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USB Micro-B OTG adapter for connecting a full-size A-type plug into a smartphone or tablet

USB On-The-Go (USB OTG) is a specification that allows certain USB devices, such as tablets or smartphones, to function either as a host or a peripheral. This enables them to connect directly to other USB devices, such as flash drives, digital cameras, mice or keyboards. USB OTG was first introduced in late 2001.

Unlike standard USB connections, which involve a fixed host (such as a computer) and a peripheral (such as a keyboard), USB OTG allows a device to switch between these roles. For example, a smartphone can act as a host when reading files from a flash drive, but function as a peripheral when connected to a computer.

USB OTG defines two device roles: the A-device, which supplies power and initially acts as the host, and the B-device, which consumes power and begins as the peripheral. These roles can be reversed using the Host Negotiation Protocol (HNP). The initial role is determined by the wiring of a specific pin, known as the ID pin, in the USB connector.[1] The A/B naming convention reflects earlier USB connector types: Type-A connectors were used with host devices, while Type-B connectors were used with peripherals.

In September 2019 USB Implementers Forum has stopped certifying new USB OTG products because of Introduction of USB-C standard.[2]

Overview

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A USB OTG setup involving a number of devices

Standard USB uses a host/device architecture; a host acts as the Host device for the entire bus, and a USB device acts as a Peripheral. If implementing standard USB, devices must assume one role or the other, with computers generally set up as hosts, while (for example) printers normally function as a Peripheral. In the absence of USB OTG, cell phones often implemented Peripheral functionality to allow easy transfer of data to and from computers. Such phones could not readily be connected to printers as they also implemented the peripheral role. USB OTG directly addresses this issue.[1]

When a device is plugged into the USB bus, the host device sets up communications with the device and handles service provisioning (the host's software enables or does the needed data-handling such as file managing or other desired kind of data communication or function). That allows the devices to be greatly simplified compared to the host; for example, a mouse contains very little logic and relies on the host to do almost all of the work. The host controls all data transfers over the bus, with the devices capable only of signalling (when polled) that they require attention. To transfer data between two devices, for example from a phone to a printer, the host first reads the data from one device, then writes it to the other.[citation needed]

While the host-device arrangement works for some devices, many devices can act either as host or as device depending on what else shares the bus. For instance, a computer printer is normally a device, but when a USB flash drive containing images is plugged into the printer's USB port with no computer present (or at least turned off), it would be useful for the printer to take on the role of host, allowing it to communicate with the flash drive directly and to print images from it.[citation needed]

USB OTG recognizes that a device can perform both Host and Peripheral roles, and so subtly changes the terminology. With OTG, a device can be either a host when acting as a link host, or a link peripheral. The choice between host and peripheral roles is handled entirely by which end of the cable the device is connected to. The device connected to the "A" end of the cable at start-up, known as the "A-device", acts as the default host, while the "B" end acts as the default peripheral, known as the "B-device".[citation needed]

After initial startup, setup for the bus operates as it does with the normal USB standard, with the A-device setting up the B-device and managing all communications. However, when the same A-device is plugged into another USB system or a dedicated host becomes available, it can become a device.[citation needed]

USB OTG does not preclude using a USB hub, but it describes host-peripheral role swapping only for the case of a one-to-one connection where two OTG devices are directly connected. Role swapping does not work through a standard hub, as one device will act as a host and the other as a peripheral until they are disconnected.[citation needed]

Specifications

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USB OTG is a part of a supplement[3] to the Universal Serial Bus (USB) 2.0 specification originally agreed upon in late 2001 and later revised.[4] The latest version of the supplement also defines behavior for an Embedded Host which has targeted abilities and the same USB Standard-A port used by PCs.[citation needed]

SuperSpeed OTG devices, Embedded Hosts and peripherals are supported through the USB OTG and Embedded Host Supplement[5] to the USB 3.0 specification.[citation needed]

Protocols

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The USB OTG and Embedded Host Supplement to the USB 2.0 specification introduced three new communication protocols:

  • Attach Detection Protocol (ADP): Allows an OTG device, embedded host or USB device to determine attachment status in the absence of power on the USB bus, enabling both insertion-based behavior and the capability to display attachment status. It does so by periodically measuring the capacitance on the USB port to determine whether there is another device attached, a dangling cable, or no cable. When a large enough change in capacitance is detected to indicate device attachment, an A-device will provide power to the USB bus and look for device connection. At the same time, a B-device will generate SRP (see below) and wait for the USB bus to become powered.
  • Session Request Protocol (SRP): Allows both communicating devices to control when the link's power session is active; in standard USB, only the host is capable of doing so. That allows fine control over the power consumption, which is very important for battery-operated devices such as cameras and mobile phones. The OTG or embedded host can leave the USB link unpowered until the peripheral (which can be an OTG or standard USB device) requires power. OTG and embedded hosts typically have little battery power to spare, so leaving the USB link unpowered helps in extending the battery runtime.
  • Host Negotiation Protocol (HNP): Allows the two devices to exchange their host/peripheral roles, provided both are OTG dual-role devices. By using HNP for reversing host/peripheral roles, the USB OTG device is capable of acquiring control of data-transfer scheduling. Thus, any OTG device is capable of initiating data-transfer over USB OTG bus. The latest version of the supplement also introduced HNP polling, in which the host device periodically polls the peripheral during an active session to determine whether it wishes to become a host.
  • The main purpose of HNP is to accommodate users who have connected the A and B devices (see below) in the wrong direction for the task they want to perform. For example, a printer is connected as the A-device (host), but cannot function as the host for a particular camera, since it does not understand the camera's representation of print jobs. When that camera knows how to talk to the printer, the printer will use HNP to switch to the device role, with the camera becoming the host so pictures stored on the camera can be printed out without reconnecting the cables. The new OTG protocols cannot pass through a standard USB hub since they are based on electrical signaling via a dedicated wire.

