USB hardware

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The initial versions of the USB standard specified connectors that were easy to use and that would have acceptable life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of the USB standard gave rise to another family of connectors to permit additional data paths. All versions of USB specify cable properties; version 3.x cables, marketed as SuperSpeed, added data paths. The USB standard included power supply to peripheral devices; modern versions of the standard allow power delivery up to 240 watts, for battery charging and powering various devices. USB has been selected as the charging format for many mobile phones and other devices, reducing the proliferation of proprietary chargers.

Unlike other data buses (such as Ethernet), USB connections are directed; a host device has downstream-facing ports that connect to the upstream-facing ports of peripherals. Only downstream-facing ports provide power; this topology was chosen to easily prevent electrical overloads and damaged equipment. Thus, USB cables have different ends, A and B, with different physical connectors for each. Each format has a plug and receptacle defined for each of the A and B ends. A USB cable, by definition, has a plug on each end—one A (or C) and one B (or C)—and the corresponding receptacle is usually on a computer or electronic device. The mini and micro formats may connect to an AB receptacle, which accepts either an A or a B plug, that plug determining the behavior of the receptacle.

The three sizes of USB connectors are the default, or standard, format intended for desktop or portable equipment, the mini intended for mobile equipment, which was deprecated when it was replaced by the thinner micro size, all of which were deprecated in USB 3.2 in favor of Type-C. There are five speeds for USB data transfer: Low Speed, Full Speed, High Speed (from version 2.0 of the specification), SuperSpeed (from version 3.0), and SuperSpeed+ (from version 3.1). The modes have differing hardware and cabling requirements. USB devices have some choice of implemented modes, and USB version is not a reliable statement of implemented modes. Modes are identified by their names and icons, and the specification suggests that plugs and receptacles be color-coded (SuperSpeed is identified by blue).

The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug.^[2] The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.^{[3] [clarification needed ]}

By design, it is difficult to insert a USB plug into its receptacle incorrectly. The USB specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation.^[2] The USB‐C plug, however, is reversible. USB cables and small USB devices are held in place by the gripping force from the receptacle, with no screws, clips, or thumb-turns as other connectors use.

The different A and B plugs prevent accidentally connecting two power sources. However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as USB On-The-Go in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables. See the USB On-The-Go connectors section below for a more detailed summary description.

There are so-called cables with A plugs on both ends, which may be valid if the "cable" includes, for example, a USB host-to-host transfer device with two ports.^[4] This is, by definition, a device with two logical B ports, each with a captive cable, not a cable with two A ends.

The standard connectors were designed to be more robust than many past connectors. This is because USB is hot-swappable, and the connectors would be used more frequently, and perhaps with less care, than previous connectors.

Standard USB connectors have a minimum rated lifetime of 1,500 cycles of insertion and removal,^[5] and this increased to 5,000 cycles for Mini-B connectors.^[5] The rating for all Micro connectors is 10,000 cycles,^[5] and the same applies to USB-C.^[6] To accomplish this, a locking device was added and a leaf spring was moved 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.^{[5] [page needed ]}

In standard USB, 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 shell.^[5]

The shell on the plug makes contact with the receptacle before any of the internal pins. The shell is typically grounded, to dissipate static electricity and to shield the wires within the connector.

The USB standards specify dimensions and tolerances for connectors, to prevent physical incompatibilities, including maximum dimensions of plug bodies and minimum clear spaces around receptacles so that adjacent ports are not blocked.

USB 1.0, 1.1, and 2.0 use two wires for power (V_BUS and GND) and two wires for one differential signal of serial data.^[7] Mini and micro connectors have their GND connections moved from pin #4 to pin #5, while their pin #4 serves as an ID pin to electrically differentiate A and B plugs when connecting to the AB receptacles of On-The-Go devices.^[8]

USB 3.0 provides two additional differential pairs (four wires, SSTx+, SSTx−, SSRx+ and SSRx−), providing full-duplex data transfers at SuperSpeed, making it similar to Serial ATA or single-lane PCI Express.

Standard USB pin assignments

Pin	Name	Wire color[a]		Description
1	VBUS	Red or	Orange	+5 V
2	D−	White or	Gold	Data−
3	D+	Green		Data+
4	GND	Black or	Blue	Ground

Mini- and Micro-USB pin assignments

Pin	Name	Wire color[a]	Description
1	VBUS	Red	+5 V
2	D−	White	Data−
3	D+	Green	Data+
4	ID	None (only used in plug)	When a cable is connected to a Mini- or Micro-AB receptacle, the ID pin indicates to the On-The-Go device whether the plug is the A (host) or B (peripheral) end of its cable, causing the device to behave as a host or peripheral accordingly. A plug (host end): connected to GND B plug (device end): not connected
5	GND	Black	Signal ground

