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Maxim > Design
support > App notes
> Interface
Circuits > APP 1822

Dec 20, 2002

Keywords: USB OTG, OTG, USB on the go, dual role
device, host negotiation protocol, session request protocol,
host, peripheral, transceiver

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APPLICATION NOTE 1822

USB On-The-Go Basics

Abstract: USB
On-The-Go (OTG) allows two USB devices to talk to each other
without requiring the services of a personal computer.
Although OTG appears to add "peer to peer" connections to USB,
it does not. Instead, USB OTG retains the standard USB
host/peripheral model, where a single host talks to USB
peripherals. OTG introduces the dual-role device (DRD),
capable of functioning as either host or peripheral. Part of
the magic of OTG is that a host and peripheral can exchange
roles if necessary.

Before OTG, the concept of an
embedded host was already established in the USB world.
Instead of duplicating the full UHCI/OHCI USB controllers and
drivers built into personal computers, most embedded host
chips provide limited hosting capabilities. This makes them
better suited to the embedded environment than to the PC with
its huge resources and infinite capacity for drivers and
application software.

Introduction
USB On-The-Go (OTG) allows two USB
devices to talk to each other without requiring the services
of a personal computer (PC). Although OTG appears to add
peer-to-peer connections to the USB world, it does not.
Instead, USB OTG retains the standard USB host/peripheral
model, in which a single host talks to USB peripherals. OTG
does introduce, however, the dual-role device, or simply
stated, a device capable of functioning as either host or
peripheral. Part of the magic of OTG is that a host and
peripheral can exchange roles if necessary.

Before OTG,
the concept of an embedded host was already established in the
USB world. Instead of duplicating the full UHCI/OHCI USB
controllers and drivers built into PCs, most embedded host
chips provide limited hosting capabilities. This makes them
better suited to the embedded environment than a PC with its
huge resources and infinite capacity for drivers and
application software.

An OTG device may, or may not be
capable of functioning as a host. It is likely, nonetheless,
that most OTG devices will be dual-role.

USB Peripherals
Figure 1 illustrates the basic
USB peripheral circuitry on which OTG builds. These example
peripherals operate at low or full speed, and are commonly
known as USB 1.1 devices. This nomenclature is used even
though the USB 2.0 Specification includes the current
USB 1.1 specification and introduces a third, higher speed.

Figure
1. A USB peripheral controller and its associated
circuitry.

The controller in Figure 1 might be a
microprocessor plus USB SIE (Serial
Interface Engine), an integrated microprocessor/USB chip,
or an ASIC
connected to a USB transceiver. A bus-powered peripheral
requires a 3.3V regulator,
both to power the logic and to supply the proper voltage to a
1500Ω resistor connected to either the D+ or D- USB pins. This
pullup resistor signals the host that a device is connected,
and indicates the device's operating speed. A pullup to D+
indicates full speed; a pullup to D- indicates low speed. The
other end of the connection—host or hub—contains 15kΩ pulldown
resistors on D+ and D- so the pullup resistor can be detected.
Finally, an ESD
protection circuit is advisable on D+, D-, and
VBUS pins because USB is designed to be
hot-plugged.

How to Be a Host
The Figure 1 circuit functions only
as a USB peripheral device. To add OTG dual-role capability,
the transceiver must be augmented to allow the OTG device to
function as either host or peripheral. Adding the following to
Figure 1 lets the system also function as a host:

15kΩ pulldown resistors on D+ and D-

A means to supply, rather than draw, power on
VBUS

The ASIC or controller must also
contain logic to function as a USB host. Some of the host
duties absent in a peripheral device are:

Send SOF (Start of Frame) packets.

Send SETUP, IN, and OUT packets.

Schedule transfers within USB 1ms frames.

Signal USB reset.

Provide USB power management.

In addition to
requiring a dual-role peripheral/host USB controller, OTG
requires additional circuitry to support two new protocols,
called HNP and SRP.

Host Negotiation Protocol
An OTG dual-role device can
operate either as a host or peripheral. In OTG nomenclature,
the initial host is called the A-Device, and the
initial peripheral is called the B-Device. The word
initial is important. Once connected, OTG dual-role
devices can exchange roles—host and peripheral—by using the
new Host Negotiation Protocol (HNP). HNP raises two obvious
questions: (a) how are the initial roles determined; and (b)
why is the role reversal necessary?

