Electronic Design

USB Branches Out

SMSC's controller lets hosts share USB devices, while Newnham research's controller links displays via USB.

USB has long played the role of a single-host, peripheral link for mice, keyboards, and flash drive dongles. With the arrival of two new chips, however, USB will be turned on its proverbial head.

Standard Microsystems' (SMSC) USB2524 lets two hosts control USB devices (Fig. 1). Newnham Research's USB-NIVO links any VGA display (Fig. 2). The video device driver in the PC drives the video adapter in the USB-NIVO module. Alone, each of these devices is likely to attract attention. Together, they will change how devices are used.

SMSC's chip will find its way into a number of embedded devices, such as docking stations and printers. It also will show up in advanced USB hubs. And, don't be surprised to see a USB2524 provide network-like access to shared devices (e.g., external hard disks).

The USB-NIVO answers one of the major complaints about monitors: "How do you hook it up?" There's no need to add an internal adapter card to a PC. Simply plug in a USBNIVOequipped adapter and connect it to a monitor. It even works with a laptop. Just try getting a laptop to drive its display and two independent monitors. It doesn't work without a USB-NIVO.

These announcements are very important for two reasons. The first is USB Hi-Speed's 480-Mbit/s bandwidth, which is high enough so that everything from video to hard disks can take advantage of it. Second, there's a push to eliminate non-USB interfaces, especially from laptops. With these products, all a laptop will need is a USB connector. Docking-station connectors and video connectors will go the way of parallel port connectors—gone for good .

The SMSC USB2524 closed up is a typical USB hub chip with four downstream ports (Fig. 3). It differs at the other end because of the extra upstream host connection. While the logic behind this is simple, the execution isn't because of the 480-Mbit/s bandwidth that may be coming down the pike from either host.

Luckily, the hub's job is a little simpler than an Ethernet switch, because each USB downstream device link is associated with only one of the upstream host links. The combinations that can occur are programmable.

Once connected, though, the logical link between a host and device remains until an event occurs. Events include the removal of either the host or device via disconnection or by powering down. Or, the links can be broken via software or hardware switches.

Probably the best feature about the USB2524 is that its dual-mode operation is effectively transparent to either USB host or any of the USB devices. Plug in a USB host or device, and the USB2524 makes the appropriate association. The association between host and devices can be fixed. For example, host A may have control over device 1 and 2 while host B has device 3 and 4.

Things can get a little more complex, too. Host A may have control of all devices until host B is activated. At this point, the USB2524 logically disconnects host A from a set of previously configured devices, and these devices are associated with host B. When B is removed from the environment, A gets access to all of the devices again.

The standard USB connect/disconnect protocol easily handles the handoff between host A and B. First, A's devices are disconnected and then connected to B. This means host A will properly shut down its associated drivers, and B will start up its drivers for the newly associated devices.

Jumpers can handle the control and associations of the USB2524 via outofband microprocessor or in-band host control. The four shared device ports can be expanded using multiple USB2524s by connecting a pair of USB2524 device ports to the upstream ports of another USB2524.

Three USB2524s can provide switched access to eight devices. Of course, conventional USB hubs are usable, too. But only one host will be able to access the devices supported by this hub.

Overall, the USB2524 is a simple device. But the implications of its features are significant, as are its number of possible uses. For instance, it may find use in a docking station that allows a laptop to gain access to devices connected to a desktop PC—and it doesn't disconnect the PC from the devices.

Another potential use would be inside a printer that has a built-in hard drive or sockets for flash memory. In this case, a USB2524 would be used instead of putting a microcontroller in between the USB host and device, enabling data to move at USB Hi Speed rates instead of being limited by the microcontroller.

A development kit is available. Pricing for the USB2524 starts at $3.75. Configuration-is possible using switches, I2C serial memory, or an SMBus interface. In addition, the chip integrates termination and short-circuit protection.

Newnham Research could change the way video is connected to the PC, simply for the fact that its method doesn't require hardware modification. Each device connected via a USB-NIVO will have its own video device driver, much like each USB device having its own driver (Fig. 4). The difference is that the interface portion of the operating system uses the video drivers.

Another difference is that a conventional video driver communicates directly with the hardware. This makes sense for running high-speed games over an x16 PCI Express link, but most applications don't require this level of performance.

In this case, the USB speeds are more than effective since the USB-NIVO driver compresses the changes made to the logical display at the device driver end. The USB-NIVO device receives the changes and adjusts its internal display memory accordingly. The attached monitor displays the results.

This compression mechanism reduces the bandwidth required for video update, as well as reduces the latency. Typical latency is on the order of 12 ms. Viewers usually will only notice the delay if it hits 120 ms. This should provide plenty of leeway, even when the USB connections are shared with other devices, such as an audio stream or mouse.

The USB-NIVO can handle resolutions in excess of the 1280 by 1024 found on today's monitors. The unit supports a 75-Hz refresh rate. Nonetheless, the USBNIVO won't compete with conventional in-system video adapters for high-performance games, such as first-person shooters. But it could be an ideal adjunct to the main screen. For instance, many games now can display their status screens continuously using a USB-NIVO display because the update frequency tends to be lower.

The USB-NIVO has no trouble keeping up with less demanding displays, such as office applications. It's an ideal platform for providing a half-dozen displays in applications such as stock trading and security-or patient-monitoring systems. Many smaller displays often are less expensive than larger displays.

Connectivity is improved via USB. Hubs can add distance between the video source (the PC) and the monitor. New wireless USB hubs supply even more flexibility without the complexity of VGA-style wireless connections. A monitor with a single USB connection leads to some interesting configurations. For example, the USB link can be used to drive a monitor with speakers, and possibly a Bluetooth adapter, to connect a wireless keyboard and mouse to the PC. In the future, monitors may come with a built-in USB-NIVO, plus a DVI or VGA interface.

Video drivers will be available for Windows and Linux platforms. Pricing isn't set for the chip or module yet, but a retail version of the USB-to-VGA adapter is expected to cost about $79. Modules and developer kits will be available soon. One feature of USB-based video via USB-NIVO is that most operating systems recognize when devices are added and removed. In the future, monitors may have to recognize their neighbors, so that the video desktop expands automatically when new units are plugged in.

Combining the USB-NIVO approach with SMSC's switching hub offers some interesting options as well. Possibly, two hosts could vie for control of the monitor with a built-in USB2524.

See Table 1
See Table 2

Newnham Research
Standard Microsystems

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