Usual USB color-coding

Color							Location	Description
Black or white	Receptacles and plugs	Type-A or Type-B
Blue (Pantone 300C)	Receptacles and plugs	Type-A or Type-B, SuperSpeed
Teal blue	Receptacles and plugs	Type-A or Type-B, SuperSpeed+
Green	Receptacles and plugs	Type-A or Type-B, Qualcomm Quick Charge (QC)
Purple	Plugs only	Type-A or Type-C, Huawei SuperCharge
Yellow or red	Receptacles only	High-current or sleep-and-charge
Orange	Receptacles only	High-retention connector, mostly used on industrial hardware

USB ports and connectors are often color-coded to distinguish their different functions and USB versions. These colors are not part of the USB specification and can vary between manufacturers; for example, the USB 3.0 specification mandates appropriate color-coding while it only recommends blue inserts for Standard-A USB 3.0 connectors and plugs.^[10]

USB connector types multiplied as the specification progressed. The original USB specification detailed standard-A and standard-B plugs and receptacles. The connectors were different so that users could not connect one computer receptacle to another. The data pins in the standard plugs are recessed compared to the power pins so that the device can power up before establishing a data connection. Some devices operate in different modes depending on whether the data connection is made. Charging docks supply power and do not include a host device or data pins, allowing any capable USB device to charge or operate from a standard USB cable. Charging cables provide power connections, but not data. In a charge-only cable, the data wires are shorted at the device end, otherwise, the device may reject the charger as unsuitable.

• The Type-A plug. This plug has an elongated rectangular cross-section, inserts into a Type-A receptacle on a downstream port on a USB host or hub, and carries both power and data. Captive cables on USB devices such as keyboards or mice terminate with a Type-A plug.
• The Type-B plug: This plug has a near square cross-section with the top exterior corners beveled. As part of a removable cable, it inserts into an upstream port on a device, such as a printer. 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 a loop.^{[11] [12]}

The maximum allowed cross-section of the overmold boot (which is part of the connector used for its handling) is 16 by 8 mm (0.63 by 0.31 in) for the Standard-A plug type, while for the Type-B it is 11.5 by 10.5 mm (0.45 by 0.41 in).^[3]

Mini-USB connectors were introduced together with USB 2.0 in April 2000, mostly used with smaller devices such as digital cameras, smartphones, and tablet computers. The Mini-A connectors and the Mini-AB receptacle were deprecated in May 2007, meaning their use in new products has been prohibited since then.^[13] Mini-B connectors are still permitted, but they are not On-The-Go–compliant and cannot be certified;^{[14] [15] [16]} the Mini-B connector was common for transferring data to and from early smartphones and PDAs. Both Mini-A and Mini-B plugs are approximately 3 by 7 mm (0.12 by 0.28 in). The Mini-AB receptacle accepts either the Mini-A or the Mini-B plug, causing the On-The-Go device to behave as a host (A) or peripheral (B) accordingly.

Micro-USB connectors, which were announced by the USB-IF on January 4, 2007,^{[17] [18]} have a similar width to Mini-USB but approximately half the thickness, enabling their integration into thinner portable devices. The Micro-A connector is 6.85 by 1.8 mm (0.270 by 0.071 in) with a maximum plug body size of 11.7 by 8.5 mm (0.46 by 0.33 in), while the Micro-B connector has the same height and width with a slightly smaller maximum plug body size of 10.6 by 8.5 mm (0.42 by 0.33 in).^[9]

The thinner Micro-USB connectors were intended to replace the Mini connectors in devices manufactured since May 2007, including smartphones, personal digital assistants, and cameras.^[19]

The Micro plug design is rated for at least 10,000 connect–disconnect cycles, which is more than the Mini plug design.^{[17] [20]} The Micro connector is also designed to reduce the mechanical wear on the device; instead, the easier-to-replace cable is designed to bear more of the mechanical wear of connection and disconnection. The Universal Serial Bus Micro-USB Cables and Connectors Specification details the mechanical characteristics of Micro-A plugs, Micro-AB receptacles (which accept both Micro-A and Micro-B plugs), and Micro-B plugs and receptacles,^[20] along with a permitted adapter with a Standard-A receptacle and a Micro-A plug, as would be used e.g. to connect a camera to an existing Standard-A–B desktop printer cable.

Micro-USB was endorsed as the standard connector for data and power on mobile devices by the cellular phone carrier group Open Mobile Terminal Platform (OMTP) in 2007.^[21]

Micro-USB was embraced as the "Universal Charging Solution" by the International Telecommunication Union (ITU) in October 2009.^[22]

In Europe, micro-USB became the defined common external power supply (EPS) for use with smartphones sold in the EU,^[23] and 14 of the world's largest mobile phone manufacturers signed the EU's common EPS Memorandum of Understanding (MoU).^{[24] [25]} Apple, one of the original MoU signers, makes Micro-USB adapters available—as permitted in the Common EPS MoU—for its iPhones equipped with Apple's proprietary 30-pin dock connector and, later, Lightning connector.^{[26] [27]} according to the CEN, CENELEC, and ETSI.

