Electronic Design
USB 3.0: A Tale Of Two Buses

USB 3.0: A Tale Of Two Buses

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.

This separation of new and old is different from how hubs handle multiple speeds. USB 2.0 must handle devices operating at 1.5 Mbits/s, 12 Mbits/s, or 480 Mbits/s. A USB 2.0 hub needs to switch speeds as different devices are used.

A USB 3.0 hub essentially keeps the USB 2.0 traffic on one set of wires and runs full-duplex, high-speed traffic on the other set. The big difference is that the USB 3.0 hub can transfer data at super speeds at the same time that USB 2.0 traffic is being transferred.

The latency of the slower device will limit a USB 2.0 system with a mouse and hard drive on the same USB hub. The hard drive transfers data at full speed, but it needs to wait until any mouse data has been transferred.

PROTOCOLS AND DEVICE CLASSES

USB 2.0 uses a half-duplex, host-based connection. The host still controls USB SuperSpeed, but the full-duplex connection enables commands and data to flow in both directions. It also allows concurrent transactions to occur. There is now distributed end-to-end and link level error detection and recovery as well.

A USB 2.0 device can support up to 32 pipes: 16 in, 16 out. Pipes are a logical connection. USB 3.0 ups the count to 65,533. USB 2.0 has stream and message pipes that support isochronous, interrupt, bulk, and control transfers. USB 3.0 keeps these transfer modes, although the bulk mode is expanded to support streams.

Pipes are connected to endpoints, which can be grouped into function units called interfaces. Endpoint 0 is special and is dedicated to the overall device. It is used to query and control the physical device. This is how a USB device provides vendor information.

The logical interfaces can contain any number of endpoints based on the type of interface. Packets are based on 8-bit bytes. They include a packet identifier, a destination, data payload, and a cyclic redundancy code (CRC). Packets include a destination device and endpoint number. Packets can be designated as IN, OUT, and TOKEN. IN packets request data while OUT packets contain data for the endpoint.

USB classes provide a standard definition for interfaces and their higher-level protocols. Standard classes include human-interface devices (HIDs), such as keyboards and mice, as well as mass storage. The video class handles Web cams. A class can be a device class, interface class, or both. The USB hub class is a device. The communication class, which handles modems and Ethernet adapters, is both.

Also, USB classes permit a standard device driver to handle any USB device of the designated class. It allows a single device driver to handle a wide range of products from various vendors such as flash memory drives. A vendor-specific class enables device-specific interfaces, which require a custom device driver.

USB 3.0 CHANGES

USB 3.0 brings a number of improvements, including power management (see “USB 3.0—The Next-Generation Interconnect”). The hubs can now deliver up to 150 mA to unconfigured or suspended devices versus 100 mA for USB 2.0. Likewise, the upper limit on power per port is 900 mA versus 500 mA. This should make hard drives easier to contend with, eliminating the special cables used to scavenge extra power from another USB port.

A Powered-B connector includes two additional wires to supply up to 1000 mA to a device associated with USB devices, such as a wireless USB adapter. USB 3.0 devices also now have four power-down states:

• U0: operational
• U1: link idle, fast exit, phase-locked loop (PLL) remains ON
• U2: link idle, slow exit, PLL may be OFF
• U3: suspend

USB 2.0 has states U0 and U3 only. U3 requires the least amount of power. The intermediate states trade lower-power operation for a faster startup time. Handshaking between hubs and devices is more effective and progressive for better power savings across the entire system. USB 2.0 devices are still limited to their usual mode of operation, even when plugged into a USB 3.0 hub. Isochronous transfers with USB 3.0 devices can take advantage of these states between service intervals.

Another power-related change is the elimination of broadcast mode and polling or rather a change to directed broadcasts. USB 2.0 would poll devices on a regular basis. It would also use a broadcast mode to communicate with a device. A hub would forward a broadcast packet to all devices, requiring them to be active and consuming power.

USB 3.0’s directed transmission only goes to the desired device, allowing hubs and devices not involved in the transaction to remain in their existing, possibly power-down, state. Mixing a USB 2.0 hub below a USB 3.0 hub may result in broadcasts from the USB 2.0 hub. Asynchronous notices have replaced polling. Overall, USB 3.0 should be more responsive and power-efficient in addition to supporting a higher bandwidth.

