Original logo |
|||
| Type | Bus | ||
|---|---|---|---|
| Production history | |||
| Designer | Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel | ||
| Designed | 1994 | ||
| Manufacturer | Intel, Compaq, Microsoft, NEC, Digital Equipment Corporation, IBM, Nortel | ||
| Superseded | Serial port, parallel port, game port, Apple Desktop Bus, PS/2 connector | ||
| General specifications | |||
| Length | 5 metres (maximum) | ||
| Width | 11.5 mm (A-plug), 8.45 mm (B-plug), | ||
| Height | 4.5 mm (A-plug), 7.78 mm (B-plug, pre-v3.0) | ||
| Hot pluggable | Yes | ||
| External | Yes | ||
| Cable | 4 wires; 8 in USB 3.0 | ||
| Pins | 4 (1 supply, 2 data, 1 ground); 8 in USB 3.0 (additional 4 for SuperSpeed technology) | ||
| Connector | Unique | ||
| Electrical | |||
| Signal | 5 volt DC | ||
| Max. voltage | 5 V(±5%) | ||
| Max. current | 500–900 mA @ 5 V (depending on version) | ||
| Data | |||
| Data signal | Packet data, defined by specifications | ||
| Width | 1 bit | ||
| Bitrate | 1.5/12/480/4,000 Mbit/s (depending on version) | ||
| Max. devices | 127 | ||
| Protocol | Serial | ||
| Pin out | |||
| The standard USB A plug (left) and B plug (right) | |||
| Pin 1 | VCC (+5 V) | ||
| Pin 2 | Data- | ||
| Pin 3 | Data+ | ||
| Pin 4 | Ground | ||
Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices.[1]
USB was designed to standardize the connection of computer peripherals, such as keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
As of 2008[update], approximately 6 billion USB devices had been sold in total, and about 2 billion were being sold each year.[2]
Contents |
A group of seven companies began development on USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. The first silicon for USB was made by Intel in 1995.[3]
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed".[3] The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.[4]
The USB 2.0 specification was released in April 2000 and was ratified by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a fortyfold increase over the original USB 1.1 specification.
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), to decrease power consumption, to increase power output, and to be backwards-compatible with USB 2.0.[5] USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.[6] For this reason, the new version is also called SuperSpeed.[7] The first USB 3.0 equipped devices were presented in January 2010.[7][8]
The USB standard evolved through several versions before its official release in 1995:
USB 3.0 was released in November 2008. The standard specifies a maximum transmission speed of up to 5 Gbit/s (625 MB/s), which is over 10 times as fast as USB 2.0 (480 Mbit/s, or 60 MB/s), although this speed is typically only achieved using powerful professional grade or developmental equipment. USB 3.0 reduces the time required for data transmission, reduces power consumption, and is backward compatible with USB 2.0. The USB 3.0 Promoter Group announced on 17 November 2008 that the specification of version 3.0 had been completed and had made the transition to the USB Implementers Forum (USB-IF), the managing body of USB specifications.[9] This move effectively opened the specification to hardware developers for implementation in future products. A new feature is the "SuperSpeed" bus, which provides a fourth transfer mode at 5.0 Gbit/s. The raw throughput is 4 Gbit/s (using 8b/10b encoding), and the specification considers it reasonable to achieve around 3.2 Gbit/s (0.4 GB/s or 400 MB/s), increasing as hardware advances in the future take hold. Two-way communication is also possible. In USB 3.0, full-duplex communications are done when using SuperSpeed (USB 3.0) transfer. In previous USB versions (i.e., 1.x or 2.0), all communication is half-duplex and directionally controlled by the host.
The design architecture of USB is asymmetrical in its topology, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. A USB host may implement multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.[10][11]
USB devices are linked in series through hubs. One hub is known as the root hub which is built into the host controller.
A physical USB device may consist of several logical sub-devices that are referred to as device functions. A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function). Such a device is called a compound device in which each logical device is assigned a distinctive address by the host and all logical devices are connected to a built-in hub to which the physical USB wire is connected. A host assigns one and only one device address to a function.
USB device communication is based on pipes (logical channels). A pipe is a connection from the host controller to a logical entity, found on a device, and named an endpoint. Because pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably. A USB device can have up to 32 endpoints: 16 into the host controller and 16 out of the host controller. The USB standard reserves one endpoint of each type, leaving a theoretical maximum of 30 for normal use. USB devices seldom have this many endpoints.
There are two types of pipes: stream and message pipes depending on the type of data transfer.