USB 3.0 introduced SuperSpeed plugs and receptacles, both Standard and Micro. All 3.0 SuperSpeed receptacles (Standard-A, Standard-B, Micro-A, Micro-B, and Micro-AB) are backward-compatible with the corresponding pre-3.0 plugs; additionally, the Standard-A SuperSpeed plug fits the pre-SuperSpeed Standard-A receptacle. (All other SuperSpeed plugs cannot be attached to pre-SuperSpeed receptacles.)

For any devices to have a SuperSpeed link, all the connectors between them must be Type‐C or SuperSpeed.

Every USB cable predating USB‐C had an A plug at one end and a B plug at the other (with the rare exception of one special A–A configuration with certain conductors omitted, for operating system debugging and other host-to-host connection applications.^{[28] : §5.5.2} In a USB‐C-to-legacy cable, the Type‐C plug is electrically marked to take the role complementary to the connector at the opposite end, A for B and B for A. When a modern C–C cable is used, the two connected devices communicate to determine which takes which role.

USB 3.0 Type-B plug

Before USB‐C, USB On-The-Go (OTG) introduced the concept of a device that could switch roles, performing either the host role or peripheral device role, as needed, depending simply on which type of plug was attached. An OTG device was required to have one, and only one, USB connector: a Micro-AB receptacle or, before Micro-USB, a Mini-AB receptacle.

The Micro-AB receptacle is capable of accepting the Micro-A or Micro-B plug of any of the allowed cables and adapters as defined in revision 1.01 of the Micro-USB specification.

Since a Type-AB receptacle allows either an A or an B plug to be attached, each corresponding A and B plug design has an ID contact to indicate electrically whether the plug is the A or the B end of its cable: In an A plug the ID contact is connected to GND, and in a B plug it is not. Typically, a pull-up resistor in the device is used to detect the presence or absence of the GND connection.

An OTG device with an 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. An OTG device with a B plug inserted is called the B-device and by default assumes the role of peripheral. If an application on the On-The-Go device requires the role of host, then the Host Negotiation Protocol (HNP) is used to temporarily transfer the host role to the OTG device.

The USB-C connector supersedes all earlier USB connectors and the Mini DisplayPort connector. It is used for all USB protocols and for Thunderbolt (3 and later), DisplayPort (1.2 and later), and others. Developed at roughly the same time as the USB 3.1 specification, but distinct from it, the USB-C Specification 1.0 was finalized in August 2014^[29] and defined a new small reversible connector for all USB and some other devices.^[30] The USB-C plug connects both to hosts and to peripheral devices, as well as to chargers and power supplies, replacing all of the preceeding USB connectors with a standard meant to be future-proof.^{[29] [31]}

The 24-pin double-sided connector provides four power–ground pairs, two differential pairs for USB 2.0 data (though only one pair is implemented in a USB-C cable), four pairs for SuperSpeed data bus (only two pairs are used in USB 3.1 mode), two "sideband use" pins, V_CONN +5 V power for active cables, and a configuration pin for cable orientation detection and dedicated biphase mark code (BMC) configuration data channel (CC).^{[32] [33]} Type-A and Type-B adaptors and cables are required for older hosts and devices to plug into USB-C hosts and devices. Adapters and cables with a USB-C receptacle are not allowed.^[34]

A Full-Featured USB cable is a Type‐C-to-Type‐C cable that supports USB 2.0, USB 3.2 and USB4 data operation, and a Full-Featured Type‐C receptacle likewise supports the same full set of protocols.^[35] It contains a full set of wires and is electronically marked (E-marked): It contains an E-marker chip that responds to the USB Power Delivery Discover Identity command, a kind of vendor-defined message (VDM) sent over the configuration data channel (CC). Using this command, the cable reports its current capacity, maximum speed, and other parameters.^{[36] : §4.9} Full-Featured USB Type-C devices are a mechanic prerequisite for multi-lane operation (USB 3.2 Gen ×ばつ2, USB 3.2 Gen ×ばつ2, USB4 ×ばつ2, USB4 ×ばつ2, USB Gen 4 Asymmetric).^[36]

USB-C devices support power currents of 1.5 A and 3.0 A over the 5 V power bus in addition to baseline 900 mA. These higher currents can be negotiated through the configuration line. Devices can also use the full Power Delivery specification using both BMC-coded configuration line and the legacy BFSK-coded V_BUS line.^{[36] : §4.6.2.1}

USB plugs fit one receptacle with notable exceptions for USB On-The-Go "AB" support and the general backward compatibility of USB 3.0 as shown.

