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nd mice usually one of each to a usb port also exist 52 these devices present two hid endpoints to the system and use a microcontroller to perform bidirectional data translation between the two standards device firmware upgrade mechanism edit device firmware upgrade dfu is a vendor and device independent mechanism for upgrading the firmware of usb devices with improved versions provided by their manufacturers offering for example a way to deploy firmware bug fixes during the firmware upgrade operation usb devices change their operating mode effectively becoming a prom programmer any class of usb device can implement this capability by following the official dfu specifications 49 53 54 dfu can also give the user the freedom to flash usb devices with alternative firmware one consequence of this is that usb devices after being re flashed may act as various unexpected device types for example a usb device that the seller intends to be just a flash drive can spoof an input device like a keyboard see badusb 55 audio streaming edit the usb device working group has laid out specifications for audio streaming and specific standards have been developed and implemented for audio class uses such as microphones speakers headsets telephones musical instruments etc the working group has published three versions of audio device specifications 56 57 audio 1 0 2 0 and 3 0 referred to as uac 58 or adc 59 uac 3 0 primarily introduces improvements for portable devices such as reduced power usage by bursting the data and staying in low power mode more often and power domains for different components of the device allowing them to be shut down when not in use 60 uac 2 0 introduced support for high speed usb in addition to full speed allowing greater bandwidth for multi channel interfaces higher sample rates 61 lower inherent latency 62 58 and 8 improvement in timing resolution in synchronous and adaptive modes 58 uac2 also introduced the concept of clock domains which provides information to the host about which input and output terminals derive their clocks from the same source as well as improved support for audio encodings like dsd audio effects channel clustering user controls and device descriptions 58 63 uac 1 0 devices are still common however due to their cross platform driverless compatibility 61 and also partly due to microsoft s failure to implement uac 2 0 for over a decade after its publication having finally added support to windows 10 through the creators update on 20 march 2017 64 65 63 uac 2 0 is also supported by macos ios and linux 58 however android also only implements a subset of uac 1 0 66 usb provides three isochronous fixed bandwidth synchronization types 67 all of which are used by audio devices 68 asynchronous the adc or dac are not synced to the host computer s clock at all operating off a free running clock local to the device synchronous the device s clock is synced to the usb start of frame sof or bus interval signals for instance this can require syncing an 11 2896 mhz clock to a 1 khz sof signal a large frequency multiplication 69 70 adaptive the device s clock is synced to the amount of data sent per frame by the host 71 while the usb spec originally described asynchronous mode being used in low cost speakers and adaptive mode in high end digital speakers 72 the opposite perception exists in the hi fi world where asynchronous mode is advertised as a feature and adaptive synchronous modes have a bad reputation 73 74 66 in reality all the types can be high quality or low quality depending on the quality of their engineering and the application 70 58 75 asynchronous has the benefit of being untied from the computer s clock but the disadvantage of requiring sample rate conversion when combining multiple sources connectors edit main article usb hardware connectors 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 female connector mounted on the host or device is called the receptacle and the male connector attached to the cable is called the plug 23 2 5 2 6 the official usb specification documents also periodically define the term male to represent the plug and female to represent the receptacle 76 the standard usb type a plug this is one of many types of usb connector the design is intended to make it 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 23 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 some 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 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 cabling edit main article usb hardware cabling a variety of usb cables for sale in hong kong the usb 1 1 standard specifies that a standard cable can have a maximum length of 5 meters 16 ft 5 in with devices operating at full speed 12 mbit s and a maximum length of 3 meters 9 ft 10 in with devices operating at low speed 1 5 mbit s 77 78 79 usb 2 0 provides for a maximum cable length of 5 meters 16 ft 5 in for devices running at high speed 480 mbit s 79 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 meters 9 ft 10 in 80 usb bridge cables edit usb bridge cables or data transfer cables can be found within the market offering direct pc to pc connections a bridge cable is a special cable with a chip and active electronics in the middle of the cable the chip in the middle of the cable acts as a peripheral to both computers and allows for peer to peer communication between the computers the usb bridge cables are used to transfer files between two computers via their usb ports popularized by microsoft as windows easy transfer the microsoft utility used a special usb bridge cable to transfer personal files and settings from a computer running an earlier version of windows to a computer running a newer version in the context of the use of windows easy transfer software the bridge cable can sometimes be referenced as easy transfer cable many usb bridge data transfer cables are still usb 2 0 but there are also a number of usb 3 0 transfer cables despite usb 3 0 being 10 times faster than usb 2 0 usb 3 0 transfer cables are only 2 3 times faster given their design the usb 3 0 specification introduced an a to a cross over cable without power for connecting two pcs these are not meant for data transfer but are aimed at diagnostic uses dual role usb connections edit usb bridge cables have become less important with usb dual role device capabilities introduced with the usb 3 1 specification under the most recent specifications usb supports most scenarios connecting systems directly with a type c cable for the capability to work however connected