PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

24.1 USB Characteristics

All
USB devices share several general characteristics. Among these are:

In
theory, at least, USB peripherals can be connected to and
disconnected from the bus at any time, without shutting down the
computer or taking any action to inform applications or the OS that a
device is being added or removed. In practice, this is not always the
case, particularly with older interfaces and devices.

USB mass-storage devices are an exception worth noting. Disconnecting the USB cable before the drive finishes writing may corrupt data. If you use a USB hard drive, tape drive, or CD/DVD writer, read and follow the directions carefully to avoid losing data.

The USB host controller chipset
installed on the PC motherboard or an add-on USB port card manages
driver software and allocates bandwidth to each USB device attached
to the bus. When a device is added or removed, the USB host
controller automatically loads or unloads the driver for that device.

Educate your USB drivers. When you connect a USB peripheral to a root hub port or external hub, the USB driver "learns" that device/port combination. If you connect and disconnect USB devices frequently, you can make things faster and easier by allowing the PC to learn all combinations of device and port. Rather than simply connecting a device to a port and leaving it connected, plug that device in turn into each available USB port. That way, you can connect any device to any port and have it recognized immediately.

A USB host controller occupies one
interrupt, which is shared among all devices attached to the bus.
This small resource footprint allows multiple USB host controllers to
be installed in a system without undue demands on scarce IRQs.
Although each USB host controller can in theory support as many as
127 devices, it's often better to distribute
multiple USB devices among host controllers to avoid conflicts.

A USB 1.1 bus provides 12 Mb/s of
bandwidth and a USB 2.0 bus provides 480 Mb/s of bandwidth, which is
shared among all devices attached to the bus. Many devices may
communicate simultaneously on a USB, provided that adequate bandwidth
is available to service all of them at the same time. Properly
designed USB peripherals and drivers use bandwidth dynamically,
releasing bandwidth they are not using so that it can be used by
other devices. For isochronous (time-critical) tasks such as audio or
video streams, USB permits dedicating bandwidth as needed to a
particular peripheral, although that dedicated bandwidth then becomes
permanently unavailable for use by other peripherals.

In addition to providing a data
connection, USB provides electrical power to peripherals, allowing
you to eliminate the tangle of power cables required by traditional
peripherals. That power, however, is limited to 500 milliamps (mA),
which must be shared by all unpowered devices connected to the USB
port. In practice, that means that only low-power peripherals, such
as keyboards and mice, can be powered directly by a USB connection.
High-power peripherals, such as printers and scanners, usually (but
not always) have their own power bricks and are powered directly from
a standard AC receptacle. For example, the Canon Canoscan 1220U
relies on the USB port for power. Despite the minimal amount of
current available on the USB port, it is possible to connect multiple
unpowered USB peripherals by connecting them to powered USB hubs,
each port of which has its own 500 milliamp supply.

Multiple USB devices connected to an unpowered hub can collectively exceed the maximum power that hub can deliver, particularly if you have connected one or more high-draw hub-powered devices. Many unpowered hubs, particularly those embedded in keyboards and similar devices, do not supply the full 500 mA called for by the USB specification. For example, the USB hub embedded in Microsoft USB keyboards can supply at most 200 to 250 mA. Connecting too many devices or devices that draw too much current is one frequent cause of USB problems.

The following sections detail other important
characteristics of USB interfaces and devices.

24.1.1 USB Versions

Three
versions of USB exist:

USB 1.0 was the original specification. Most
systems produced from 1996 through mid-1998 have USB 1.0 ports. USB
1.0 supports data rates of 1.5 Mb/s and 12 Mb/s. Relatively few USB
1.0 peripherals were produced because by the time USB peripherals
began shipping in volume, USB 1.0 had been superseded by USB 1.1. USB
1.0-compliant peripherals generally operate properly when connected
to a USB 1.1 or USB 2.0 interface, but USB 1.1 or USB 2.0 peripherals
may not function properly when connected to a USB 1.0 interface. USB
1.0 interfaces are primitive and buggy, so if your motherboard has
USB 1.0 ports, we recommend you disable those ports in BIOS Setup and
install an add-on PCI USB port card. The first release of Windows 98
included USB 1.0 support.

