Datasheet

Application Note
www.tektronix.com/oscilloscopes10
The USB Implementers Forum (USB-IF) manages and
promotes USB standards and USB technology. USB
specifications are available at the USB-IF web site at
www.usb.org.
How It Works
The USB configuration is one host controller with 1 to 127
devices. USB is a tiered-star topology with optional hubs
to expand the bus (Figure 14). The host is the only master
and it controls all bus traffic. The host initiates all device
communications and devices do not have the capability to
interrupt the host.
There are four USB speeds as shown in Table 2. A high-speed
device starts out at full-speed and then transitions to high-
speed. The speed of a USB 2.0 bus is limited by the slowest
device connected to the host controller.
With SuperSpeed USB, two host controllers are used: one for
SuperSpeed USB devices and one for USB 2.0 devices. Like
a USB 2.0 system, the speed of the bus with USB 2.0 devices
is limited by the slowest device.
USB
Background
The Universal Serial Bus (USB) has become a dominant
interface on today’s personal computers, replacing many of
the external serial and parallel buses previously used. Since
its introduction in 1995, USB has grown beyond its original
personal computer usage and has become a ubiquitous
interface used in many types of electronic devices.
The USB 2.0 specification released in 2000 covers most of
the USB devices that are being used today. USB 2.0 replaced
the USB 1.1 specification, adding a high-speed interface (see
Table 2) to the low-speed and full-speed interfaces in the USB
1.1 specification.
USB has expanded beyond just system-to-system
communication. For example, the Inter-Chip USB (IC_USB)
and the High-Speed Inter-Chip (HSIC) USB are used for
chip-to-chip communications. Supplements to the USB 2.0
specification cover IC_USB, HSIC and other enhancements.
In 2008, the USB 3.0 specification was released. USB
3.0 is called SuperSpeed USB and is ten times faster
than high-speed USB 2.0. SuperSpeed USB preserves
backward compatibility with USB 2.0 devices. USB 3.0 is an
additional specification that is used in conjunction with the
USB 2.0 specification and does not replace it. SuperSpeed
USB devices must implement USB 2.0 device framework
commands and descriptors.
Table 2. USB speeds.
Figure 14. Correct synthesizer configuration messages.
Host Controller
Hub Device Device
DeviceDeviceDeviceDevice
Device Device Device Device
Hub
Hub
USB Speed Bit Rate Bit Period
Low-Speed USB 2.0 1.5 Mbps 667 ns
Full-Speed USB 2.0 12 Mbps 83.3 ns
High-Speed USB 2.0 480 Mbps 2.08 ns
SuperSpeed USB 3.0 5 Gbps 200 ps