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Hub lets hosts easily share USB peripherals

Posted: 16 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Dave Bursky? EE Times? USB2524 MultiSwitch hub? Standard Microsystems Corp.? USB peripherals?

Sharing a USB-based peripheral between two computers has traditionally meant unplugging the USB cable from one computer and plugging it into the other, or using an A/B switch that connects both computers to the peripheral. But when more than one peripheral must be connected to multiple computers, juggling the cables becomes a headache.

Eliminating that migraine and providing more flexibility than an A/B switch, the MultiSwitch USB hub developed by Standard Microsystems Corp. allows users to connect and share multiple USB peripherals without having to touch a cable. A collection of peripherals, such as a mouse, keyboard, printer and scanner, can be switched en masse or selectively from computer to computer (between a laptop and desktop, for example), with more flexibility than is possible with a two-channel KVM switch.

The USB2524 MultiSwitch hub contains four USB downstream peripheral ports and two upstream USB ports to connect to the computers. All ports comply with USB 2.0 specification and can operate as full-speed or full/high-speed ports. The hub appears as a standard USB hub to either host, and requires no drivers and no software installation.

The four downstream ports can be individually programmed to stay connected to the computer linked to the first upstream port or, alternatively, to switch to the other computer when that computer is turned on or connected to the second upstream port.

Ports can be switched using mechanical or electronic switches, or they can be electronically controlled by the host systems. Three select lines can be decoded to set eight default or programmable configurations, or four port-assignment lines can be used to set the host assignment directly for each downstream port. Configuration options can also be pre-assigned using a low-cost serial I2C EEPROM or SMBus slave port.

If more than four peripherals are needed, a standard USB hub can be plugged into any of the downstream ports to allow multiple peripherals to connect to a single USB port. All peripherals attached to the standard hub, however, will switch simultaneously to the second computer when the port on the MultiSwitch hub is switched to that second host.

An on-chip 24MHz crystal driver and an internal PLL allow the circuit to handle the 480MHz USB 2.0 sampling rate. To reduce the number of external components needed to support the chip, SMSC's designers integrated USB termination and pull-up and pull-down resistors. Internal short-circuit protection for the USB differential signal pins has also been integrated on the chip.

Future implementations might include the use of wireless USB connectivity between the MultiSwitch hub and the host systems--eliminating the need to insert a laptop into a docking station or even to plug in the upstream USB cable, for example.

Samples of the USB2524 are immediately available. It comes in a 56-lead 8-by-8mm QFN package and sells for $3.75 in large quantities.

- Dave Bursky
EE Times

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