USB connector mating matrix (images not to scale)

Plug Receptacle	Standard‐A	Standard‐A SuperSpeed	Standard‐B	Standard‐B SuperSpeed	Mini‐A	Mini‐B	Micro‐A†	Micro‐B	Micro‐B SuperSpeed	C
Standard‐A	Yes	Only non- SuperSpeed	No	No	No	No	No	No	No	No
Standard‐A SuperSpeed	Only non- SuperSpeed	Yes	No	No	No	No	No	No	No	No
Standard‐B	No	No	Yes	No	No	No	No	No	No	No
Standard‐B SuperSpeed	No	No	Only non- SuperSpeed	Yes	No	No	No	No	No	No
Mini‐A	No	No	No	No	Yes	No	No	No	No	No
Mini‐B	No	No	No	No	No	Yes	No	No	No	No
Mini‐AB	No	No	No	No	Deprecated	Deprecated	No	No	No	No
Micro‐B	No	No	No	No	No	No	No	Yes	No	No
Micro‐AB	No	No	No	No	No	No	Yes	Yes	No	No
Micro‐B SuperSpeed	No	No	No	No	No	No	No	Only non- SuperSpeed	Yes	No
C	No	No	No	No	No	No	No	No	No	Yes
^† Mates with Micro‐AB receptacle. No Micro‐A–only receptacle was ever designed.

USB cables

Plugs, each end	Standard‐A	Mini‐A	Micro‐A	Standard‐B	Mini‐B	Micro‐B	Micro‐B	C
Standard‐A	Prohibited, hazardous[37] [1]	Prohibited, hazardous[1]	Prohibited, hazardous[1]	Yes	Yes	Yes	Yes	Yes
Mini‐A	Prohibited, hazardous[1]	No	No	Deprecated	Deprecated	Non- standard	No	No
Micro‐A	Prohibited, hazardous[1]	No	No	Non- standard	Non- standard	Yes	No	No
Standard‐B	Yes	Deprecated	Non- standard	No	No	No	No	Yes
Mini‐B	Yes	Deprecated	Non- standard	No	OTG non- standard	OTG non- standard	No	Yes
Micro‐B	Yes	Non- standard	Yes	No	OTG non- standard	OTG non- standard	No	Yes
Micro‐B	Yes	No	No	No	No	No	OTG non- standard	Yes
C	Yes	No	No	Yes	Yes	Yes	Yes	Yes

  Prohibited, hazardous

Not inter-operable with USB-IF–compliant equipment and possibly damaging to both devices when connected. A special Standard-A-to-Standard-A cable omitting the power (V_BUS) and legacy data (D−, D+) conductors is allowed for operating system debugging and other host-to-host connection applications; this exception, while safe, has no common application.^{[28] [37]} Also there are valid A-to-A assemblies, referred to loosely as cables (such as the Easy Transfer Cable), which are actually not simply cables but active peripheral devices: In USB terms, such a product is two peripherals, each one seen by one of the hosts to which the "cable" is connected.

  Non-standard

The USB standards do not exhaustively list all combinations with one Type‐A and one Type‐B connector, however, most such cables have good chances of working.

  OTG non-standard

Commonly available cables that address the problem of defectively designed devices that can function as USB hosts except that they have Micro‐B and Mini‐B receptacles instead of -AB receptacles, e.g. some smartphones. While not compliant with the USB standards, these cables at least do not pose a risk of physical damage since Type‐B ports are unpowered by default.^[38]

  Deprecated

Some older devices and cables with Mini‐A connectors have been certified by USB-IF. The Mini‐A connector is obsolete: no new Mini‐A connectors and neither Mini‐A nor Mini‐AB receptacles will be certified.^[13] Note: Mini‐B is not deprecated, although it is less and less used since the arrival of Micro‐B. Micro‐A and Micro‐B have one more contact than Standard‐A and Standard‐B in order for hardware with a Micro‐AB receptacle to discern Micro‐A from Micro‐B and behave as a host or peripheral accordingly.

In addition to the above cable assemblies comprising two plugs, receptacles are allowed in three adapter assemblies:

• Two legacy adapter assemblies for compatibility with equipment that predates USB‐C:
  • USB 3.1 Standard‐A receptacle to Type‐C plug, to connect a legacy Standard-A plug to a modern Type‐C receptacle^[39]
  • USB 2.0 Micro‐B receptacle to Type-C plug, to connect a legacy Micro-B plug to a modern Type‐C receptacle^[39]
• One older adapter, itself designated legacy, predating USB‐C: Standard-A receptacle to Micro‐A plug, giving a compact On-The-Go device, such as a camera or smartphone, a Standard-A port for connecting peripherals, such as printers and mass storage devices. That is, to connect a Standard‐A plug to a Micro‐AB receptacle.^{[40] [9]} (All USB connectors except Type‐C were designated legacy in 2014.^[39] )