USB 1.1 was formalized in September 1998,
although many manufacturers produced USB 1.1-compliant motherboards
and peripherals based on the proposed standard long before the formal
standard was adopted. USB 1.1 also supports data rates of 1.5 Mb/s
and 12 Mb/s, and was largely a clarification of ambiguities in the
USB 1.0 specification. A few functional definitions were changed in
USB 1.1, including minor changes to hub specifications, removing
provision for battery-powered hubs, adding interrupt-out mode, and
changes to recommended enumeration to eliminate the requirement for
an 8-byte endpoint zero. Most changes, however, merely tightened up
the existing requirements because experience had shown that there
were enough ambiguities in the USB 1.0 specification to allow
producing interfaces and devices that complied with the standard but
were not interoperable. USB 1.1 interfaces and devices began shipping
in mid-1998 and are still in production. Early USB 1.1 interfaces and
devices suffered many incompatibilities, but current production
models have relatively fewer such issues. You can download the
Universal Serial Bus Revision 1.1 Specification from http://www.usb.org/developers/docs/usbspec.zip.

USB 2.0 was formalized in April 2000, with
various errata and Engineering Change Notices later incorporated as
supplements. USB 2.0 supports data rates of 1.5 Mb/s, 12 Mb/s, and
480 Mb/s, and provides full backward compatibility with USB 1.0 and
USB 1.1 devices.

The uptake of USB 2.0 was slower than expected because USB 2.0
chipsets were slower in arriving than expected and because Microsoft
initially did not provide USB 2.0 Windows drivers. Microsoft shipped
native USB 2.0 drivers for Windows XP in early 2002 and for Windows
2000 in late 2002. Microsoft has no plans to provide Windows 9X USB
2.0 drivers. Using USB 2.0 under Windows 9X requires drivers supplied
by the manufacturer of the motherboard (or PCI/USB card) and USB 2.0
peripherals.

With so many of the major players in the computer industry backing
it, USB 2.0 saw a fast ramp-up during late 2002 and into 2003. Most
motherboards introduced during fall 2002 and later have chipset-level
USB 2.0 support, and by early 2003 USB 2.0 peripherals such as
external hard drives and optical drives were readily available. Older
systems can be upgraded to support USB 2.0 by adding an inexpensive
adapter.

In addition to its much higher speed, the attraction of USB 2.0 is
its standardization. Earlier USB versions had frequent compatibility
problems, not the least because two different and slightly
incompatible controller standards existed. USB 2.0 defines one
controller interface, called the Enhanced Host Controller
Interface (EHCI), which eliminates
many compatibility issues. You can download the most recent complete
Universal Serial Bus Revision 2.0 Specification from http://www.usb.org/developers/docs/usb_20.zip.

24.1.2 USB Speeds

USB
defines the following three speeds, all of which can coexist on one
bus:

Low Speed
USB peripherals operate at a data
rate of 1.5 Mb/s, and are supported by USB 1.1 and USB 2.0
interfaces. Low Speed USB is intended for such low-bandwidth devices
as mice and keyboards, and is designed to be inexpensive to
implement. Low Speed USB devices use a captive cable that can be no
longer than 3 meters. Actual throughput on Low Speed USB is typically
limited by overhead and other factors to about 1.2 Mb/s, or 150 KB/s.

Full Speed
USB peripherals operate at a data
rate of 12.0 Mb/s, and are supported by USB 1.1 and USB 2.0
interfaces. Full Speed is the fastest speed supported by USB 1.0 and
USB 1.1, and is intended for such moderate-bandwidth devices as
printers and scanners. Full Speed USB devices use active components,
which are more expensive to implement than the passive components
used by Low Speed USB. Full Speed USB devices use a detachable cable
that can be no longer than 5 meters. Full Speed USB seldom exceeds
actual throughput of 900 KB/s or so.