A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an isochronous, interrupt, or bulk transfer. A message pipe is a bi-directional pipe connected to a bi-directional endpoint that is exclusively used for control data flow. An endpoint is built into the USB device by the manufacturer and therefore exists permanently. An endpoint of a pipe is addressable with a tuple (device_address, endpoint_number) as specified in a TOKEN packet that the host sends when it wants to start a data transfer session. If the direction of the data transfer is from the host to the endpoint, an OUT packet (a specialization of a TOKEN packet) having the desired device address and endpoint number is sent by the host. If the direction of the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g., the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet will be ignored. Otherwise, it will be accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.
Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and which is not associated with any interface. A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function.
When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.
The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The slowest device connected to a controller sets the bandwidth of the interface. For SuperSpeed USB (defined since USB 3.0), connected devices can request service from host. Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices will transmit and receive at USB 3.0 data rates regardless of USB 2.0 or earlier devices connected to that host. Operating data rates for them will be set in the legacy manner.
The functionality of USB devices is defined by class codes, communicated to the USB host to effect the loading of suitable software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers.
Device classes include:[12]
| Class | Usage | Description | Examples, or exception |
|---|---|---|---|
| 00h | Device | Unspecified[13] | Device class is unspecified, interface descriptors are used to determine needed drivers |
| 01h | Interface | Audio | Speaker, microphone, sound card, MIDI |
| 02h | Both | Communications and CDC Control | Modem, Ethernet adapter, Wi-Fi adapter |
| 03h | Interface | Human interface device (HID) | Keyboard, mouse, joystick |
| 05h | Interface | Physical Interface Device (PID) | Force feedback joystick |
| 06h | Interface | Image | Webcam, scanner |
| 07h | Interface | Printer | Laser printer, inkjet printer, CNC machine |
| 08h | Interface | Mass storage | USB flash drive, memory card reader, digital audio player, digital camera, external drive |
| 09h | Device | USB hub | Full bandwidth hub |
| 0Ah | Interface | CDC-Data | Used together with class 02h: communications and CDC control |
| 0Bh | Interface | Smart Card | USB smart card reader |
| 0Dh | Interface | Content security | Fingerprint reader |
| 0Eh | Interface | Video | Webcam |
| 0Fh | Interface | Personal Healthcare | Pulse monitor (watch) |
| DCh | Both | Diagnostic Device | USB compliance testing device |
| E0h | Interface | Wireless Controller | Bluetooth adapter, Microsoft RNDIS |
| EFh | Both | Miscellaneous | ActiveSync device |
| FEh | Interface | Application-specific | IrDA Bridge, Test & Measurement Class (USBTMC),[14] USB DFU (Direct Firmware update)[15] |
| FFh | Both | Vendor-specific | Indicates that a device needs vendor specific drivers |
USB implements connections to storage devices using a set of standards called the USB mass storage device class (MSC or UMS). This was at first intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices, particularly flash drives, because many systems can be controlled with the familiar metaphor of file manipulation within directories. The process of making a novel device look like a familiar device is also known as extension. The ability to boot a write-locked SD card with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.
Though most post-2005 computers are capable of booting from USB mass storage devices, USB is not intended to be a primary bus for a computer's internal storage: buses such as Parallel ATA (PATA or IDE), Serial ATA (SATA), or SCSI fulfill that role in PC class computers. However, USB has one important advantage in that it is possible to install and remove devices without rebooting the computer (hot-swapping), making it useful for mobile peripherals, including drives of various kinds. Originally conceived and still used today for optical storage devices (CD-RW drives, DVD drives and so on), several manufacturers offer external portable USB hard disk drives, or empty enclosures for disk drives, which offer performance comparable to internal drives, limited by the current number and type of attached USB devices and by the upper limit of the USB interface (in practice about 30 MB/s for USB 2.0 and potentially 400 MB/s or more[16] for USB 3.0). These external drives have typically included a "translating device" that bridges between a drive's interface to a USB interface port. Functionally, the drive appears to the user much like an internal drive. Other competing standards for external drive connectivity include eSATA, ExpressCard (now at version 2.0), and FireWire (IEEE 1394).
Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.[17][18]
Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, and PC game port connectors to USB.
