USB 3.0 (Universal Serial Bus) is moving quickly into the mainstream along with related high-speed serial technologies like 6-Gbit/s SATA III and 5-Gbit/s PCI Express 2.1. This meshes nicely with USB 3.0’s 5-Gbit/s clock rate, especially since it primarily targets storage devices such as magnetic hard disks and flash-based solid-state disk drives.
USB 3.0 is arriving just in time to address high-capacity storage such as 25-Gbyte Blu-ray disks. Copying this much information to a hard drive takes only 70 seconds with USB 3.0. The chore would require 14 minutes using USB 2.0 and almost half a day using USB 1.1.
Developed by Intel’s Ajay Bhatt, USB has grown since its inception in 1996. USB 1.0 got off to a rocky start, but USB 1.1 became the dominate interface for keyboards and mice. It delivered 1.5-Mbit/s (low speed) and 12-Mbit/s (full-speed) transfer rates sufficient for many peripherals, including scanners and printers.
The USB connection included 5-V power and ground in addition to a bidirectional, twisted-pair differential data cable with 90 Ω ±15% for a total of four wires. The host controls the half-duplex, self-clocking serial protocol. In full-speed mode, the cable is not terminated. With USB 2.0, the 480-Mbit/s (high speed) mode requires the cable to be terminated. The controllers handle this with active termination.
USB 1.1 devices select low-speed or full-speed mode by selecting the D– or D+ line to toggle high. USB 2.0 devices negotiate the high-speed mode during reset negotiation by driving both lines low for 10 to 20 ms.
USB is a host managed peripheral bus that uses a point-to-point protocol. A USB host has a root hub controller. Additional hubs are used to expand access to a maximum of 127 devices. Four hub levels are possible in theory, but it is rare to find a system that has more than two.
POWER TO PERIPHERALS
A USB device can use up to 500 mA from a hub port. An unpowered hub will distribute its power from its host to its devices, but it is limited to the total power the host provides. A powered hub can provide up to 500 mA per port.
Low-power devices like mice require significantly less than 500 mA, while hard drives often require the maximum. Some external hard drives even come with cables that tap a second USB port simply to gain access to the extra half-amp of current. USB ports are current-limited and can detect shorts, allowing a bad port connection to be powered down.
A host manages the initial power-up sequence. It also detects and manages changes that occur while the system is running. It enumerates each device connected to its hub and then each level below until all devices are reached as well. A hub is really a USB device.
USB 2.0 kicked the top bandwidth up to 480 Mbits/s. It uses the same connectors and cables as USB 1.1, with additional connectors included with the new standard (see “USB Connector Gallery”). The USB standard and connectors are designed for hot-plug support. A close examination of the USB connectors shows that the power and ground connections will be completed before the signal connections when inserting a plug.
USB On-The-Go (OTG) came about to address devices such as digital cameras that needed to act as a host when connected to a printer and as a device when connected to a host such as a PC. USB OTG 1.3 provided this support at USB 1.x speeds, and it is found on many devices. The OTG device needs to determine whether it is connected to a host or device and act accordingly.
The USB OTG 2.0 spec adds power-saving features and support for the micro USB connectors. It also allows OTG devices to communicate with each other. USB 3.0 (SuperSpeed) is the latest incarnation. Although USB 3.0 uses a single cable, it is actually two buses in one.
TWO PERIPHERAL BUSES IN ONE
SuperSpeed USB (Fig. 1) adds two pairs of high-speed differential signals and a shielded cable to the mix, upping the four-wire USB standard to nine wires. The initial four wires include power, ground, and a pair of 480-Mbit/s differential pairs. The high-speed differential pairs provide a full-duplex, bidirectional 4.8-Gbit/s communication link between a host and an active device.
A USB 3.0 device essentially has an independent USB 2.0 controller and a USB 3.0 controller. It will only use one set or the other, depending on the capabilities of the upstream hub. A USB 3.0 hub must support both, so the USB 3.0 host must support both as well.
This allows a hub port to be connected to a USB 3.0 or USB 2.0 device (Fig. 2). A USB 3.0 hub can use both upstream links when communicating with a USB 3.0 host. This highlights the duality of the USB 3.0 architecture since the two are essentially operating independently even though a single cable is used.
The USB device driver at the host is where the two USB hierarchies are merged. This makes the USB subsystem appear as a single entity to applications running on the host.