Hi-Speed
USB peripherals operate at a data
rate of 480 Mb/s, and are supported only by USB 2.0 interfaces.
Hi-Speed USB is intended for such high-bandwidth devices as external
drives. Hi-Speed USB devices use active components that are more
expensive than Full Speed USB components. Also, Hi-Speed USB hubs
require additional circuitry to arbitrate between mixed Hi-Speed and
Full Speed devices connected to that hub. Accordingly, Hi-Speed USB
devices, particularly hubs, were initially more expensive than USB
1.1 devices, although that differential had largely disappeared by
early 2003. Hi-Speed USB devices use the same detachable cable used
by Full Speed USB devices, which can be no longer than 5 meters.
Hi-Speed USB typically achieves actual maximum throughput of 35 to 40
MB/s, which is fast enough to keep up with all but the fastest hard
drives.

In late June 2003, computer trade publications reported a brouhaha concerning the relabeling of USB components. Apparently, some component makers decided that labeling Low Speed and Full Speed USB components as USB 1.1 put them at a competitive disadvantage relative to USB 2.0 devices. With USB 2.0 the current standard, purchasers were avoiding USB 1.1 components in droves, even though such devices of course run properly with either a USB 1.1 or USB 2.0 interface. Accordingly, the manufacturers convinced the USB Forum to change labeling. Devices formerly known as "USB 1.1" (1.5 and 12 Mb/s) are now to be labeled simply "UBS". Hi-Speed USB 2.0 devices are now to be labeled simply "Hi-Speed USB" (without the 2.0 designation). The only change is semantic. The technology remains the same. We use the original terminology in this chapter for charity. Also note that even official standards documents published by the USB Forum have inconsistent spelling and usages. Low Speed USB is referred to variously as Low Speed, low speed, Lo Speed, lo-speed, and other variants. Hi-Speed (the currently preferred form) may be referred to as High Speed, high speed, high-speed, hi-speed, and so on. As is the case with many standards documents, the authors appear to have gone to great lengths to get the important stuff right, but not paid much attention to consistent spellings and usages.

Note that USB bandwidth is shared among all devices connected
to the bus, and that the system reserves some bandwidth (typically
10%) for control signals and other administrative purposes. Although
many Hi-Speed USB devices require much less than 480 Mb/s, if you do
connect more than one high-bandwidth Hi-Speed device to a single USB
channel, you may throttle the bandwidth available to each when more
than one are operating. In that situation, if your system has more
than one USB 2.0 HCI, we recommend splitting your high-bandwidth
devices among different host controllers.

24.1.3 USB Topology

USB uses a
tiered-star topology, shown in Figure 24-1. At the
center of the star is the USB host, which
defines the USB, and only one of which is permitted per USB. (Note,
however, that more than one USB host may be installed in a PC, and in
fact most recent motherboards have multiple USB hosts installed.) The
USB host resides inside the PC, and is implemented as a combination
of hardware, firmware, and software. The USB host has one or more USB
root hubs, which provide attachment points called USB
ports to which USB hubs and
USB functions may be connected. (Loosely
speaking, a USB function is a peripheral such as a scanner, printer,
mouse, digital camera, etc.)

Figure 24-1. USB topology.

USB hubs use two types of connections. An upstream
connection links the USB hub to another USB hub in the
next-higher tier. A downstream connection links
the USB hub to another USB hub or to a USB function located in the
next-lower tier. Each USB hub has one upstream port, and may have as
many as seven downstream ports. Via daisy-chaining, USB allows
connecting a maximum of 127 devices (USB hubs and USB functions) in a
maximum of seven tiers. The limitation on the number of tiers is
required to ensure that the most distant USB device can communicate
within the maximum allowable propagation delay defined in the USB
specification.

In practical terms, you seldom need more than two or three USB tiers to connect a typical assortment of USB peripherals to a PC. However, it's worth noting that the USB limit of 5 meters on cable length applies between tiers. For example, if you need to locate a USB peripheral some distance away from a PC, you can extend the separation to as far as 30 meters by putting the USB peripheral in Tier 7 and installing USB hubs in Tiers 2 through 6. Note that Tier 7 can contain only USB functions. A USB hub that resides in Tier 7 is useless because any device connected to that hub resides in the nonexistent Tier 8.