Manufacturers of personal electronic devices might not include a USB standard connector on their product for technical or marketing reasons.^[41] E.g. Olympus has been using a special cable called CB-USB8 one end of which has a special contact. Some manufacturers provide proprietary cables, such as Lightning, that permit their devices to physically connect to a USB standard port. Full functionality of proprietary ports and cables with USB standard ports is not assured; for example, some devices only use the USB connection for battery charging and do not implement any data transfer functions.^[42]

The D± signals used by low, full, and high speed are carried over a twisted pair (typically unshielded) to reduce noise and crosstalk. SuperSpeed uses separate transmit and receive differential pairs, which additionally require shielding (typically, shielded twisted pair but twinax is also mentioned by the specification). Thus, to support SuperSpeed data transmission, cables contain twice as many wires and are larger in diameter.^[43]

The USB 1.1 standard specifies that a standard cable can have a maximum length of 5 metres (16 ft 5 in) with devices operating at full speed (12 Mbit/s), and a maximum length of 3 metres (9 ft 10 in) with devices operating at low speed (1.5 Mbit/s).^{[44] [45] [46]}

USB 2.0 provides for a maximum cable length of 5 metres (16 ft 5 in) for devices running at high speed (480 Mbit/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.^[46] The USB 2.0 specification requires that cable delay be less than 5.2 ns/m (1.6 ns/ft, 192000 km/s), which is close to the maximum achievable transmission speed for standard copper wire.

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 ft 10 in).^[47]

Downstream USB connectors supply power at a nominal 5 V DC via the V_BUS pin to upstream USB devices.

The tolerance on V_BUS at an upstream (or host) connector was originally ±5% (i.e. could lie anywhere in the range 4.75 V to 5.25 V). With the release of the USB Type-C specification in 2014 and its 3 A power capability, the USB-IF elected to increase the upper voltage limit to 5.5 V to combat voltage droop at higher currents.^[48] The USB 2.0 specification (and therefore implicitly also the USB 3.x specifications) was also updated to reflect this change at that time.^[49] A number of extensions to the USB Specifications have progressively further increased the maximum allowable V_BUS voltage: starting with 6.0 V with USB BC 1.2,^[50] to 21.5 V with USB PD 2.0^[51] and 50.9 V with USB PD 3.1,^[51] while still maintaining backwards compatibility with USB 2.0 by requiring various forms of handshake before increasing the nominal voltage above 5 V.

USB PD continues the use of the bilateral 5% tolerance, with allowable voltages of PDO ±5% ±0.5 V (eg. for a PDO of 9.0 V, the maximum and minimum limits are 9.95 V and 8.05 V, respectively).^[51]

There are several minimum allowable voltages defined at different locations within a chain of connectors, hubs, and cables between an upstream host (providing the power) and a downstream device (consuming the power). To allow for voltage drops, the voltage at the host port, hub port, and device are specified to be at least 4.75 V, 4.4 V, and 4.35 V respectively by USB 2.0 for low-power devices,^[a] but must be at least 4.75 V at all locations for high-power^[b] devices (however, high-power devices are required to operate as a low-powered device so that they may be detected and enumerated if connected to a low-power upstream port). The USB 3.x specifications require that all devices must operate down to 4.00 V at the device port.

Unlike USB 2.0 and USB 3.2, USB4 does not define its own VBUS-based power model. Power for USB4 operation is established and managed as defined in the USB Type-C Specification and the USB PD Specification.

USB power standards

Specification	Current (max.)	Voltage	Power (max.)
Low-power device	100 mA	5 V	0.50 W
Low-power SuperSpeed (USB 3.0) device	150 mA	5 V	0.75 W
High-power device	500 mA[a]	5 V	2.5 W
High-power SuperSpeed (USB 3.0) device	900 mA[b]	5 V	4.5 W
Battery Charging (BC) 1.2	1.5 A	5 V	7.5 W
Single-lane SuperSpeed+ (USB 3.2 Gen2x1, and former USB 3.1 Gen2) device	1.5 A[c]	5 V	7.5 W
Power Delivery 3.0 SPR	3 A	5 V	15 W
Power Delivery 3.0 SPR	3 A	9 V	27 W
Power Delivery 3.0 SPR	3 A	15 V	45 W
Power Delivery 3.0 SPR	3 A	20 V	60 W
Power Delivery 3.0 SPR Type-C	5 A[d]	20 V	100 W
Power Delivery 3.1 EPR Type-C	5 A[d]	28 V[e]	140 W
Power Delivery 3.1 EPR Type-C	5 A[d]	36 V[e]	180 W
Power Delivery 3.1 EPR Type-C	5 A[d]	48 V[e]	240 W
^ Up to 5 unit loads; with non-SuperSpeed devices, one unit load is 100 mA. ^ Up to 6 unit loads; with SuperSpeed devices, one unit load is 150 mA. ^ Up to 6 unit loads; with multi-lane devices, one unit load is 250 mA. ^ a b c d >3 A (>60 W) operation requires an electronically marked cable rated at 5 A. ^ a b c >20 V (>100 W) operation requires an electronically marked Extended Power Range (EPR) cable.