USB mice and keyboards can usually be used with older computers that have PS/2 connectors with the aid of a small USB-to-PS/2 adapter. Such adaptors contain no logic circuitry: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Converters also exist to allow PS/2 keyboards and mice (usually one of each) to be connected to a USB port. These devices present two HID endpoints to the system and use a microcontroller to perform bidirectional translation of data between the two standards.
| Pin | Name | Cable color | Description |
|---|---|---|---|
| 1 | VBUS | Red | +5 V |
| 2 | D− | White | Data − |
| 3 | D+ | Green | Data + |
| 4 | GND | Black | Ground |
| Pin | Name | Cable color | Description |
|---|---|---|---|
| 1 | VBUS | Red | +5 V |
| 2 | D− | White | Data − |
| 3 | D+ | Green | Data + |
| 4 | ID | None | Permits distinction of A plug from B plug * A plug: connected to Signal ground * B plug: not connected |
| 5 | GND | Black | Signal ground |
The connectors specified by the USB committee were designed to support a number of USB's underlying goals, and to reflect lessons learned from the menagerie of connectors which have been used in the computer industry. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug.[20] In the case of an extension cable, the connector on one end is a receptacle. The official USB specification documents periodically define the term male to represent the plug, and female to represent the receptacle.
By design, it is difficult to attach a USB connector incorrectly. Connectors cannot be plugged in upside down and it is clear from the physical act of making a connection, when the plug and receptacle are correctly mated. The USB specification states that the required USB Icon is to be "embossed" on the "topside" of the USB plug, which "provides easy user recognition and facilitates alignment during the mating process". The specification also shows that the "recommended" (optional) "Manufacturer's logo" ("engraved" on the diagram but not specified in the text) is on the opposite side of the USB Icon. The specification further states "the USB Icon is also located adjacent to each receptacle. Receptacles should be oriented to allow the icon on the plug to be visible during the mating process". However, the specification does not consider the height of the device compared to the eye level height of the user, so the side of the cable that is "visible" when mated to a computer on a desk can depend on whether the user is standing or kneeling.[20]
Only moderate insertion/removal force is needed. USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumb-turns other connectors have required). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances (i.e., behind a floor-mounted chassis, or from below) or by those with motor disabilities.
The standard connectors were deliberately intended to enforce the directed topology of a USB network: type A connectors on host devices that supply power and type B connectors on target devices that receive power. This prevents users from accidentally connecting two USB power supplies to each other, which could lead to dangerously high currents, circuit failures, or even fire. USB does not support cyclical networks and the standard connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. network cabling) gender changers make little sense with USB and are almost never used.
The standard connectors were designed to be robust. Many previous connector designs were fragile, specifying embedded component pins or other delicate parts which proved vulnerable to bending or breakage, even with the application of modest force. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal sheath.
The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise twisted data pair travels in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping.
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.
The USB standard specifies relatively loose tolerances for compliant USB connectors to minimize physical incompatibilities in connectors from different vendors. To address a weakness present in some other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to the size of the cable strain relief mechanism (usually molding integral with the cable outer insulation) at the connector. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does.
In general, cables have only plugs (very few have a receptacle on one end, although extension cables with a standard A plug and jack are sold), and hosts and devices have only receptacles. Hosts almost universally have type-A receptacles, and devices one or another type-B variety. Type-A plugs mate only with type-A receptacles, and type-B with type-B; they are deliberately physically incompatible. However, an extension to USB standard specification called USB On-The-Go allows a single port to act as either a host or a device—chosen by which end of the cable plugs into the receptacle on the unit. Even after the cable is hooked up and the units are communicating, the two units may "swap" ends under program control. This capability is meant for units such as PDAs in which the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.
USB 3.0 receptacles are electrically compatible with USB Standard 2.0 device plugs if they physically match. USB 3.0 type-A plugs and receptacles are completely backward compatible, and USB 3.0 type-B receptacles will accept USB 2.0 and earlier plugs. However, USB 3.0 type-B plugs will not fit into USB 2.0 and earlier receptacles. eSATAp (eSATA/USB) port is also compatible with USB 2.0 devices.
There are several types of USB connectors, including some that have been added while the specification progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles.
The data connectors in the Standard-A plug are actually recessed in the plug as compared to the outside power connectors. This permits the power to connect first which prevents data errors by allowing the device to power up first and then transfer the data. Some devices will operate in different modes depending on whether the data connection is made. This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.
To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections, and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to be charged and/or operated from a standard USB cable.
The USB 2.0 Standard-A type of USB plug is a flattened rectangle which inserts into a "downstream-port" receptacle on the USB host, or a hub, and carries both power and data. This plug is frequently seen on cables that are permanently attached to a device, such as one connecting a keyboard or mouse to the computer via usb connection.
USB connections eventually wear out as the connection loosens through repeated plugging and unplugging. The lifetime of a USB-A male connector is approximately 1,500 connect/disconnect cycles.[21]
A Standard-B plug—which has a square shape with bevelled exterior corners—typically plugs into an "upstream receptacle" on a device that uses a removable cable, e.g. a printer. A Type B plug delivers power in addition to carrying data. On some devices, the Type B receptacle has no data connections, being used solely for accepting power from the upstream device. This two-connector-type scheme (A/B) prevents a user from accidentally creating an electrical loop.[22]
Various connectors have been used for smaller devices such as PDAs, mobile phones or digital cameras. These include the now-deprecated[23] (but standardized) Mini-A and the currently standard Mini-B,[24] Micro-A, and Micro-B connectors.