The USB OTG and Embedded Host Supplement to the USB 3.0 specification introduces an additional communication protocol:

  • Role Swap Protocol (RSP): RSP achieves the same purpose as HNP (i.e., role swapping) by extending standard mechanisms provided by the USB 3.0 specification. Products following the USB OTG and Embedded Host Supplement to the USB 3.0 specification are also required to follow the USB 2.0 supplement in order to maintain backward compatibility. SuperSpeed OTG devices (SS-OTG) are required to support RSP. SuperSpeed Peripheral Capable OTG devices (SSPC-OTG) are not required to support RSP since they can only operate at SuperSpeed as a peripheral; they have no SuperSpeed host and so can only role swap using HNP at USB 2.0 data rates.

Device roles

[edit]

USB OTG defines two roles for devices: OTG A-device and OTG B-device, specifying which side supplies power to the link, and which initially is the host. The OTG A-device is a power supplier, and an OTG B-device is a power consumer. In the default link configuration, the A-device acts as a USB host with the B-device acting as a USB peripheral. The host and peripheral modes may be exchanged later by using HNP or RSP. Because every OTG controller supports both roles, they are often called "Dual-Role" controllers rather than "OTG controllers".

For integrated circuit (IC) designers, an attractive feature of USB OTG is the ability to achieve more USB capabilities with fewer gates.

A "traditional" approach includes four controllers, resulting in more gates to test and debug:

  • USB high speed host controller based on EHCI (a register interface)
  • Full/low speed host controller based on OHCI (another register interface)
  • USB device controller, supporting both high and full speeds
  • Fourth controller to switch the OTG root port between host and device controllers

Also, most gadgets must be either a host or a device. OTG hardware design merges all of the controllers into one dual-role controller that is somewhat more complex than an individual device controller.

Targeted peripheral list (TPL)

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A manufacturer's targeted peripheral list (TPL) serves the aim of focusing a host device towards particular products or applications, rather than toward its functioning as a general-purpose host, as is the case for typical PCs. The TPL specifies products supported by the "targeting" host, defining what it needs to support, including the output power, transfer speeds, supported protocols, and device classes. It applies to all targeted hosts, including both OTG devices acting as a host and embedded hosts.

Plug

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Standard, mini, and micro USB plugs (not to scale). The white areas in the drawings represent hollow spaces. As the plugs are shown here, the USB logo (with optional letter A or B) is on the top of the overmold in all cases. Pin numbering (looking into receptacles) is mirrored from plugs, such that pin 1 on plug connects to pin 1 on the receptacle.

OTG mini plugs

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The original USB OTG standard introduced a plug receptacle called mini-AB that was replaced by micro-AB in later revisions (Revision 1.4 onwards). It can accept either a mini-A plug or a mini-B plug, while mini-A adapters allows connection to standard-A USB cables coming from peripherals. The standard OTG cable has a mini-A plug on one end and a mini-B plug on the other end (it can not have two plugs of the same type).

The device with a mini-A plug inserted becomes an OTG A-device, and the device with a mini-B plug inserted becomes a B-device (see above). The type of plug inserted is detected by the state of the ID pin (the mini-A plug's ID pin is grounded, while the mini-B plug's is floating).

Pure mini-A receptacles also exist, used where a compact host port is needed, but OTG is not supported.

OTG micro plugs

[edit]

With the introduction of the USB micro plug, a new plug receptacle called micro-AB was also introduced. It can accept either a micro-A plug or a micro-B plug. Micro-A adapters allow for connection to standard-A plugs, as used on fixed or standard devices. An OTG product must have a single micro-AB receptacle and no other USB receptacles.[6][7]

An OTG cable has a micro-A plug on one end, and a micro-B plug on the other end (it cannot have two plugs of the same type). OTG adds a fifth pin to the standard USB connector, called the ID-pin; the micro-A plug has the ID pin grounded, while the ID in the micro-B plug is floating. A device with a micro-A plug inserted becomes an OTG A-device, and a device with a micro-B plug inserted becomes a B-device. The type of plug inserted is detected by the state of the pin ID.

Three additional ID pin states are defined[6] at the nominal resistance values of 124 kΩ, 68 kΩ, and 36.5 kΩ, with respect to the ground pin. These permit the device to work with USB Accessory Charger Adapters that allows the OTG device to be attached to both a charger and another device simultaneously.[8]

These three states are used in the cases of:

  • A charger and either no device or an A-device that is not asserting VBUS (not providing power) are attached. The OTG device is allowed to charge and initiate SRP but not connect.[8]
  • A charger and an A-device that is asserting VBUS (is providing power) are attached. The OTG device is allowed to charge and connect but not initiate SRP.[8]
  • A charger and a B-device are attached. The OTG device is allowed to charge and enter host mode.[8]

USB 3.0 introduced a backwards compatible SuperSpeed extension of the micro-AB receptacle and micro-A and micro-B plugs. They contain all pins of the non-Superspeed micro connectors and use the ID pin to identify the A-device and B-device roles, also adding the SuperSpeed pins.

OTG micro cables

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USB OTG adapters, hubs and card readers

When an OTG-enabled device is connected to a PC, it uses its own USB-A or USB Type-C cable (typically ending in micro-B, USB-C or Lightning plugs for modern devices). When an OTG-enabled device is attached to a USB device, such as a flash drive, the device must either end in the appropriate connection for the device, or the user must supply an appropriate adapter ending in USB-A. The adapter enables any standard USB peripheral to be attached to an OTG device. Attaching two OTG-enabled devices together requires either an adapter in conjunction with the device's USB-A cable, or an appropriate dual-sided cable and a software implementation to manage it. This is becoming commonplace with USB Type-C devices.

