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BEING MERRY WITH USB

by Tom Cantrell

Start ý Pride Before the Fall ý Universal Sales Booster ý The Mouse that Roared ý Wire Wars ý Kiss the Cook ý Sources and PDF

WIRE WARS

Even if the old ports start to disappear, it doesnýt mean the new configurations will be more rational or simpler than the old. The real advantage will be more speed and ubiquitous Internet enabling. Otherwise, the need to move on is mainly a result of how badly outdated, overworked, and un-plug-and-play the old setup is.

It would be nice to imagine that the current incarnations of Ethernet (10/100) and USB (1.1) could suffice. Indeed, I expect most designs will be able to make do with one, the other, or both in the near future. The tough questions emerge when you contemplate going one step beyond. Options include IEEE-1394 (a.k.a., Apple FireWire and Sony iLink), 1-Gbps Ethernet, and now with the recent official blessing of the spec, USB 2.0.

Although Ethernet 10/100 and USB 1.1 are obvious, the prospect for the next generation high-bandwidth contenders are far less clear. In this case, the outcome of the battle will be determined as much by marketing issues as bit and byte technical differences.

For USB 2.0, the story is simple: speed, as in a whopping 480 Mbps! Yet, 2.0 remains compatible with existing V.1.1 cables, connectors, and devices, which is no mean feat.

Itýs achieved by putting the burden of the upgrade in the USB 2.0 hubs, which must be more sophisticated than their 1.1 counterparts. While a 1.1 hub is rather passive, acting as little more than a switch, a 2.0 hub must be capable of speed matching (e.g., downshifts high-speed traffic on the host side to and from a low-speed attached device). This calls for a more sophisticated hub design with extra processing power and larger memory and FIFO buffers.

Thus, the only real user concern for a 2.0 upgrade is to ensure a full-speed path exists between 480-Mbps devices and the host (see Figure 2). Indeed, the designers have built detection of nonoptimal configurations (i.e., a full-speed 2.0 device connected to a 1.1 hub) into the protocol, allowing software to advise the user accordingly.

Figure 2ýUSB 2.0 must be an easy and compatible upgrade with 1.1, and 2.0 hubs bear much of the burden of making that happen.

The fact that USB is now ubiquitous means the upgrade to 2.0 should require less bootstrapping in the market than 1.1 did. Just as with 1.1, full-fledged takeoff will only occur after 2.0 is built-in to PCs and Windows, something which apparently wonýt happen for a year or two. The difference with 2.0 is that there are incremental upgrade alternatives, such as add-in cards and 2.0 hubs, unlike the all-or-nothing proposition 1.1 faced. Thus, the first generation of USB 2.0 chips to emerge look more like Ethernet than mouse chips. Notably, they are not designed for stand-alone operation, but rather to connect with a processor, be it the ýx86 brain in the PC or a powerful processor in a full-speed hub or device.

The Philips ISP1581 (see Figure 3) is a good example of the 2.0 breed, with a PLL 40x clock multiplier (i.e., 480 Mbps from a 12-MHz crystal), large on-chip memory/FIFO (8 KB), a high-speed 16-bit data bus with DMA, and a built-in USB transceiver.

Figure 3ýThere are still D+ and Dý pins, but otherwise, the new USB 2.0 chips like this ISP1581 from Philips, are a far cry from their 1.1 mouse and keyboard class counterparts. (click to enlarge)

The NetChip NET2290 (see Photo 2) is similarly high-end and features a 32-bit DMA local bus interface with programmable modes (Intel asynchronous, Hitachi SH3 and 4, and MPC860) that blasts data at up to 200 MBps.

Photo 2ýUntil it migrates onto the motherboard, a plug-in card (such as this one based on the new NetChip NET2290) is the way to evaluate and develop USB 2.0 designs.

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