The Mini-A and Mini-B plugs are approximately 3 by 7 mm. The micro-USB plugs have a similar width and approximately half the thickness, enabling their integration into thinner portable devices. The mini-A connector is 6.85 by 1.8 mm with a maximum overmold size of 11.7 by 8.5 mm. The mini-B connector is 6.85 by 1.8 mm with a maximum overmold size of 10.6 by 8.5 mm.[19]
The Micro-USB connector was announced by the USB-IF on 4 January 2007.[25] The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007.[26] As of February 2009[update], many currently available devices and cables still use Mini plugs, but the newer Micro connectors are being widely adopted and as of December 2010, the Micro connectors are the most widely used. The thinner micro connectors are intended to replace the Mini plugs in new devices including smartphones and personal digital assistants. The Micro plug design is rated for at least 10,000 connect-disconnect cycles which is significantly more than the Mini plug design.[27] The Universal Serial Bus Micro-USB Cables and Connectors Specification details the mechanical characteristics of Micro-A plugs, Micro-AB receptacles, and Micro-B plugs and receptacles,[27] along with a Standard-A receptacle to Micro-A plug adapter.
The cellular phone carrier group, Open Mobile Terminal Platform (OMTP) in 2007 have endorsed Micro-USB as the standard connector for data and power on mobile devices.[28] These include various types of battery chargers, allowing Micro-USB to be the single external cable link needed by some devices.
As of 30 January 2009 Micro-USB has been accepted and is being used by almost all cell phone manufacturers as the standard charging port (including Hewlett-Packard, HTC, LG, Motorola, Nokia, Research In Motion, Samsung, Sony Ericsson) in most of the world.
On 29 June 2009, following a request from the European Commission and in close co-operation with the Commission services, major producers of mobile phones have agreed in a Memorandum of Understanding ("MoU") to harmonise chargers for data-enabled mobile phones sold in the European Union. Industry commits to provide charger compatibility on the basis of the Micro-USB connector. Consumers will be able to purchase mobile phones without a charger, thus logically reducing their cost.[29] Following a mandate from the European Commission, the European Standardisation Bodies CEN-CENELEC and ETSI have now made available the harmonised standards needed for the manufacture of data-enabled mobile phones compatible with the new common External Power Supply (EPS) based on micro-USB.[30]
In addition, on 22 October 2009 the International Telecommunication Union (ITU) has also announced that it had embraced micro-USB as the Universal Charger Solution its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger".[31]
A USB On-The-Go device is required to have one, and only one USB connector: a Mini-AB or Micro-AB receptacle. This receptacle is capable of accepting both Mini-A and Mini-B plugs, and alternatively, Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in Micro-USB1.01.
The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral. An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the HNP protocol is used to temporarily transfer the host role to the B-device.
OTG devices attached either to a peripheral-only B-device or a standard/embedded host will have their role fixed by the cable since in these scenarios it is only possible to attach the cable one way around.
The following receptacles accept the following plugs:
| Receptacle | Plug | ||||
|---|---|---|---|---|---|
| Yes | No | No | No | No | |
| No | Yes | No | No | No | |
| No | No | Yes | No | No | |
| No | No | No | Yes | Yes | |
| No | No | No | No | Yes | |
Cables exist with pairs of plugs:
| Plug | Plug | ||||
|---|---|---|---|---|---|
| Yes | NS | Yes | Yes | NS | |
| No | NS | No | No | ||
| No | NS | No | |||
| Yes | No | ||||
| No | |||||
NS: non-standard, existing for specific proprietary purposes, and not interoperable with USB-IF compliant equipment.
In addition to the above cable assemblies comprising two plugs, an "adapter" cable with a Micro-A plug and a Standard-A receptacle is compliant with USB specifications.[19] Other combinations of connectors are not compliant. However, some older devices and cables with Mini-A connectors have been certified by USB-IF. The Mini-A connector has been deprecated: there will be no new certification of assemblies using Mini-A connector.[23]
The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. USB 3.0 cables contain twice as many wires as USB 2.x to support SuperSpeed data transmission, and are thus larger in diameter.[33]
The USB 1.1 Standard specifies that a standard cable can have a maximum length of 3 meters with devices operating at Low Speed (1.5 Mb/s), and a maximum length of 5 meters with devices operating at Full Speed (12 Mb/s).
USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mb/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns.[34] The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (192,000 km/s, which is close to the maximum achievable transmission speed for standard copper wire).[35]
The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires the maximum practical length is 3 metres (9.8 ft).[36]
The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 3.0, the voltage supplied by low-powered hub ports is 4.45–5.25 V.[37]
A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws the maximum number of unit loads permitted by the standard. Every device functions initially as low-power but the device may request high-power and will get it if the power is available on the providing bus.[38]
Some devices, such as high-speed external disk drives, require more than 500 mA of current[39] and therefore cannot be powered from one USB 2.0 port. Such devices usually come with Y-shaped cable that has two USB connectors to be plugged into a computer. With such a cable, a device can draw power from two USB ports simultaneously.[40]
A bus-powered hub initializes itself at 1 unit load and transitions to maximum unit loads after it completes hub configuration. Any device connected to the hub will draw 1 unit load regardless of the current draw of devices connected to other ports of the hub (i.e. one device connected on a four-port hub will draw only 1 unit load despite the fact that more unit loads are being supplied to the hub).[38]
A self-powered hub will supply maximum supported unit loads to any device connected to it. In addition, the VBUS will present 1 unit load upstream for communication if parts of the Hub are powered down.[38]
The Battery Charging Specification of 2007 defines new types of USB ports, e.g., charging ports.[41] As compared to standard downstream ports, where a portable device can only draw more than 100 mA current after digital negotiation with the host or hub, charging ports can supply currents above 0.5 A without digital negotiation. A charging port supplies up to 500 mA at 5 V, up to the rated current at 3.6 V or more, and drop its output voltage if the portable device attempts to draw more than the rated current. The charger port may shut down if the load is too high.
Charging ports exist in two flavors: downstream charging ports, supporting data transfers as well, and dedicated charging ports, without data support. A portable device can recognize the type of USB port from the way the D+ and D- pins are connected. For example, on a dedicated charging port, the D+ and D- pins are shorted. With downstream charging ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. A dedicated charge port may have a rated current between 0.5 and 1.5 A. There is no upper limit for the rated current of a downstream charge port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A).
Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhone chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2V, the device may pull up to 500mA. When D+ = 2.0V and D- = 2.8V, the device may pull up to 1000mA of current.[42]
Dedicated charging ports can be found on USB power adapters that convert utility power and/or another power source (e.g., a car's electrical system) to run attached devices and battery packs. On a host (such as a laptop computer) with both standard and charging USB ports, the charging ports should be labeled as such.[41]
Sleep-and-charge USB ports can be used to charge electronic devices even when the computer is switched off. Normally when a computer is powered off the USB ports are powered down. This prevents phones and other devices from being able to charge unless the computer is powered on. Sleep-and-charge USB ports remain powered even when the computer is off. On laptops, charging devices from the USB port when it is not being powered from AC will drain the laptop battery faster. Desktop machines need to remain plugged into AC power for Sleep-and-charge to work.[43]
As of 14 June 2007, all new mobile phones applying for a license in China are required to use the USB port as a power port.[44][45] This was the first standard to use the convention of shorting D+ and D-.[46]
In September 2007, the Open Mobile Terminal Platform group (a forum of mobile network operators and manufacturers such as Nokia, Samsung, Motorola, Sony Ericsson and LG) announced that its members had agreed on micro-USB as the future common connector for mobile devices.[47][48]
On 17 February 2009, the GSM Association (GSMA) announced[49] that they had agreed on a standard charger for mobile phones. The standard connector to be adopted by 17 manufacturers including Nokia, Motorola and Samsung is to be the micro-USB connector (several media reports erroneously reported this as the mini-USB). The new chargers will be much more efficient than existing chargers.[49] Having a standard charger for all phones means that manufacturers will no longer have to supply a charger with every new phone. The basis of the GSMA's Universal Charger Solution (UCS) is the technical recommendation from OMTP and the USB-IF battery charging standard.[50][51][52]
On 22 April 2009, this was further endorsed by the CTIA – The Wireless Association.[53]
In June 2009, many of the world's largest mobile phone manufacturers signed a Memorandum of Understanding (MoU), agreeing to make most data-enabled mobile phones marketed in the European Union compatible with a common External Power Supply (EPS) based on the GSMA / OMTP Universal Charging Solution.[54][55]
On 22 October 2009, the International Telecommunication Union (ITU) announced that it had embraced the Universal Charger Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger".[56]
Some USB devices require more power than is permitted by the specifications for a single port. This is common for external hard and optical disc drives, and generally for devices with motors or lamps. Such devices can use an external power supply, which is allowed by the standard, or use a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some USB ports and external hubs can, in practice, supply more power to USB devices than required by the specification but a standard-compliant device may not depend on this.