Smartphone and tablet implementation

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BlackBerry 10.2 implements Host Mode (like in the BlackBerry Z30 handset).[9] Nokia has implemented USB OTG in many of their Symbian cellphones such as Nokia N8, C6-01, C7, Oro, E6, E7, X7, 603, 700, 701 and 808 Pureview. Some high-end Android phones produced by HTC, and Sony under Xperia series also have it.[10] Samsung[11][12] Android version 3.1 or newer supports USB OTG, but not on all devices.[13][14]

Specifications listed on technology web sites (such as GSMArena, PDAdb.net, PhoneScoop, and others) can help determine compatibility. Using GSMArena as an example, one would locate the page for a given device, and examine the verbiage under Specifications → Comms → USB. If "USB Host" is shown, the device should be capable of supporting OTG-type external USB accessories.[15][16]

In many of the above implementations, the host device has only a micro-B receptacle rather than a micro-AB receptacle. Although non-standard, micro-B to micro-A receptacle adapters are widely available and used in place of the mandated micro-AB receptacle on these devices.[17]

Backward compatibility

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USB OTG devices are backward-compatible with USB 2.0 (USB 3.0 for SuperSpeed OTG devices) and will behave as standard USB hosts or devices when connected to standard (non-OTG) USB devices. The main exception is that OTG hosts are only required to provide enough power for the products listed on the TPL, which may or may not be enough to connect to a peripheral that is not listed. A powered USB hub may sidestep the issue, if supported, since it will then provide its own power according to either the USB 2.0 or USB 3.0 specifications.

Some incompatibilities in both HNP and SRP were introduced between the 1.3 and 2.0 versions of the OTG supplement, which can lead to interoperability issues when using those protocol versions.

Charger compatibility

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Main articles: USB Battery Charging Specification and USB Power Delivery Specification

Some devices can use their USB ports to charge built-in batteries, while other devices can detect a dedicated charger and draw more than 500 mA (0.5 A), allowing them to charge more rapidly. OTG devices are allowed to use either option.[8]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

USB On-The-Go

View on Grokipedia
from Grokipedia
USB On-The-Go (USB OTG) is a supplement to the Universal Serial Bus (USB) specification that enables portable devices and non-PC hosts to implement targeted host functionality, allowing direct connections to USB peripherals and other OTG devices without requiring a . Introduced in 2001 as an extension to the USB specification, OTG defines dual-role devices (DRDs) capable of operating as either a host or a peripheral, with role switching facilitated by the Host Negotiation Protocol (HNP) to support dynamic connections. These devices typically use a Micro-AB receptacle connector, which allows automatic detection based on the cable's orientation, and support low-speed, full-speed, and high-speed USB operations while incorporating power-saving features to extend battery life in mobile applications. In 2012, the OTG and Embedded Host Supplement for (Revision 1.1) extended these capabilities to SuperSpeed rates up to 5 Gbit/s, introducing the Role Swap Protocol (RSP) for seamless at higher speeds and defining new device classes such as SuperSpeed OTG (SS-OTG) and SuperSpeed Peripheral Capable OTG (SSPC-OTG). Key concepts include the A-device (default host) and B-device (default peripheral) roles, along with mechanisms like the Targeted Peripheral (TPL) to specify supported peripherals, ensuring efficient and standardized in embedded and portable systems.

Overview and History

Definition and Purpose

USB On-The-Go (USB OTG) is a supplement to the USB 2.0 specification (and subsequent versions) that enables portable electronic devices to dynamically switch between host and peripheral roles in a USB connection. Introduced in 2001 by the (USB-IF), it extends the standard USB to support flexible connectivity without relying on a traditional PC as the host. This allows battery-powered devices, such as smartphones and tablets, to initiate and manage directly. The primary purpose of USB OTG is to facilitate the connection and control of USB peripherals by mobile devices, eliminating the need for intermediary hardware like a computer. For instance, a can function as a host to interface with devices like keyboards, flash drives, or digital cameras, enabling data transfer, input, or storage expansion on the go. By supporting this host capability in compact, power-constrained devices, OTG reduces hardware requirements and enhances usability for users in mobile scenarios. Key benefits of USB OTG include improved portability and convenience, as it allows seamless integration of peripherals into everyday mobile workflows without bulky adapters or dedicated hosts. It also enables connections between compatible devices, such as direct between two smartphones, fostering more versatile and efficient device ecosystems. At its core, the operational principle involves a single USB port on each device that can handle both upstream (as a peripheral) and downstream (as a host) connections, with role detection managed through electrical signaling to determine the active host.

Development History

The development of USB On-The-Go (OTG) began in 2001 as a supplement to the USB 2.0 specification, aimed at overcoming limitations in mobile device connectivity by enabling direct peer-to-peer communication between portable devices without requiring a traditional PC host. This initiative was driven by the USB Implementers Forum (USB-IF), which coordinated efforts among industry stakeholders to address the growing demand for flexible USB roles in emerging mobile electronics. The USB OTG 1.0 specification was released in December 2001, marking the formal introduction of features like dual-role device capability, session request protocol, and host negotiation protocol to support on-the-go scenarios. Key input came from device manufacturers such as Nokia and Palm, who contributed to the working group alongside other members including Intel, Microsoft, Motorola, and Texas Instruments, ensuring the standard met the needs of portable computing and communication devices. Subsequent revisions refined these capabilities; for instance, version 1.3, released on December 5, 2006, incorporated errata and clarifications to improve interoperability and compatibility with USB 2.0 ecosystems. As USB technology advanced toward higher speeds, OTG evolved to integrate with SuperSpeed capabilities. The On-The-Go and Embedded Host Supplement to the specification, version 1.0, was issued on July 1, 2011, extending OTG functionality to support up to 5 Gbit/s data rates while maintaining with USB 2.0 OTG devices. However, adoption of these higher-speed OTG implementations lagged due to hardware constraints in early mobile devices, such as limited pin support and challenges. Market dynamics played a pivotal role in OTG's trajectory, with the rise of smartphones in the mid-2000s—exemplified by devices offering internet browsing, , and —creating a strong impetus for direct device interconnectivity. By 2010, as smartphones became ubiquitous, OTG saw widespread implementation in mobile platforms, particularly Android ecosystems, facilitating features like peripheral attachments and file transfers on the go.