The limit to device power draw is stated in terms of a unit load which is 100 mA for USB 2.0, or 150 mA for SuperSpeed (i.e. USB 3.x) devices. Low-power devices may draw at most 1 unit load, and all devices must act as low-power devices before they are configured. A high-powered device must be configured, after which it may draw up to 5 unit loads (500 mA), or 6 unit loads (900 mA) for SuperSpeed devices, as specified in its configuration because the maximum power may not always be available from the upstream port.^{[52] [53] [54] [55]}

A bus-powered hub is a high-power device providing low-power ports. It draws 1 unit load for the hub controller and 1 unit load for each of at most 4 ports. The hub may also have some non-removable functions in place of ports. A self-powered hub is a device that provides high-power ports by supplementing the power supply from the host with its own external supply. Optionally, the hub controller may draw power for its operation as a low-power device, but all high-power ports must draw from the hub's self-power.

Where devices (for example, high-speed disk drives) require more power than a high-power device can draw,^[56] they function erratically, if at all, from bus power of a single port. USB provides for these devices as being self-powered. However, such devices may come with a Y-shaped cable that has two USB plugs (one for power and data, the other for only power), so as to draw power as two devices.^[57] Such a cable is non-standard, with the specification stating that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered."^[58]

USB Battery Charging (BC) defines a charging port, which may be a charging downstream port (CDP), with data, or a dedicated charging port (DCP), without data. Dedicated charging ports can be found on USB power adapters to run attached devices and battery packs. Charging ports on a host with both kinds will be labeled.^[59]

The charging device identifies a charging port by non-data signaling on the D+ and D− terminals. A dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals.^{[59] : §1.4.7; table 5-3}

Per the base specification, any device attached to a standard downstream port (SDP) must initially be a low-power device, with high-power mode contingent on later USB configuration by the host. Charging ports, however, can immediately supply between 0.5 and 1.5 A of current. The charging port must not apply current limiting below 0.5 A, and must not shut down below 1.5 A or before the voltage drops to 2 V.^[59]

Since these currents are larger than in the original standard, the extra voltage drop in the cable reduces noise margins, causing problems with High Speed signaling. Battery Charging Specification 1.1 specifies that charging devices must dynamically limit bus power current draw during High Speed signaling;^[60] 1.2 specifies that charging devices and ports must be designed to tolerate the higher ground voltage difference in High Speed signaling.

Revision 1.2 of the specification was released in 2010. It made several changes and increased limits, including allowing 1.5 A on charging downstream ports for unconfigured devices—allowing High Speed communication while having a current up to 1.5 A. Also, support was removed for charging-port detection via resistive mechanisms.^[61]

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 (sometimes also called "Apple Brick ID"). When D+ = D− = 2.0 V, the device may pull up to 900 mA. When D+ = 2.0 V and D− = 2.8 V, the device may pull up to 1 A of current.^[62] When D+ = 2.8 V and D− = 2.0 V, the device may pull up to 2 A of current.^[63]

A USB On-The-Go (OTG) device has a single Micro-AB port (or, formerly, a Mini-AB port) for charging as well as for connecting either to a host or to peripheral devices. An Accessory Charger Adapter (ACA) allows simultaneous connection to a charger and either to a host or to peripheral devices, with the charger providing power to both the OTG device and any connected peripheral devices. For example, a keyboard can connect to a smartphone, or a printer, a keyboard, and a flash drive can connect to a smartphone through a USB hub, with the ACA capable of charging the smartphone and powering the keyboard, flash drive, and hub; or the smartphone can connect to a computer (host) that does not provide full power for charging, while the ACA provides full charging power.

An Accessory Charger Adapter has three ports: OTG, Charger, and Accessory. The OTG port connects to the On-The-Go device through a permanently-attached (captive) cable with a (mechanically) Micro-A plug. The Charger port is visibly marked Charger Only and does not support USB communication with the OTG device. It is either a Micro-B receptacle or a captive cable; such a captive cable either has a Standard-A plug or is permanently attached to a charger. The Accessory port is either a Micro-AB or Standard-A receptacle. An A receptacle by definition can only connect to peripheral devices; the Micro-AB receptacle can be used to connect either a host or peripheral devices. The captive plug of the OTG port is unusual in that, unlike a normal Micro-A plug, which is not only mechanically identifiable as an A plug but also electrically marked as such (causing an OTG device to behave as a host), the Micro-A plug of the Accessory Charger Adapter electrically becomes B when a Micro-B plug is connected to the (Micro-AB) Accessory port, causing the OTG device to behave as a peripheral.^{[59] : §6}

USB PD Rev. 1.0 source profiles^[64]