In addition to limiting the total average power used by the device, the USB specification limits the inrush current (i.e., that used to charge decoupling and filter capacitors) when the device is first connected. Otherwise, connecting a device could cause problems with the host's internal power. USB devices are also required to automatically enter ultra low-power suspend mode when the USB host is suspended. Nevertheless, many USB host interfaces do not cut off the power supply to USB devices when they are suspended since resuming from the suspended state would become a lot more complicated if they did.
Some non-standard USB devices use the 5 V power supply without participating in a proper USB network which negotiates power draws with the host interface. These are usually referred to as USB decorations. The typical example is a USB-powered keyboard light; fans, mug coolers and heaters, battery chargers, miniature vacuum cleaners, and even miniature lava lamps are available. In most cases, these items contain no digital circuitry, and thus are not Standard compliant USB devices at all. This can theoretically cause problems with some computers, such as drawing too much current and damaging circuitry; prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which would then permit the device to switch into high-power mode.
Some devices, when plugged into charging ports, draw even more power (10 watts or 2.1 Amps) than the Battery Charging Specification allows. The iPad and MiFi 2200 are two such devices. [57]
Powered USB uses standard USB signaling with the addition of extra power lines. It uses four additional pins to supply up to 6 A at either 5 V, 12 V, or 24 V (depending on keying) to peripheral devices. The wires and contacts on the USB portion have been upgraded to support higher current on the 5 V line, as well. This is commonly used in retail systems and provides enough power to operate stationary barcode scanners, printers, PIN pads, signature capture devices, etc. This modification of the USB interface is proprietary and was developed by IBM, NCR, and FCI/Berg. It is essentially two connectors stacked such that the bottom connector accepts a standard USB plug and the top connector takes a power connector.
USB supports the following signaling rates. The terms speed and bandwidth are used interchangeably. "high-" is alternatively written as "hi-".
USB signals are transmitted on a twisted-pair data cable with 90Ω ±15% Characteristic impedance,[60] labeled D+ and D−. Prior to USB 3.0, these collectively use half-duplex differential signaling to reduce the effects of electromagnetic noise on longer lines. Transmitted signal levels are 0.0 to 0.3 volts for low and 2.8 to 3.6 volts for high in full-bandwidth and low-bandwidth modes, and −10 to 10 mV for low and 360 to 440 mV for high in hi-bandwidth mode. In FS mode, the cable wires are not terminated, but the HS mode has termination of 45 Ω to ground, or 90 Ω differential to match the data cable impedance, reducing interference due to signal reflections. USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.
A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end. Originally, this was a "B' connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables which permitted erroneous connections (and potential damage to the circuitry). USB interconnections are not as fool-proof or as simple as originally intended.
The host includes 15 kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and indicates a reset or disconnected connection.
A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates a device's bandwidth support; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D− high.
USB data is transmitted by toggling the data lines between the J state and the opposite K state. USB encodes data using the NRZI convention; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions remains in the bitstream, USB uses bit stuffing; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits. Seven consecutive received 1 bits is always an error. USB 3.0 has introduced additional data transmission encodings.
A USB packet begins with an 8-bit synchronization sequence '00000001'. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame. For high bandwidth USB, the packet begins with a 32-bit synchronization sequence.
A USB packet's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D− both below max) and 1 bit time of J state. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to hubs can add as much as one bit time before the SE0 of the end of packet. This extra bit can also result in a "bit stuff violation" if the six bits before it in the CRC are '1's. This bit should be ignored by receiver.
A USB bus is reset using a prolonged (10 to 20 milliseconds) SE0 signal.
USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the high bandwidth mode with the host/hub. A device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D− driven LOW by host for 10 to 20 ms) it pulls the D− line high, known as chirp K. This indicates to the host that the device is high bandwidth. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D− and D+ lines) letting the device know that the hub will operate at high bandwidth. The device has to receive at least 3 sets of KJ chirps before it changes to high bandwidth terminations and begins high bandwidth signaling. Because USB 3.0 uses wiring separate and additional to that used by USB 2.0 and USB 1.x, such bandwidth negotiation is not required.
Clock tolerance is 480.00 Mbit/s ±500 ppm, 12.000 Mbit/s ±2500 ppm, 1.50 Mbit/s ±15000 ppm.
Though high bandwidth devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are high bandwidth. The USB-IF certifies devices and provides licenses to use special marketing logos for either "basic bandwidth" (low and full) or high bandwidth after passing a compliance test and paying a licensing fee. All devices are tested according to the latest specification, so recently compliant low bandwidth devices are also 2.0 devices.
