finally up to USB4

    2.1.1.5 USB4 Host
    A USB4 Host contains:
    • A Host Router.
    • A USB 2.0 Host.
    • An Enhanced SuperSpeed Host.
    • A DisplayPort Source.

    2.1.1.4.2 USB4 Hub
    A USB4 Hub contains:
    • A Device Router.
    • An Enhanced SuperSpeed USB Hub.
    • A PCIe Switch.
    • A USB 2.0 Hub.

    2.1.1.4.1 USB4 Peripheral Device
    A USB4 Peripheral Device contains a Device Router and can also optionally contain one or more of the following:
    • An Enhanced SuperSpeed Function.
    • A USB 2.0 Function.
    • A PCIe Function.
    • A DisplayPort Source or Sink.
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  28. ^ "How to Test and Troubleshoot USB4" (PDF). Archived (PDF) from the original on 2022-09-10. Retrieved 2022-07-25.
  29. ^ USB Power Delivery Base Specification

    2.1.2 Power Delivery Contracts
    Note [USB4] does not define a default power, rather relies on a USB PD power contract. When first attached the [USB4] device operates in [USB 3.2] mode which is its USB Default Operation.

    6.4.8 Enter_USB Message
    The Enter_USB Message Shall be sent by the DFP to its UFP Port Partner and to the Cable Plug(s) of an Active Cable, when in an Explicit Contract, to enter a specified USB Mode of operation.
    ...
    When entering [USB4] operation, the Enter_USB Message Shall be sent by a [USB4] PDUSB Hub’s DFP(s) or [USB4] PDUSB Host’s DFP(s)...
  30. ^ USB Type-C® Cable and Connector Specification Release 2.3

    4.6 Power
    Prior to entering a USB PD explicit contract, a USB4 port operates as a USB 3.2 port regarding power.
  31. ^ USB 3.2 Revision 1.1 - June 2022

    9.8 Constants
    UNIT_LOAD
    The amount a device, operating in single lane mode, can draw in the unconfigured state 150mA
    The amount a device, operating in multilane mode, can draw in the unconfigured state 250mA

    9.2.5.1 Power Budgeting
    Devices shall limit the power they consume from VBUS to one unit load or less until configured.
  32. ^ USB4 Specification V1.0 August 2019 Chapter 13: "A USB4 host and USB4 peripheral device may optionally support TBT3-Compatibility. If a USB4 host or USB4 peripheral device supports TBT3-Compatibility, it shall do so as defined in this chapter".
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  42. ^ a b https://www.usb.org/products  USB-IF certified product search tool
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  47. ^ USB4 Specification V1.0 August 2019 Chapter 13: "A USB4 host and USB4 peripheral device may optionally support TBT3-Compatibility. If a USB4 host or USB4 peripheral device supports TBT3-Compatibility, it shall do so as defined in this chapter".
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  50. ^ intel JHL8440 datasheet
  51. ^ https://www.usb.org/single-product/9068  intel USB4 search result: Meteor Lake
  52. ^ https://en.wikipedia.org/wiki/Meteor_Lake

External links[edit]