Profile	+5 V	+12 V	+20 V
0	Reserved
1	3.0 A, 15 W[a]	—	—
2		1.5 A, 18 W
3		3.0 A, 36 W
4			3.0 A, 60 W
5		5.0 A, 60 W	5.0 A, 100 W
^ Default start-up profile

USB Power Delivery rev. 2.0/3.x power rules

Power	Minimum USB‐C cable required	Voltage	Current
≤ 15 W	Any[A] [65] [66] [67]	5 V	≤ 3.0 A
≤ 27 W		9 V
≤ 45 W		15 V
≤ 60 W		20 V
≤ 100 W	5 A, or 100 W[B]	20 V	≤ 5.0 A
≤ 140 W[C]	240 W[B] [D] [67]	28 V	≤ 5.0 A
≤ 180 W[C]		36 V
≤ 240 W[C]		48 V
^ 60 W label required on both plug bodies by current standard, not required on older cables ^ a b Electronically marked ^ a b c USB PD Extended Power Range ^ 240 W label required on both plug bodies

In July 2012, the USB Promoters Group announced the finalization of the USB Power Delivery (USB-PD) specification (USB PD rev. 1), an extension that specifies using certified PD aware USB cables with standard USB Type-A and Type-B connectors to deliver increased power (more than 7.5 W maximum allowed by the previous USB Battery Charging specification) to devices with greater power demands. (USB-PD A and B plugs have a mechanical mark while Micro plugs have a resistor or capacitor attached to the ID pin indicating the cable capability.) USB-PD Devices can request higher currents and supply voltages from compliant hosts—up to 2 A at 5 V (for a power consumption of up to 10 W), and optionally up to 3 A or 5 A at either 12 V (36 W or 60 W) or 20 V (60 W or 100 W).^[68] In all cases, both host-to-device and device-to-host configurations are supported.^[69]

The intent is to permit uniformly charging laptops, tablets, USB-powered disks and similarly higher-power consumer electronics, as a natural extension of existing European and Chinese mobile telephone charging standards. This may also affect the way electric power used for small devices is transmitted and used in both residential and public buildings.^{[70] [64]} The standard is designed to coexist with the previous USB Battery Charging specification.^[71]

The first Power Delivery specification (Rev. 1.0) defined six fixed power profiles for the power sources. PD-aware devices implement a flexible power management scheme by interfacing with the power source through a bidirectional data channel and requesting a certain level of electrical power, variable up to 5 A and 20 V depending on supported profile. The power configuration protocol can use BMC coding over the configuration channel (CC) wire if one is present, or a 24 MHz BFSK-coded transmission channel on the V_BUS line.^[64]

The USB Power Delivery specification revision 2.0 (USB PD Rev. 2.0) has been released as part of the USB 3.1 suite.^{[65] [72] [73]} It covers the USB-C cable and connector with a separate configuration channel, which now hosts a DC coupled low-frequency BMC-coded data channel that reduces the possibilities for RF interference.^[74] Power Delivery protocols have been updated to facilitate USB-C features such as cable ID function, Alternate Mode negotiation, increased V_BUS currents, and V_CONN-powered accessories.

As of USB Power Delivery specification revision 2.0, version 1.2, the six fixed power profiles for power sources have been deprecated.^[75] USB PD Power Rules replace power profiles, defining four normative voltage levels at 5 V, 9 V, 15 V, and 20 V. Instead of six fixed profiles, power supplies may support any maximum source output power from 0.5 W to 100 W.

The USB Power Delivery specification revision 3.0 defines an optional Programmable Power Supply (PPS) protocol that allows granular control over V_BUS output, allowing a voltage range of 3.3 to 21 V in 20 mV steps, and a current specified in 50 mA steps, to facilitate constant-voltage and constant-current charging. Revision 3.0 also adds extended configuration messages and fast role swap and deprecates the BFSK protocol.^{[66] : Table 6.26 [76] [77]}

On January 8, 2018, USB-IF announced the Certified USB Fast Charger logo for chargers that use the Programmable Power Supply (PPS) protocol from the USB Power Delivery 3.0 specification.^[78]

In May 2021, the USB PD promoter group launched revision 3.1 of the specification.^[67] Revision 3.1 adds Extended Power Range (EPR) mode which allows higher voltages of 28, 36, and 48 V, providing up to 240 W of power (48 V at 5 A), and the "Adjustable Voltage Supply" (AVS) protocol which allows specifying the voltage from a range of 15 to 48 V in 100 mV steps.^{[79] [80]} Higher voltages require electronically marked EPR cables that support 5 A operation and incorporate mechanical improvements required by the USB Type-C standard rev. 2.1; existing power modes are retroactively renamed Standard Power Range (SPR). In October 2021 Apple introduced a 140 W (28 V 5 A) GaN USB PD charger with new MacBooks,^[81] and in June 2023 Framework introduced a 180 W (36 V 5 A) GaN USB PD charger with the Framework 16.^[82]