USB 3 uses tinned copper stranded AWG-28 cables with 90±7 Ω impedance for its high-speed differential pairs and linear feedback shift register and 8b/10b encoding sent with a voltage of 1 V nominal with a 100 mV receiver threshold; the receiver uses equalization.[61] SSC clock and 300 ppm precision is used. Packet headers are protected with CRC-16, while data payload is protected with CRC-32.[62] Power up to 3.6 W may be used. One unit load in superspeed mode is equal to 150 mA.[62]
The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the southbridge.
Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet.[63] This is 70% of the total bandwidth available.
According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead—the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards".[64] Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec.
For isochronous devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission.
During USB communication data is transmitted as packets. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.
After the sync field, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus will never need bit-stuffing, even when combined with the final 1 bit in the sync byte. However, trailing 1 bits in the PID may require bit-stuffing within the first few bits of the payload.)
| Type | PID value (msb-first) |
Transmitted byte (lsb-first) |
Name | Description |
|---|---|---|---|---|
| Reserved | 0000 | 0000 1111 | ||
| Token | 1000 | 0001 1110 | SPLIT | High-bandwidth (USB 2.0) split transaction |
| 0100 | 0010 1101 | PING | Check if endpoint can accept data (USB 2.0) | |
| Special | 1100 | 0011 1100 | PRE | Low-bandwidth USB preamble |
| Handshake | ERR | Split transaction error (USB 2.0) | ||
| 0010 | 0100 1011 | ACK | Data packet accepted | |
| 1010 | 0101 1010 | NAK | Data packet not accepted; please retransmit | |
| 0110 | 0110 1001 | NYET | Data not ready yet (USB 2.0) | |
| 1110 | 0111 1000 | STALL | Transfer impossible; do error recovery | |
| Token | 0001 | 1000 0111 | OUT | Address for host-to-device transfer |
| 1001 | 1001 0110 | IN | Address for device-to-host transfer | |
| 0101 | 1010 0101 | SOF | Start of frame marker (sent each ms) | |
| 1101 | 1011 0100 | SETUP | Address for host-to-device control transfer | |
| Data | 0011 | 1100 0011 | DATA0 | Even-numbered data packet |
| 1011 | 1101 0010 | DATA1 | Odd-numbered data packet | |
| 0111 | 1110 0001 | DATA2 | Data packet for high-bandwidth isochronous transfer (USB 2.0) | |
| 1111 | 1111 0000 | MDATA | Data packet for high-bandwidth isochronous transfer (USB 2.0) | |
Packets come in three basic types, each with a different format and CRC (cyclic redundancy check):
Handshake packets consist of a PID byte, and are generally sent in response to data packets. The three basic types are ACK, indicating that data was successfully received, NAK, indicating that the data cannot be received and should be retried, and STALL, indicating that the device has an error condition and will never be able to successfully transfer data until some corrective action (such as device initialization) is performed.
USB 2.0 added two additional handshake packets, NYET which indicates that a split transaction is not yet complete. A NYET packet is also used to tell the host that the receiver has accepted a data packet, but cannot accept any more due to buffers being full. The host will then send PING packets and will continue with data packets once the device ACK's the PING. The other packet added was the ERR handshake to indicate that a split transaction failed.
The only handshake packet the USB host may generate is ACK; if it is not ready to receive data, it should not instruct a device to send any.
Token packets consist of a PID byte followed by 2 payload bytes: 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device.
IN and OUT tokens contain a 7-bit device number and 4-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively.
An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.
An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, NYET, or STALL, as appropriate.
SETUP operates much like an OUT token, but is used for initial device setup. It is followed by an 8-byte DATA0 frame with a standardized format.
Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-bandwidth USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe" (60000 high-bandwidth bit times each).
USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK.
USB 2.0 also added a larger 3-byte SPLIT token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-bandwidth USB bus sending data to a slower USB device, the nearest high-bandwidth capable hub receives a SPLIT token followed by one or two USB packets at high bandwidth, performs the data transfer at full or low bandwidth, and provides the response at high bandwidth when prompted by a second SPLIT token.
A data packet consists of the PID followed by 0–1,023 bytes of data payload (up to 1,024 in high bandwidth, at most 8 at low bandwidth), and a 16-bit CRC.
There are two basic data packets, DATA0 and DATA1. They must always be preceded by an address token, and are usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.
To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.
When a device is reset with a SETUP packet, it expects an 8-byte DATA0 packet next.
USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-bandwidth devices doing high-bandwidth isochronous transfers which need to transfer more than 1024 bytes per 125 µs microframe (8,192 kB/s).