If you mix USB 1.1 and USB 2.0 devices on a
bus, it's important to understand the following
points:

USB 2.0 hubs support
USB 1.0/1.1 and USB 2.0 devices, including downstream USB 1.0/1.1
hubs and USB 2.0 hubs, providing full bandwidth to each device
according to its version.

USB
1.0/1.1 hubs support USB 1.0/1.1 and USB 2.0 devices, but USB 2.0
devices connected to a USB 1.1 hub function as USB 1.1 Full Speed
devices (i.e., at 12 Mb/s maximum), which means that it is pointless
to connect a USB 2.0 hub downstream from a USB 1.1 hub.

Although USB 2.0 hubs provide transparent
support for Low Speed and Full Speed USB devices, that support incurs
significant overhead on the USB 2.0 hub and is intended only for
limited use, such as connecting a USB mouse and keyboard. Connecting
multiple Low Speed or Full Speed USB devices to a USB 2.0 hub, either
directly or downstream, degrades the ability of the USB 2.0 hub to
support Hi-Speed USB 2.0 devices. If you have many Low Speed or Full
Speed USB devices, connect them to a USB 1.1 root hub port and
reserve your USB 2.0 ports for Hi-Speed devices.

24.1.4 USB Cables and Connectors

USB
connectors and cables are simple and rigidly standardized. USB
defines three plug/jack combinations, designated Series
A, Series B, and Series
mini-B.

Series mini-B connectors are smaller versions of the standard Series B connector, designed for devices such as digital cameras, for which a standard Series B connector is physically too large. Most devices that use Series mini-B connectors come with a Series-A to-Series-mini-B cable.

The narrow Series A jacks, shown in Figure 24-2, appear on the back of a PC or USB hub, and
may be labeled "down" jacks. These
provide connection points for USB peripherals, including USB hubs.
Some USB peripherals have permanently connected cables that terminate
in a Series A plug. Series A plugs always face upstream, toward the
host system, and Series A jacks always face downstream, toward the
device.

Figure 24-2. USB Series A
connector

Peripherals that do not have a permanent cable instead have a
Series B jack, shown in Figure 24-3. Series B plugs
always face downstream, and Series B jacks always face upstream. Use
a standard USB A-B device cable to connect these peripherals to the
PC or hub.

Figure 24-3. USB Series B
connector

A USB cable uses four wires, two each for data and power. The
data wires are a green/white twisted pair that carry +Data and -Data,
respectively. USB uses differential digital signaling, which means
that the same signal is present on each data wire, but with different
polarity. This allows electrical noise to be eliminated from the
circuit because induced voltages affect the + and - signals equally,
netting to zero. The power wires may or may not be twisted. The red
wire carries nominal +5V DC, and the black is a ground return for the
power circuit. The USB specification permits cables as long as 5
meters (~16 feet), a limitation enforced by the allowable propagation
delay between a port and a connected device.

The USB
specification defines only three types of USB cable:

Keyboards and similar low-speed
devices use a USB low-speed captive cable. The
maximum allowable length for a low-speed USB cable is 3 meters (9
feet, 10 inches). This limit is determined by the rise and fall times
of low-speed USB signaling, which restricts low-speed USB cables to a
maximum length only 60% that of standard full-speed/hi-speed cables.
A USB standards-compliant low-speed cable must be

captive or

hardwired , which
is to say that the cable either must be permanently connected to the
device or must use a nonstandard or proprietary connector on the end
that connects to the device. The concern is that if a low-speed USB
device used a standard USB device connector, a standard detachable
USB cable longer than acceptable for low-speed USB devices could be
used to connect that device.

Most full-speed and hi-speed
USB devices use a USB standard detachable cable.
This cable is terminated on one end with a Series A plug and on the
other with a Series B plug or Series mini-B plug. The maximum
allowable length for a standard detachable cable is 5 meters (16
feet, 5 inches).

Some full-speed and hi-speed USB
devices use a USB hi-/full-speed captive cable.
This cable is terminated on one end with a Series A plug. The other
end terminates either as a hard-wired connection to the device or
with a vendor-proprietary connector. The maximum allowable length for
a hi-/full-speed captive cable is also 5 meters.