Technical Specifications

Standard Versions

The USB On-The-Go (OTG) specification originated as a supplement to the USB 2.0 standard, with its initial version, OTG 1.0, released on December 18, 2001, by the (USB-IF). This version introduced core features for enabling dual-role functionality in portable devices, including the use of an ID pin on Mini-B connectors to detect and switch between host and peripheral roles without requiring additional hardware. It supported low-speed, full-speed, and high-speed USB 2.0 operations at up to 480 Mbps, focusing on power-efficient session management for battery-powered gadgets. In 2006, OTG 1.3 was released on December 5, refining protocols from the prior version while remaining a supplement to USB 2.0. Key additions included enhanced support for the Session Request Protocol (SRP), which allows a peripheral to request the host to initiate a session for power conservation, and improvements to the Host Negotiation Protocol (HNP) for smoother role switching between connected devices. These refinements addressed issues and better managed power states, ensuring more reliable operation in embedded systems. The specification advanced to OTG 2.0 in July 2011, as the On-The-Go and Embedded Host Supplement to the USB 2.0 Specification, maintaining with earlier OTG implementations. This version enhanced dual-role and embedded host features for USB 2.0, emphasizing dynamic role detection and power negotiation, though limited to 480 Mbps due to USB 2.0 constraints. In May 2012, the OTG and Embedded Host Supplement to the Specification (Revision 1.1) was released, extending OTG capabilities to SuperSpeed rates up to 5 Gbit/s. It introduced the Role Swap Protocol (RSP) for role negotiation at SuperSpeed and defined new classes such as SuperSpeed OTG (SS-OTG) and SuperSpeed Peripheral Capable OTG (SSPC-OTG), while supporting with USB 2.0 OTG. Subsequent developments integrated OTG principles into broader USB standards rather than standalone revisions. The USB 3.1 Gen 2 specification, released in July 2013, incorporated enhanced role-switching mechanisms compatible with OTG for SuperSpeed+ operations at 10 Gbps, leveraging USB Type-C connectors for improved flexibility. Similarly, the USB 3.2 specification, finalized in September 2017, extended multi-lane configurations to support OTG-like dual-role ports at up to 20 Gbps, focusing on cable assemblies and protocol efficiency. As of 2025, the USB-IF maintains the OTG specifications without issuing major new standalone revisions, having ceased certification for version 1.3 after December 31, 2011, and prioritizing OTG 2.0 compliance. Development emphasis has shifted to embedded OTG features within the specification, released in August 2019 and updated to version in October 2022, which natively supports asymmetric dual-role ports and tunneling protocols for up to 120 Gbps in USB Type-C ecosystems.

Electrical and Power Specifications

USB On-The-Go (OTG) operates on a standard 5 V bus power supply, with specific voltage thresholds defined for session validity and device operation to ensure compatibility with portable, battery-powered implementations. The VBUS line, which carries the power, must be driven by the A-device (host) to a minimum of 4.4 V under loads up to 100 mA, rising to 4.75 V for higher loads, with an upper limit of 5.25 V; session validity for the A-device is maintained between 0.8 V and 2.0 V, while for the B-device (peripheral), it extends to 4.0 V, ending below 0.8 V. These tolerances accommodate variations in portable devices, including a rise time of no more than 100 ms for VBUS when activating a session under a 10 µF load, and a minimum input impedance of 40 kΩ on unpowered A-devices to prevent excessive leakage. Current limits in OTG prioritize low-power operation to extend battery life, with the A-device required to supply at least 8 mA at 4.4 V to support session detection via the Session Request Protocol (SRP), though full host operation aligns with USB 2.0 standards allowing up to 100 mA for unconfigured peripherals and 500 mA for configured ones. B-devices, when unconfigured, are restricted to drawing no more than 150 µA (averaged over 1 ms) if dual-role capable or 8 mA if peripheral-only, ensuring minimal drain during idle states. Power budgeting occurs through protocol negotiations, where the B-device can initiate a session by pulsing VBUS or data lines to request power without constant supply, and the A-device may discharge VBUS through a minimum 656 Ω resistance to end sessions efficiently; dual-role devices maintain a VBUS decoupling capacitance between 1.0 µF and 6.5 µF to stabilize transients. Core OTG does not support advanced high-power delivery like USB Power Delivery, focusing instead on these constrained profiles. The ID pin facilitates role detection and grounding, with a resistance of less than 10 Ω to ground on Mini-A plugs to signal host mode, and greater than 100 kΩ (typically floating) on Mini-B plugs for peripheral mode, enabling automatic switching without software intervention in many cases. is maintained through requirements like data line pull-down resistors of 14.25 kΩ to 24.8 kΩ and leakage voltages below 0.342 V, with B-devices tolerating VBUS transients up to 400 mV at slew rates under 100 mA/µs to prevent disruption during connections. Battery charging in OTG builds on USB Battery Charging 1.2 support in revisions like OTG , permitting up to 1.5 A when the device acts as a peripheral (B-role), though operation emphasizes low-power modes—such as suspending VBUS after inactivity—to minimize thermal load and efficiency losses during role switches.
ParameterSymbolMinMaxUnitsNotes
VBUS Output (A-Device, 0-100 mA)V_A_VBUS_OUT4.45.25VStandard bus power
VBUS Output (A-Device, >100 mA)V_A_VBUS_OUT4.755.25VConfigured operation
A-Device Session ValidV_A_SESS_VLD0.82.0VFor SRP detection
B-Device Session ValidV_B_SESS_VLD0.84.0VFull peripheral support
A-Device Output CurrentI_A_VBUS_OUT8-mAMinimum for host mode
B-Device Unconfigured Current (Dual-Role)I_B_SRP-150µA1 ms average
ID Pin Resistance (Mini-A)R_ID-10ΩGrounded for host
ID Pin Resistance (Mini-B)R_ID100k-ΩFloating for peripheral