USB 3.0 HOSTS, HUBS, AND BRIDGES

USB 3.0 showed up first in high-performance motherboards like Gigabyte’s GA-X58A-UD7 ATX motherboard (see “Gigabyte’s Core i7 Motherboard”). MSI’s 890GXM-G65 motherboard supports the latest six-core AMD Phenom II X6 processors in addition to USB 3.0 and SATA III.

The motherboards may include the USB 3.0 controllers in the south bridge or one of the multifunction peripheral controllers. Alternatively, a USB 3.0 host controller like the Renesas Electronics µPD720200 can be connected to the host via its PCI Express x1 link.

The µPD720200 has its operational registers mapped to the PCI memory space, giving the device direct, full control over the interface. The chip’s serial peripheral interface (SPI) is used to access an external SPI flash memory. It runs off 3.3 V and a 24-MHz crystal, and it’s available in a 10- by 100-mm, 176-pin plastic fine-pitch ball-grid array (FBGA) package.

The Texas Instruments TUSB9260 SuperSpeed USB 3.0 to Serial ATA Bridge (Fig. 3) is likely to be on the device side since hard-disk drives will initially be the most common USB 3.0 devices. It offers an Arm Cortex-M3 microcontroller in addition to a SATA controller and a full USB 3.0 interface.

Physical layers (PHYs) for all three connections (SATA, USB 2.0, USB 3.0) are on-chip. The SPI can be connected to an external flash device for booting, or firmware information can be delivered via a USB HID interface. The SATA advanced host controller interface (AHCI) host bus adapter can be used after the firmware is running.

PLX’s OXU3102 takes a more conventional approach with its USB 3.0 to Dual SATA RAID Controller with Encryption. The system loads its configuration from an SPI flash memory. The RAID controller handles RAID 0, 1 and JBOD configurations. The on-chip cypher engine supports 128- and 256-bit AES encryption. There are 16 GPIO ports as well.

USB 3.0 ACCELERATES STORAGE

Full-duplex operation is just one of the changes with USB 3.0. Another is the addition of a bulk transfer mode and a bulk stream mode specifically designed to improve overall transfer speeds with devices such as hard drives. USB 2.0’s half-duplex operation and block size imposed a significant burden on the system, so hard-drive transfers were never close to the transfer rate of 480 Mbits/s.

The bulk transfer mode block size has been increased to 1024 bytes. The bulk stream mode, which is new to USB 3.0, supports multistream transfers. USB 3.0 hard drives are now available. As with most USB 3.0 devices, these hard drives will operate with USB 2.0 hosts and hubs. Of course, the transfer rates will be significantly slower, but the data will be accessible.

Western Digital’s My Book 3.0 comes with an optional PCI Express Gen 2 x1 adapter and a USB 3.0 Micro-B cable (Fig. 4). The PCI Express adapter is designed for motherboards that lack native USB 3.0 support.

Seagate’s USB 3.0 BlackArmor PS 110 is a 7200-rpm, 500-Gbyte hard drive. The drive is only 12.5 mm high. Like the Western Digital drive, Seagate recognizes that upcoming transition period. Seagate’s drive targets laptops, so it includes an ExpressCard-based USB 3.0 adapter (Fig. 5). The USB 3.0 cable can be used alone with the adapter or a PC equipped with a USB 3.0 host controller. The extra USB power-scavenging cable is needed when a USB 2.0 controller and cable is used.

The preloaded software from Acronis targets Windows users. It provides automatic data protection with full-system backup. Also, it includes a copy of SafetyDrill+ recovery (Bare Metal Restore).

USB 3.0 is not without its challengers. SATA drives are one of its primary rivals, and eSATA is gaining traction. There are now eSATA connectors on many motherboards that provide power, which is an issue when using a basic SATA cable. A native SATA connection essentially eliminates the need for the USB controller and its overhead.

The eSATA cables are limited to 2 m, while USB is 3 m plus hubs. USB 3.0 also provides flexibility that the point-to-point eSATA connection lacks, so both are likely to coexist. In fact, many motherboards are cropping up with eSATA and USB 3.0 connectors on the back panel.

USB 2.0 and USB 3.0 are likely to coexist for a decade. Devices such as hard drives are likely to migrate quickly, but cost and host availability will initially be a consideration. The SuperSpeed bandwidth remains overkill for low-speed devices. But eventually, the cost considerations will disappear as volumes ramp up and USB 3.0 costs fall. 

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