Although the USB standard states that only these three cable
assemblies are acceptable, it further emphasizes it by specifically
prohibiting the following cable assemblies, all of which have been
manufactured and sold despite their
non-compliance:

Some vendors produce cables that terminate with two Series A plugs,
two Series B receptacles, or two Series mini-B receptacles, which
allow connecting USB ports and devices in prohibited combinations.

Cables with two Series A plugs are particularly dangerous, because they can be used to link two powered ports with differing ground potentials, which may cause a fire. In theory, such a cable could be used to link two PCs directly via USB. In practice, this does not work and is dangerous. The only acceptable way to link two PCs via USB is to use a USB bridge. Note that some cables have USB bridge circuitry built into them, and so are in reality more than a simple cable. This prohibition does not apply to such USB bridge devices.

A cable that terminates with a Series A plug and a Series A
receptacle, a Series B plug and a Series B receptacle, or a Series
mini-B plug and a Series mini-B receptacle is specifically prohibited
by the USB specification. However, many vendors sell such cables,
including some vendors who should know better. The purpose of these
cables is to extend the distance between port and device by joining
multiple cable segments. The risk is that the joined cables will
exceed the maximum 5 meters permissible under the standard, which can
cause problems, from sporadic operation to complete failure of the
entire USB. Even if the joined cables total less than 5 meters, the
electrical characteristics of the extended cable may fall outside
specifications. Avoid using extension cables under any circumstances.

The USB specification does not permit low-speed devices to use
standard detachable cables. Standard detachable cables are used only
to connect hi-speed/full-speed devices. Their capacitive load exceeds
the maximum allowable for a low-speed device.

There is much confusion about which USB cables can be used at which speeds. Some vendors have confused the issue by producing so-called USB 2.0 cables. In fact, any detachable USB 1.1 cable is also a USB 2.0 cable, and can be used at 480 Mb/s. Note, however, that many of the cables supplied with USB 1.1 devices are technically not USB 1.1 cables because they don't meet USB requirements. These out-of-spec cables may work reliably with USB 1.1 devices, but are simply not good enough for the higher data rate of Hi-Speed USB 2.0 connections. By some estimates, 30% or more of the cables supplied with USB 1.1 devices do not meet USB specifications.

24.1.5 USB Data Transfer Modes

USB
uses a set of unidirectional and bidirectional pipes to transfer user
data and control information between the host and USB devices. Each
device may support multiple pipes for different purposes, and data
transferred in one pipe is independent from data transferred in other
pipes. For example, a USB printer might have one pipe that it uses to
receive page data from the host, and a second pipe that it uses to
transfer status information to the host. USB defines the following
data flow types:

Isochronous Data
Transfers
are used for periodic, continuous
communication between the host and a device, typically time-critical
data such as audio or video streams. Isochronous Data Transfers are
enabled by reserving the required amount of bandwidth for the
isochronous device, which the USB host controller makes unavailable
to other devices whether the isochronous device happens to be using
that bandwidth at any given time. Isochronous Data Transfers have the
highest priority for bandwidth. If all available bandwidth is
reserved for Isochronous Data Transfers, no other device can use the
USB.

Interrupt Data
Transfers
are used for small, limited-latency
transfers when timely, reliable delivery of data is
requiredfor example, to receive coordinate changes from a
mouse or status changes from a modem. Interrupt Data Transfers have
lower priority for available bandwidth than do Isochronous Data
Transfers.

Control Transfers
are used to configure a device when
it is connected to the USB, and may be used for other device-specific
control, configuration, and status commands, including controlling
other pipes on the device. Control Transfers usually comprise small
amounts of data that are not time-critical, and have lower priority
for available bandwidth than do Interrupt Data Transfers.

Bulk Data Transfers
are used to communicate large amounts
of nonperiodic, bursty data with relaxed timing constraints to a
devicee.g., sending page data to a USB printer. Bulk Data
Transfers are not time-critical, and have the lowest priority for
available bandwidth. Some early HCIs implemented bulk mode poorly,
and so work properly with USB devices such as keyboards and mice but
are unsuitable for use with devices such as scanners and printers.