Protocols and Functionality

Communication Protocols

USB On-The-Go (OTG) introduces specific communication protocols that extend the standard USB framework to enable dual-role functionality, allowing devices to dynamically negotiate and switch between host and peripheral roles without requiring a dedicated PC host. These protocols build upon USB packet structures while adding OTG-specific mechanisms for role management and session control, ensuring efficient connection establishment and maintenance in scenarios. The Host Negotiation Protocol (HNP) enables the transfer of the host role from the A-device (initial host) to the B-device (initial peripheral) during an active session, facilitating dynamic role switching without disconnecting the cable. To initiate HNP, the current host suspends the bus and sets the b_hnp_enable feature using a SET_FEATURE request; the peripheral then detects the suspend and signals a disconnect by driving SE0 (single-ended zero) for a short period, after which the former host enables its D+ pull-up within a maximum of 3 ms to assume the peripheral role. The new host must then assert a bus reset within 1 ms to complete the swap, ensuring seamless transition. HNP support is indicated in the OTG descriptor's bmAttributes field, where the hnp_support bit (bit 1) signals capability during . This protocol is limited to high-speed and full-speed operations and requires both devices to support it for successful negotiation. Complementing HNP, the Session Request Protocol (SRP) allows a B-device to request the activation of VBUS power from the A-device, enabling a new communication session without constant power consumption by the peripheral. SRP operates when the bus is idle (SE0 state) and VBUS is below the session valid threshold; the B-device initiates by either pulsing the data line (enabling D+ pull-up for 5-10 ms) or pulsing VBUS (driving it to 2.1-5.25 V with up to 8 mA current). The A-device detects this signal and must respond by turning on VBUS within a maximum of 30 seconds; failure to respond prompts the B-device to retry after a 5-second minimum wait. SRP capability is flagged in the OTG descriptor via the srp_support bit (bit 0), and it supports both data-line and VBUS pulsing methods depending on device hardware. This protocol reduces power draw in battery-operated devices by keeping VBUS off during idle periods. The default connection sequence in OTG relies on the ID pin in the Mini-AB or Micro-AB receptacle to initially assign roles upon cable insertion, providing a plug-and-play mechanism for role detection. When a Mini-A or Micro-A plug is inserted, the ID pin is grounded (resistance <10 Ω), signaling the device to become the A-device (host), which then enables VBUS and its D+ pull-up. Conversely, a Mini-B or Micro-B plug leaves the ID pin open (resistance >100 kΩ), designating the device as the B-device (peripheral), which enables its D- pull-up and waits for VBUS. This resistance-based detection triggers state machines in both devices to enter appropriate idle or peripheral/host modes, initiating enumeration once VBUS is valid. The sequence ensures automatic role assignment based on cable orientation, with no software intervention required at connection time. OTG incorporates robust error handling within these protocols to manage timeouts, resets, and issues, maintaining link reliability in dynamic environments. For SRP, timeouts include a 30-second maximum response window (T_A_SRP_RSPNS) for the A-device and a 5-second failure retry interval (T_B_SRP_FAIL) for the B-device; exceeding these leads to session abandonment and state transitions to idle. HNP errors, such as failure to detect role swap signals, trigger bus resets or disconnects, with the B-device interpreting prolonged SE0 (>3.125 ms) as a reset signal. During , if an OTG device lacks required capabilities (e.g., no HNP support indicated in the descriptor), the host may requests or display user prompts, transitioning to error states like a_wait_bcon_timeout after 100-200 ms without connection activity. These mechanisms use standard USB reset signaling (SE0 for at least 3 ms at full-speed) extended with OTG-specific timers to recover from failures without full reconnection. At the packet level, OTG protocols extend standard USB 2.0 packets with dedicated descriptors and control requests to advertise and manage role support. The OTG descriptor, a 3-byte class-specific configuration descriptor (bDescriptorType=9), includes bmAttributes to denote SRP and HNP capabilities, placed after the interface descriptors during . Key requests include SET_FEATURE with b_hnp_enable (selector 3) to activate HNP on the peripheral and a_hnp_support (selector 4) on the host, both using standard USB control transfers over endpoint zero. These extensions ensure that OTG devices can query and enable dual-role features transparently, with failures handled via responses on unsupported requests. In later revisions like OTG, additional notifications such as Device Notification packets with Role Swap Protocol phases further refine these mechanisms, but the core packet structures remain backward-compatible with USB 2.0.

Role Switching and Device Classes

In USB On-The-Go (OTG), devices assume distinct roles to facilitate dynamic connectivity without dedicated hosts. The A-device serves as the initial host, supplying power to the VBUS line (typically up to 500 mA at 5 V) and managing bus operations, while the B-device operates as the peripheral, drawing power and responding to host commands. Dual-role devices (DRDs) incorporate capabilities for both roles, connecting via a Mini-AB or Micro-AB receptacle that determines the initial role based on the cable's ID pin orientation. Role switching occurs primarily through the Host Negotiation Protocol (HNP), initiated after the initial connection and enumeration. Once the A-device enables HNP via the SetFeature(b_hnp_enable) request and suspends the bus, the B-device can request the host role by disconnecting and reconnecting, prompting the original A-device to transition to peripheral mode within a defined timeout (e.g., 3 ms acknowledgment). This process allows peripherals to temporarily act as hosts for specific tasks, such as data transfer between peer devices, before reverting roles. The Session Request Protocol (SRP) complements HNP by enabling a powered-off B-device to signal the A-device to activate VBUS, using techniques like data-line or VBUS pulsing. OTG implementations support standard USB device classes through a vendor-defined Targeted Peripheral List (TPL), which specifies compatible peripherals to ensure reliable operation in resource-constrained embedded hosts. Common supported classes include for USB drives, for keyboards and mice, Printer Class for printing functions, and Audio Class for basic audio devices. High-bandwidth classes, such as Video Class, face limitations due to OTG's focus on low-power, full-speed operations and restricted host buffering, often requiring specialized implementations. The Dual-Role Device (DRD) concept, refined in revisions beyond the initial OTG 1.0a (such as the OTG and Embedded Host Supplement to USB 2.0), enables symmetric connections between similar devices by supporting bidirectional role negotiation without fixed host-peripheral asymmetry. During enumeration, OTG devices advertise capabilities via the OTG descriptor (a three-byte structure in the configuration descriptor), indicating support for HNP (bit 1) and SRP (bit 0), along with other features like power budgeting. The host then configures the connection based on these descriptors, ensuring compatibility before enabling role switching.