In October 2023, the USB PD promoter group launched revision 3.2 of the specification. The AVS protocol now works with the old standard power range (SPR), down to a minimum of 9 V.^{[83] : §10.2.2}

Prior to Power Delivery, mobile phone vendors used custom protocols to exceed the 7.5 W cap on the USB Battery Charging Specification (BCS). For example, Qualcomm's Quick Charge 2.0 is able to deliver 18 W at a higher voltage, and VOOC delivers 20 W at the normal 5 V.^[84] Some of these technologies, such as Quick Charge 4, eventually became compatible with USB PD again.^[85]

As of 2024 mainstream USB PD charging controllers support up to 100 W through a single port, with a few up to 140 W^{[86] [87]} and custom built up to 180 W.^[88]

Sleep-and-charge USB ports can be used to charge electronic devices even when the computer that hosts the ports is switched off. Normally, when a computer is powered off the USB ports are powered down. This feature has also been implemented on some laptop docking stations allowing device charging even when no laptop is present.^[89] On laptops, charging devices from the USB port when it is not being powered from AC drains the laptop battery; most laptops have a facility to stop charging if their own battery charge level gets too low.^[90]

On Dell, HP and Toshiba laptops, sleep-and-charge USB ports are marked with the standard USB symbol with an added lightning bolt or battery icon on the right side.^[91] Dell calls this feature PowerShare,^[92] and it needs to be enabled in the BIOS. Toshiba calls it USB Sleep-and-Charge.^[93] On Acer Inc. and Packard Bell laptops, sleep-and-charge USB ports are marked with a non-standard symbol (the letters USB over a drawing of a battery); the feature is called Power-off USB.^[94] Lenovo calls this feature Always On USB.^[95]

As of 14 June 2007^[update] , all new mobile phones applying for a license in China are required to use a USB port as a power port for battery charging.^{[96] [97]} This was the first standard to use the convention of shorting D+ and D− in the charger.^[98]

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.^{[99] [100]}

The GSM Association (GSMA) followed suit on February 17, 2009,^{[101] [102] [103] [104]} and on April 22, 2009, this was further endorsed by the CTIA – The Wireless Association,^[105] with the International Telecommunication Union (ITU) announcing on October 22, 2009, that it had also embraced the Universal Charging 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."^[106]

In June 2009, the European Commission organized a voluntary Memorandum of Understanding (MoU) to adopt micro-USB as a common standard for charging smartphones marketed in the European Union. The specification was called the common external power supply. The MoU lasted until 2014. The common EPS specification (EN 62684:2010) references the USB Battery Charging Specification and is similar to the GSMA/OMTP and Chinese charging solutions.^{[107] [108]} In January 2011, the International Electrotechnical Commission (IEC) released its version of the (EU's) common EPS standard as IEC 62684:2011.^[109]

In 2022, the Radio Equipment Directive 2022/2380 made USB-C compulsory as a mobile phone charging standard from 2024, and for laptops from 2026.^[110]

A variety of (non-USB) standards support charging devices faster than the USB Battery Charging standard. When a device doesn't recognize the faster-charging standard, generally the device and the charger fall back to the USB battery-charging standard of 5 V at 1.5 A (7.5 W). When a device detects it is plugged into a charger with a compatible faster-charging standard, the device pulls more current or the device tells the charger to increase the voltage or both to increase power (the details vary between standards).^[111]

Such standards include:^{[111] [112]}

• Anker PowerIQ
• Google fast charging
• Huawei SuperCharge
• MediaTek Pump Express
• Motorola TurboPower
• Oppo Super VOOC Flash Charge, are also known as Dash Charge or Warp Charge on OnePlus devices and Dart Charge on Realme devices
• Qualcomm Quick Charge (QC)
• Samsung Adaptive Fast Charging

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 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., the current 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.^[113]

Some non-standard USB devices use the 5 V power supply without participating in a proper USB network, which negotiates power draw with the host interface. Examples include USB-powered keyboard lights, fans, mug coolers and heaters, battery chargers, miniature vacuum cleaners, and even miniature lava lamps. In most cases, these items contain no digital circuitry, and thus are not standard-compliant USB devices. This may cause problems with some computers, such as drawing too much current and damaging circuitry. Prior to the USB 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 then permits the device to switch into high-power mode.

Some devices, when plugged into charging ports, draw even more power (10 watts) than the Battery Charging Specification allows—the iPad is one such device;^[114] it negotiates the current pull with data pin voltages.^[62] Barnes & Noble Nook Color devices also require a special charger that runs at 1.9 A.^[115]

PoweredUSB is a proprietary extension that adds four pins supplying up to 6 A at 5 V, 12 V, or 24 V. It is commonly used in point of sale systems to power peripherals such as barcode readers, credit card terminals, and printers.

• USB adapter
• USB communications

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