Figure
2. Fifth ID pin determines default host.

The cable
orientation determines the initial roles (Figure 2).
Dual-role devices use a new receptacle called the mini-AB. The
mini-A plug, the mini-B plug and the mini-AB receptacle add a
fifth pin (ID) to give different electrical identities to the
cable ends. This fifth ID pin is connected to ground inside
the mini-A plug and left floating in the mini-B plug. The OTG
device receiving the grounded ID pin is the default A-Device
(host); the device with the floating ID pin is the default
B-Device (peripheral).

Figure
3. OTG cable is inserted backwards.

To understand
the need for the HNP and host/peripheral role reversal, the
example in Figure 3 shows two dual-role devices, a PDA
and a printer. The PDA has a printer driver inside. The two
devices are connected with the new OTG cable as shown, making
the printer the default host (A-Device) and the PDA the
default peripheral (B-Device). But this setup is backwards.
The PDA, which has the printer driver, needs to act as USB
host to the printer, which contains no driver. Rather than
bothering the user to reverse the cable, HNP allows the
devices' roles to reverse automatically and silently.

Session Request Protocol
The OTG Specification adds a
second new protocol to USB, called Session Request Protocol
(SRP). SRP allows a B-Device to request an A-Device to turn on
VBUS power and start a session.

An OTG
session is defined as the time that the A-Device is furnishing
VBUS power. (Note: the A-Device always supplies
VBUS power, even if it is functioning as a
peripheral due to HNP.) The A-Device can end a session by
turning off VBUS to conserver power, a very
important requirement in a battery-powered device such as a
cell phone.

Figure
4. OTG Session Request Protocol (SRP).

Figure
4 shows a common OTG application: two cell phones
connected together to exchange information. The right phone
received the mini-A end of the cable, making it the A-Device
and thus defaulting into the host role. The left phone is the
B-Device, defaulting to peripheral. If there is no need to
communicate over USB, the A-Device can power down the
VBUS wire, which the B-Device can detect so that it
too can enter a low-power state.

Now suppose that the
user of the left phone presses a button to synchronize address
books, or any other action that requires a USB session. The
‘SRP Pulse' block in the left phone pulses first the D+ wire,
and then the VBUS wire to wake up the A-Device.
(The A-Device can respond either to D+ or VBUS
pulsing.) The A-Device then detects the pulse, causing it to
switch
on VBUS and start a session.

The SRP
protocol is more complex than this simple illustration. The
B-Device, for example, must first measure VBUS to
ensure that a session is not in progress. It must also be able
to differentiate between a classic PC or an OTG device at the
other end of the cable. It does this by delivering measured
amounts of current to the VBUS wire and noting the
resulting voltage.

Once a session is underway, the
devices may or may not use HNP.

OTG Transceiver
We are now ready to examine the
requirements for an OTG transceiver, illustrated in Figure
5.

Figure
5. An OTG transceiver.

The Figure 5 system builds
on the Figure 1example circuit. The ASIC block could also be a
microprocessor or DSP with USB capability. Three additions
make the transceiver OTG compatible:

Switchable pull-up and pull-down resistors on D+/D- to
allow peripheral or host functionality.

Circuitry to monitor and supply 5V power on
VBUS as an A-Device, and to monitor and pulse
VBUS as a B-Device initiating SRP.

An ID input pin, which is made available as an output to
the ASIC.

For this system to operate as a dual-role
OTG device, the ASIC, DSP, or whatever is connected to the
transceiver must be capable both of functioning as a
peripheral or host, and of switching roles on-the-fly as a
result of HNP.

Most of the added transceiver circuitry
manages the VBUS pin, which now must also supply 5V
power at 8mA as a host, and perform VBUS pulsing as
a peripheral. Analog switches configure the transceiver for
the various roles that it must play.

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Download,
PDF format (52kB)
APP 1822: Dec 20, 2002

AN1822, AN 1822, APP1822,
Appnote1822, Appnote 1822

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