Hardware Components

Connectors and Plugs

USB On-The-Go (OTG) initially utilized Mini-USB connectors to enable dual-role functionality in portable devices. Introduced with the OTG supplement to the USB 2.0 specification in late 2001, these connectors incorporated a 5-pin Mini-B design, extending the standard 4-pin USB configuration with an additional ID pin for role detection. The Mini-A plug, intended for host mode, featured the ID pin shorted to ground with a resistance of less than 10 Ω, while the Mini-B plug left the ID pin floating with a resistance greater than 100 kΩ to signal peripheral mode. Devices typically employed a Mini-AB receptacle to accept either plug type, allowing dynamic role switching based on the connected cable end. In 2007, the (USB-IF) introduced Micro-USB connectors for OTG applications, starting certification in December of that year to support more compact mobile devices. The Micro-AB receptacle became mandatory for consumer OTG products, capable of accepting both Micro-A and Micro-B plugs. Similar to the design, the Micro-A plug shorted the ID pin to ground to indicate host mode, while the Micro-B plug kept it floating for peripheral operation. This evolution addressed the need for smaller form factors in smartphones and accessories, replacing the bulkier Mini connectors in new designs. The pin assignments for both and OTG connectors follow a consistent layout to maintain compatibility with standard USB signaling while adding detection. Pin 1 carries VBUS for power delivery, pins 2 and 3 handle differential data lines D- and D+ respectively, pin 5 provides ground (GND), and pin 4 serves as the ID pin for OTG-specific functionality. detection relies on resistance measurements at the ID pin: OTG devices incorporate a 200 kΩ from ID to VBUS, resulting in a low voltage (near ground) when shorted (host mode) or a high voltage (near VBUS) when floating (peripheral mode). This simple resistive scheme enables the connected device to automatically assume the appropriate without software intervention.
PinNameFunctionTypical Wire Color
1VBUS+5V Power SupplyRed
2D-Data - (Negative)White
3D+Data + (Positive)Green
4IDIdentification/Role DetectN/A (internal)
5GNDGroundBlack
Following the announcement of USB Type-C in 2014, Mini and Micro OTG plugs were designated as legacy connectors by the USB-IF, with Type-C's reversible design and integrated OTG support via configuration channel (CC) pins rendering them obsolete for new developments. Despite this, Mini and Micro connectors persist in legacy devices as of 2025, particularly in budget and older peripherals where cost and existing hardware compatibility outweigh the benefits of upgrading. To enable host mode on devices with standard Micro-USB ports lacking native OTG support, specialized adapters are required that internally short the ID pin to ground, mimicking a Micro-A plug insertion. These adapters typically feature a Micro-B male connector on one end and a USB-A female on the other, allowing peripherals like flash drives to connect while signaling the device to supply VBUS power. Without this ID grounding, the device remains in peripheral mode and cannot act as a host.

Cables and Adapters

USB On-The-Go (OTG) relies on specialized cables and adapters to enable role switching between host and device modes, primarily through the use of an ID pin that signals the connection type. Micro-USB OTG cables feature a Micro-A plug at one end, where the ID pin (pin 4) is internally connected to ground (pin 5) with a resistance of less than 10 Ω, forcing the attached OTG device into host mode. The other end typically uses a Micro-B plug, where the ID pin remains floating (resistance greater than 100 kΩ to ground), designating it for peripheral connection. These cables ensure proper VBUS power direction from the host end to the peripheral, maintaining for data lines (D+ and D-). For older implementations, Mini-OTG cables follow a similar design using Mini-A and Mini-B plugs, each with five pins including the ID pin. In the Mini-A plug, the ID pin connects to ground to indicate host intent, while the Mini-B plug leaves it open. Mini-OTG cables often include variants for power passthrough, allowing simultaneous host operation and external to the OTG device via an additional connector, which helps mitigate limited battery drain during extended use. Simple OTG adapters, such as those with a Micro-B male connector to a Micro-AB receptacle or Standard-A female port, incorporate the ID pin grounded internally to emulate a Micro-A plug and trigger host mode on the device. Powered hubs serve as extension adapters, connecting via an OTG cable to the host device and providing multiple downstream ports with independent power sources to support power-hungry peripherals without overloading the OTG device's battery. These adapters adhere to wiring standards that align VBUS, ground, and signals correctly, with cable lengths limited to 2 meters maximum to preserve signal quality. Using non-OTG cables poses compatibility risks, as the absence of the grounded ID connection prevents role detection, causing the OTG device to remain in peripheral mode and fail to enumerate attached devices. Additionally, mismatched power handling in standard cables can lead to insufficient VBUS supply or reverse polarity issues, potentially damaging connected peripherals or the host device. For reference, these cable designs build on the plug pinouts defined in USB connector standards, ensuring seamless integration. Standard OTG cables, such as micro-USB or USB-C to USB-A female, are designed to place the mobile device in host mode for connecting peripherals like flash drives, keyboards, or mice. However, they do not support video transmission or direct connection to another host device such as a television for screen mirroring or media playback. Both the OTG-enabled mobile device and the TV typically function as USB hosts, resulting in no proper device detection or communication beyond possible charging. To connect a smartphone or tablet to a TV for display mirroring or content playback, specialized video output adapters are required, such as micro-USB or USB-C to HDMI adapters supporting protocols like Mobile High-Definition Link (MHL) or SlimPort, which repurpose the connector pins for audiovisual signals, or adapters utilizing DisplayPort Alternate Mode for USB-C devices. These adapters convert the signal to HDMI for connection to the TV's HDMI port. Wireless alternatives include Miracast, Chromecast, or built-in smart TV features.