Low-bandwidth devices are supported with a special PID value, PRE. This marks the beginning of a low-bandwidth packet, and is used by hubs which normally do not send full-bandwidth packets to low-bandwidth devices. Since all PID bytes include four 0 bits, they leave the bus in the full-bandwidth K state, which is the same as the low-bandwidth J state. It is followed by a brief pause during which hubs enable their low-bandwidth outputs, already idling in the J state, then a low-bandwidth packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-bandwidth devices other than hubs can simply ignore the PRE packet and its low-bandwidth contents, until the final SE0 indicates that a new packet follows.
At first, USB was considered a complement to FireWire (IEEE 1394) technology, which was designed as a high-bandwidth serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment. In the initial design, USB operated at a far lower data rate and used less sophisticated hardware. It was suitable for small peripherals such as keyboards and pointing devices.
The most significant technical differences between FireWire and USB include the following:
These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum transfer rate, FireWire 400 is faster than USB 2.0 Hi-Bandwidth in real-use,[65] especially in high-bandwidth use such as external hard-drives.[66][67][68][69] The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 Hi-Bandwidth both theoretically and practically.[70] The chipset and drivers used to implement USB and Firewire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.[71]
The IEEE 802.3af Power over Ethernet (PoE) standard specifies a more elaborate power negotiation scheme than powered USB. It operates at 48 V DC and can supply more power (up to 12.95 W, PoE+ 25.5 W) over a cable up to 100 meters compared to USB 2.0 which provide 2.5 W with a maximum cable length of 5 meters. This has made PoE popular for VoIP telephones, security cameras, wireless access points and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short, and power demand is low.
Ethernet standards requires electrical isolation between the networked device (computer, phone, etc.) and the network cable up to 1500 V AC or 2250 V DC for 60 seconds.[72] USB has no such requirement as it was designed for peripherals closely associated with a host computer, and in fact it connects the peripheral and host grounds. This gives Ethernet a significant safety advantage over USB with peripherals such as cable and DSL modems connected to external wiring that can assume hazardous voltages under certain fault conditions.[73]
Digital musical instruments are another example for which USB is competitive for low-cost devices. However Power over Ethernet and the MIDI plug standard are preferred in high-end devices that may have long cables. USB can cause ground loop problems between equipment because it connects the ground references on both transceivers. By contrast, the MIDI plug standard and Ethernet have built-in isolation to 500V or more.
The eSATA connector is a more robust SATA connector, intended for connection to external hard drives and SSDs. It has a far higher transfer rate (3 Gbit/s or 6 Gbit/s, bi-directional) than USB 2.0. A device connected by eSATA appears as an ordinary SATA device, giving both full performance and full compatibility associated with internal drives.
eSATA does not supply power to external devices. This is an increasing disadvantage compared to USB. Even though USB's 2.5 W is sometimes insufficient to power external hard drives, technology is advancing and external drives gradually need less power, exacerbating the eSATA disadvantage. eSATAp (power over eSATA; aka ESATA/USB) is a connector introduced in 2009 that supplies power to attached devices using a new, backwards-compatible, connector. On a notebook eSATAp usually supplies only 5 V to power a 2.5 in HDD/SSD; on a desktop workstation it can additionally supply 12 V to power larger devices including 3.5 in HDD/SSD and 5.25 in optical drives.
eSATAp support can be added to a desktop machine in the form of a bracket connecting to motherboard SATA, power, and USB resources.
eSATA, like USB, supports hot plugging, although this might be limited by OS drivers and device firmware.
Thunderbolt combines PCI Express and DisplayPort into a new serial data interface that can be carried over longer and less costly cables. Thunderbolt has twice the transfer speed of USB 3.0 over copper wire.
The PictBridge standard allows for interconnecting consumer imaging devices. It typically uses USB for its underlying communication layer.
The USB Implementers Forum is working on a wireless networking standard based on the USB protocol. Wireless USB is intended as a cable-replacement technology, and will use ultra-wideband wireless technology for data rates of up to 480 Mbit/s.
USB 2.0 High Speed Inter Chip (HSIC) is a chip-to-chip variant of USB 2.0 that eliminates the conventional analog transceivers found in normal USB. It was adopted as a standard by the USB Implementers Forum in 2007. The HSIC physical layer uses about 50% less power and 75% less board area compared to traditional USB 2.0. HSIC uses two signals at 1.2 V and has a throughput of 480 Mbit/s using 240 MHz DDR signaling. Maximum PCB trace length for HSIC is 10 cm. It does not have low enough latency to support RAM memory sharing between two chips.[74][75]
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