systems must support role switching dual role capabilities requires there be two controllers within the system as well as a role controller while this can be expected in a mobile platform such as a tablet or a phone desktop pcs and laptops often will not support dual roles 81 power edit main article usb hardware power upstream usb connectors supply power at a nominal 5v dc via the v_bus pin to downstream usb devices low power and high power devices edit low power devices may draw at most 1 unit load and all devices must act as low power devices when starting out as unconfigured 1 unit load is 100 ma for usb devices up to usb 2 0 while usb 3 0 defines a unit load as 150 ma high power devices such as a typical 2 5 inch usb hard disk drive draw at least 1 unit load and at most 5 unit loads 5x100ma 500 ma for devices up to usb 2 0 or 6 unit loads 6x150ma 900 ma for superspeed usb 3 0 and up devices usb power standards specification current voltage power max low power device 100 ma 5 v a 0 50 w low power superspeed usb 3 0 device 150 ma 5 v a 0 75 w high power device 500 ma b 5 v 2 5 w high power superspeed usb 3 0 device 900 ma c 5 v 4 5 w multi lane superspeed usb 3 2 gen 2 device 1 5 a d 5 v 7 5 w battery charging bc 1 1 1 5 a 5 v 7 5 w battery charging bc 1 2 1 5 a 5 v 7 5 w usb c 1 5 a 5 v 7 5 w 3 a 5 v 15 w power delivery 1 0 2 0 3 0 type c 5 a e 20 v 100 w power delivery 3 1 type c 5 a e 48 v f 240 w a b the v bus supply from a low powered hub port may drop to 4 40 v up to five unit loads with non superspeed devices one unit load is 100 ma up to six unit loads with superspeed devices one unit load is 150 ma up to six unit loads with multi lane devices one unit load is 250 ma a b 3 a 60 w operation requires an electronically marked cable rated at 5 a 20 v 100 w operation requires an electronically marked extended power range epr cable to recognize battery charging mode a dedicated charging port places a resistance not exceeding 200 ω across the d and d terminals shorted or near shorted data lanes with less than 200 ω of resistance across the d and d terminals signify a dedicated charging port dcp with indefinite charging rates 82 83 in addition to standard usb there is a proprietary high powered system known as poweredusb developed in the 1990s and mainly used in point of sale terminals such as cash registers signaling edit main article usb communications signaling usb phy usb signals are transmitted using differential signaling on a twisted pair data wires with 90 ω 15 characteristic impedance 84 usb 2 0 and earlier specifications define a single pair in half duplex hdx usb 3 0 and later specifications define one pair for usb 2 0 compatibility and two or four pairs for data transfer two pairs in full duplex fdx for single lane variants requires superspeed connectors four pairs in full duplex for dual lane 2 variants requires usb c connector rate name old name first publication standard encoding data pairs nominal rate usb if marketing name 85 86 87 logo low speed does not appear usb 1 0 nrzi 1 hdx 1 5 mbit s basic speed usb full speed 12 mbit s high speed usb 2 0 480 mbit s hi speed usb usb 3 2 gen 1 1 usb 3 0 usb 3 1 gen 1 usb 3 0 8b 10b 2 fdx 5 gbit s superspeed usb 5gbps usb 3 2 gen 2 1 usb 3 1 gen 2 usb 3 1 128b 132b 2 fdx 10 gbit s superspeed usb 10gbps usb 3 2 gen 1 2 does not appear usb 3 2 8b 10b 4 fdx 2 10 gbit s usb 3 2 gen 2 2 128b 132b 4 fdx 2 20 gbit s superspeed usb 20gbps usb4 gen 2 1 usb4 64b 66b a 2 fdx 10 gbit s usb4 gen 2 2 64b 66b a 4 fdx 2 20 gbit s usb4 20gbps usb4 gen 3 1 128b 132b a 2 fdx 20 gbit s usb4 gen 3 2 128b 132b a 4 fdx 2 40 gbit s usb4 40gbps a b c d usb4 can use optional reed solomon forward error correction rs fec in this mode 12 16 b 128 bit symbols are assembled together with 2 b 12 bit 4 bit reserved synchronisation bits indicating the respective symbol types and 4 b of rs fec to allow to correct up to 1 b of errors anywhere in the total 198 b block low speed ls and full speed fs modes use a single data pair labelled d and d in half duplex transmitted signal levels are 0 0 0 3 v for logical low and 2 8 3 6 v for logical high level the signal lines are not terminated high speed hs mode uses the same wire pair but with different electrical conventions lower signal voltages of 10 to 10 mv for low and 360 to 440 mv for logical high level and termination of 45 ω to ground or 90 ω differential to match the data cable impedance superspeed ss adds two additional pairs of shielded twisted wire and new mostly compatible expanded connectors these are dedicated to full duplex superspeed operation the superspeed link operates independently from usb 2 0 channel and takes a precedence on connection link configuration is performed using lfps low frequency periodic signalling approximately at 20 mhz frequency and electrical features include voltage de emphasis at transmitter side and adaptive linear equalization on receiver side to combat electrical losses in transmission lines and thus the link introduces the concept of link training superspeed ss uses increased data rate gen 2 1 mode and or the additional lane in the usb c connector gen 1 2 and gen 2 2 mode 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 protocol layer edit main article usb communications protocol layer 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 transactions edit main article usb communications transaction the basic transactions of usb are out transaction in transaction setup transaction control transfer exchange related standards edit the wireless usb logo the usb implementers forum introduced the media agnostic usb v 1 0 wireless communication standard based on the usb protocol on july 29 2015 wireless usb is a cable replacement technology and uses ultra wideband wireless technology for data rates of up to 480 mbit s 88 the usb if used wigig serial extension v1 2 specification as its initial foundation for the ma usb specification and is compliant with superspeed usb 3 0 and 3 1 and hi speed usb usb 2 0 devices that uses ma usb will be branded as powered by ma usb provided the product qualifies its certification program 89 interchip usb is a chip to chip variant that eliminates the conventional transceivers found in normal usb the hsic physical layer uses about 50 less power and 75 less board area compared to usb 2 0 90 comparisons with other connection methods edit ieee 1394 edit at first usb was considered a complement to ieee 1394 firewire technology which was designed as a high bandwidth serial bus that efficiently interconnects peripherals such as disk drives audio interfaces and video equ...