Device Implementations

In Smartphones and Tablets

USB On-The-Go (OTG) functionality was first widely adopted in Android smartphones starting in 2011, with devices such as the Samsung Galaxy S II enabling direct connections to USB peripherals like flash drives for file transfer and input devices such as keyboards or mice. This support allowed these early models to act as USB hosts without requiring a PC intermediary, marking a significant step in mobile device versatility. In contrast, earlier Apple iOS devices implemented a proprietary alternative to standard USB OTG through the Lightning connector, introduced in 2012 alongside the iPhone 5. The Lightning to USB Camera Adapter provided OTG-like capabilities, primarily for importing photos and videos from digital cameras, but with limitations on peripheral compatibility and requiring Made for iPhone/iPad (MFi) certification for broader accessory support. This approach restricted full USB host functionality to approved devices, differing from the more open Android implementation. However, since the iPhone 15 series in 2023, iOS devices with USB-C ports (including later iPhones and iPads) support standard USB OTG for connecting peripherals such as storage drives, keyboards, and cameras, requiring the device to be unlocked and Wired Accessories enabled in Settings > Privacy & Security for automatic allowance when unlocked. While this provides more direct compatibility, some accessories still require MFi certification or app-based support. Software support for USB OTG in Android began with the USB host API introduced in API level 12 (Android 3.1, released in 2011), which allows applications to detect, communicate with, and manage connected USB peripherals programmatically. Developers can use classes like UsbManager to enumerate devices and handle data transfer, enabling custom apps for tasks like file management or device control. On iOS, the External Accessory framework offers analogous functionality for connecting MFi-certified USB accessories via the Lightning or USB-C ports, facilitating protocol-based communication but limited to authenticated hardware. Hardware integration of USB OTG in smartphones and tablets often relies on dedicated controllers embedded in system-on-chips (SoCs), such as those in Qualcomm's Snapdragon series, which support USB 2.0 OTG modes with integrated PHY layers for host and device switching. These controllers incorporate features, including link power management (LPM) to reduce battery drain during idle states and dynamic voltage scaling for efficient peripheral powering, ensuring compatibility with mobile form factors. For instance, Snapdragon SoCs like the 845 and later models enable seamless OTG operation while optimizing energy use through PMIC integration. Common usage scenarios for USB OTG in smartphones and tablets include attaching USB flash drives for direct file access and transfer, connecting game controllers for mobile gaming, and interfacing with devices for music production apps. These capabilities expand device utility beyond options, particularly in environments without . By 2025, the market for OTG accessories, such as adapters and pen drives tailored for mobile use, has seen significant growth, with the OTG pen drive segment estimated at USD 2.8 billion, driven by increasing penetration and demand for portable storage solutions. USB OTG also supports direct file transfer between Android smartphones and tablets, such as copying photos from a tablet to a smartphone. The receiving smartphone acts as the USB host using a USB OTG adapter or cable, while the source tablet operates in file transfer (MTP) mode. The devices are connected using a compatible USB cable (e.g., USB-C to USB-C or micro-USB to USB-C with OTG support). On the tablet, the user unlocks the device and selects "File Transfer" (MTP) mode if prompted. On the smartphone, a file manager app (built-in or third-party, such as CX File Explorer) is opened, where the tablet appears as an external device. The user navigates to the tablet's DCIM or Pictures folder, selects the desired photos, and copies them to the smartphone's storage. Direct connections without OTG host capability on the receiving device may not work reliably, as both devices are typically designed as USB peripherals. For Samsung Galaxy devices, the Smart Switch app provides a streamlined alternative for such transfers: the devices are connected via USB cable (an OTG adapter may be required depending on ports), Smart Switch is launched on both devices, send/receive options are selected, and photos or other data are transferred. Standard USB OTG cables cannot directly connect smartphones or tablets to televisions for screen mirroring, media transfer, or display output. Both the mobile device (in OTG host mode) and the TV's USB port act as hosts for peripherals or storage, resulting in role conflict that prevents proper detection and communication. In such cases, the mobile device may only receive charging power or fail to connect altogether. To enable video output or screen mirroring to a TV, specialized adapters supporting video transmission protocols are required. For devices with micro-USB ports, adapters using MHL or SlimPort technologies connect to the TV's HDMI port. For USB-C ports, adapters leveraging DisplayPort Alternate Mode provide HDMI output. Compatibility varies by device and TV model, and high-quality adapters are recommended to avoid issues such as lag or non-detection. Wireless alternatives include Miracast for screen mirroring on supported Android devices, Google Cast via Chromecast, or built-in Smart TV features such as Samsung's Smart View.

Support for Peripherals

USB On-The-Go (OTG) enables dual-role devices to act as hosts for a targeted set of USB peripherals, leveraging standard USB device classes to facilitate connections without requiring full host controller capabilities. This targeted approach allows portable devices to interact with peripherals such as storage, input, and multimedia devices, provided they are enumerated successfully during the standard USB attachment process. Storage devices, including USB flash drives and external hard disk drives (HDDs), are commonly supported through the USB Class (MSC), which operates in bulk-only transport mode to enable read/write access to file systems like FAT32. This class allows OTG hosts to mount and manage storage as if connected to a traditional PC, facilitating data transfer and backup operations on portable devices. Input devices such as keyboards, mice, and gamepads utilize the (HID) class to provide direct control inputs, with boot protocol support ensuring compatibility for basic functions like typing and pointing without custom drivers. The HID class defines standardized descriptors for these devices, allowing OTG hosts to interpret reports for cursor movement, key presses, and button actions seamlessly. Other peripherals supported via OTG include imaging devices like cameras, which connect using the (PTP) or (MTP) for photo and video import, and printers using the USB Printer Class for direct printing, though mobile OTG implementations often rely on apps for compatibility as built-in support is limited. Audio interfaces, such as microphones and speakers, operate under the USB Audio Class, supporting streaming of uncompressed PCM audio for playback and recording in compatible OTG implementations. The (USB-IF) introduced the Targeted Peripheral List (TPL) in the original 2001 On-The-Go Supplement to USB 2.0, requiring manufacturers to specify supported peripherals by vendor ID/product ID (VID/PID) pairs or device classes to limit scope and ensure reliable operation. This list, initially focused on essentials like keyboards, mice, and storage drives, has expanded in practice to encompass networking adapters via the Communications Device Class (CDC) for Ethernet connectivity and sensors using HID or custom classes for . Despite these capabilities, OTG implementations face limitations, including insufficient power delivery for high-power peripherals—typically capped at 8 mA minimum and up to 500 mA after —necessitating external power sources for devices like certain HDDs or charged peripherals. Additionally, enumeration challenges arise with complex or non-targeted devices, where the OTG host may fail to assign addresses or configure endpoints if exceeds the targeted support scope, potentially leading to connection failures.

Compatibility and Integration

Backward Compatibility with Standard USB

USB On-The-Go (OTG) devices are designed to seamlessly integrate with the existing USB ecosystem by defaulting to peripheral mode when connected to a standard USB host, such as a personal computer. In this configuration, an OTG device behaves identically to a conventional USB peripheral, utilizing the standard USB Type-B plug orientation for connection. This fallback ensures that OTG-enabled devices, like smartphones or tablets, can connect to legacy hosts without requiring additional configuration or role negotiation, maintaining full compatibility with USB 2.0 and USB 3.0 protocols. When operating in host mode, OTG devices can connect to standard USB peripherals, such as keyboards, mice, or storage drives, using the conventional USB process. The OTG host initiates communication as a full-speed or high-speed USB host, enumerating the peripheral through standard USB descriptors and class protocols without any modifications needed on the peripheral side. This allows OTG hosts to support a wide range of non-OTG USB devices through the standard USB process. To distinguish between data-capable hosts and dedicated chargers, OTG devices employ voltage sensing on the D+ and D- data lines as defined in the USB Battery Charging (BC) 1.2 specification. During primary detection, the device applies a current sink to D+ and measures the resulting voltage; a voltage between 2.0 V and 2.25 V indicates a dedicated charging port (DCP) with shorted D+ and D- lines, while a lower voltage around 0.325 V to 0.425 V on D+ (with corresponding response on D-) signals a charging downstream port (CDP) that supports both data and higher charging currents. For proprietary chargers, such as certain Apple models, OTG devices may detect specific voltages like 2.0 V on D+ to enable up to 1 A charging. This mechanism allows OTG devices to optimize power draw while in peripheral mode, drawing up to 1.5 A from compliant CDPs after secondary detection confirms data line integrity. OTG implementations comply with the USB 2.0 and 3.0 battery charging specifications, including BC 1.2 released in 2010, to ensure interoperability for charging during data sessions. Targeted OTG hosts must adhere to these specs when providing power to peripherals, enabling seamless operation in mixed environments where OTG and non-OTG devices coexist. This compliance supports up to 500 mA from standard downstream ports (SDPs) and higher currents from charging ports, without disrupting USB data transfer. In mixed OTG and non-OTG setups, potential issues may arise, such as power draw mismatches where an OTG host's limited VBUS output (typically 100-500 mA) fails to meet a peripheral's requirements, leading to failures or device instability. Enumeration delays can also occur due to role detection timeouts or incompatible SuperSpeed negotiations when connecting SuperSpeed OTG devices to USB peripherals, potentially requiring fallback to high-speed mode. These challenges are mitigated through adherence to OTG state machine protocols but may necessitate external power sources for high-draw peripherals.

Integration with USB Type-C and Modern Standards

The introduction of the USB Type-C connector in 2014 marked a significant evolution for USB On-The-Go (OTG) functionality, replacing the legacy ID pin used for detection in earlier OTG implementations with Configuration Channel (CC) pins (CC1 and CC2). This reversible connector design enables automatic orientation detection and embeds dual- capabilities natively, allowing devices to dynamically switch between host (Downstream Facing Port, DFP) and peripheral (Upstream Facing Port, UFP) without specialized OTG cables or plugs. Dual-Role Port (DRP) configurations in Type-C further integrate OTG-like behavior by supporting both Rp (pull-up for sourcing) and Rd (pull-down for sinking) resistors on the CC pins, facilitating seamless negotiation after a debounce period typically around 30-50 ms. Integration with USB Power Delivery (PD) enhances OTG devices' power management, enabling negotiated power delivery up to 100 W through the CC pins, far exceeding the original OTG's 5 V/500 mA limit. In OTG scenarios, PD allows dual-role devices to perform power role swaps independently of data roles, using commands like Power Role Swap (PRS) to ensure the host supplies power while peripherals draw only what's needed. This is particularly useful in mobile OTG applications, where a smartphone acting as host can source up to 15 W (5 V/3 A) or higher via PD contracts, preventing battery drain during peripheral connections. With the release of the USB4 specification in 2020, OTG role-switching mechanisms have been unified under a broader framework that supports higher data rates up to 40 Gbit/s (with , released in 2022, extending to 80 Gbit/s) and alternate modes like 3/4 and . USB4 supports DRP for dual-role ports, effectively superseding traditional OTG protocols by incorporating structured role detection and swapping via PD extensions, such as Data Role Swap (DRS), to handle complex multi-protocol tunneling. As of 2025, adoption is increasing among flagship smartphones and tablets, with growing support for USB4-compliant Type-C ports that enable OTG-equivalent functionality for peripherals, displays, and high-speed storage without legacy constraints. Legacy OTG connectors, including Mini-AB and Micro-AB plugs, have been deprecated by the (USB-IF) since 2007 for Mini-AB and in the mid-2010s for Micro-AB, due to mechanical fragility and the shift toward universal Type-C adoption. Type-C OTG adapters, often combining a Type-C plug with a USB-A receptacle, are now prevalent for , allowing older OTG peripherals to connect to modern hosts while leveraging CC pin detection for role assignment. A simple OTG Type-C adapter is used to connect a regular USB flash drive to a Type-C phone; models with USB 3.0 support are recommended for higher speeds. In the current market as of 2025, dedicated OTG transceivers like those from (e.g., TUSB320 series) continue production primarily for legacy system support and transitional designs, but new implementations overwhelmingly favor integrated Type-C DRP controllers from vendors such as Microchip and , which consolidate role detection, PD negotiation, and USB4 compatibility in single-chip solutions. This shift reflects the standardization of Type-C across , reducing the need for discrete OTG hardware while maintaining .

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