[Design View / Design Solution]
Upgrade To High-Speed USB Handsets Without A Complete Redesign
Migrating to high-speed USB in stages can reduce the risks associated with completely redesigning a handset’s USB subsystem.
Cellular-service providers continually strive to make more features available to their subscribers, features that will increase the average revenue per user (ARPU) in a market reaching saturation.
Over the last few years, handsets have integrated digital-still-camera (DSC) functionality. Now, most of the handsets found at your local store have cameras equipped as standard. The idea behind this is that subscribers will take pictures and share them with their friends using airtime in the process, thereby creating new revenue streams for the cellular-service providers.
The next trend that’s begun to take off in the last couple of years involves the integration of portable-media-player (PMP) functionality into the handset. As a result, cellular-service providers can charge for music and video content—as well as for airtime—as subscribers download their favorite media using the network.
As mobile handsets continue to integrate features like higher-resolution digital cameras, PMPs, and PDA functionality, subscribers need a convenient method of transferring files to and from the handset and their PCs. The ubiquity of USB—the standard method of data transfer employed in MP3 players (PMPs), digital still cameras (DSCs), flash drives, hard disk drives, etc.—makes it a top candidate.
FULL SPEED VS HIGH SPEED USB Today, most handsets support full-speed (FS) USB (12 Mbits/s), which is sufficient for small data transfers like address-books. By adding MP3 players and high-resolution digital cameras, FS USB can no longer cut it, considering the large amount of data that now need to be moved between the handset and PC. Consumers are spoiled by the high-speed (HS) USB 480-Mbit/s transfers they enjoy with their dedicated PMPs and DSCs. Having to resort to full-speed transfers when transferring MP3 and picture files will prove to be a disappointing experience.
Compare the difference in device-to-PC data transfer between a HS USB and a FS USB, It takes the high-speed USB device approximately 33 seconds to transfer 105 Mbytes of data from the host PC to the handheld device. The full-speed USB device, on the other hand, takes almost 13 minutes to perform the same transfer.
With cutting-edge, flash-based handheld devices currently supporting up to 8 Gbytes of data storage, it could take over 17 hours using full-speed, versus 44 minutes using high-speed USB to transfer that amount of data. Cutting-edge, hard-drive-based handheld devices support 80 Gbytes; therefore, the transfer time would increase by 10 times to 170 hours using full-speed versus 7.3 hours (440 minutes) using high-speed USB.
Today’s handsets use FS USB for a variety of reasons, including diagnostics and manufacturing testability, as well as modem connectivity. The former affords the handset OEM a convenient method of testing handsets on the manufacturing line to ensure quality, thereby minimizing or eliminating field failures. FS USB’s bandwidth is more than sufficient for these tasks. The latter provides a means for subscribers to use their phones as a modem when connected to a laptop PC to provide wireless Internet access. FS USB provides up to 12 Mbits/s of bandwidth, which is enough (at least in theory) to support existing 2G data standards, such as GSM’s GPRS and EGDE, and CDMA’s 1xEV-DO and 1xEV-DO Rev. A, as well as emerging 3G standards like HSDPA and HSUPA.
One key design issue when upgrading features to support HS USB is that one must abandon field-proven software and come up with a whole new software suite. This takes time and resources, two factors that are in limited supply in the fast-moving handset market.
Because FS USB provides enough bandwidth for these functions, handset OEMs are more inclined to keep the existing approach and simply add HS USB support in the form of a HS USB controller or a PHY, to the design. This adds a high-bandwidth pipe for mass storage requirements only, thereby enabling a better consumer experience with respect to integrated PMP and DSC functionality. Such an approach also enables existing platforms to be more simply upgraded to HS USB as needed by adding this support alongside the FS USB. By introducing HS USB in two stages, OEMs can bring HS USB connectivity for multimedia data transfers to market much quicker than would be the case with a complete redesign around HS USB.
Another reason to add HS USB in this fashion is the limited number of endpoints in current HS USB controllers. In PC applications, HS USB controllers generally have a specific application defined and require a small number of endpoints (i.e., as few as 4 or 8 endpoints are sufficient for most applications). Mobile handsets use USB to provide many functions, so the demand for endpoints increases dramatically—12 or 16 or even upwards of 20 endpoints have been suggested. Examples of functions supported by mobile handsets (each requiring single or multiple endpoints) include: Mass Storage, Media Transfer Protocol (MTP), Modem (CDC), Device Management, Object Exchange (OBEX) and Debug/Test. For this reason, handset designers can effectively support more endpoints using both FS and HS USB datapaths than is possible with a HS USB datapath alone.
FULL-SPEED AND HIGH-SPEED TOGETHER So how do you add HS USB functionality to an existing handset OEM design that supports FS USB? Obviously, it can’t simply be added as a separate entity. Otherwise you’d need two mini- or mirco-USB connectors on the handset, which would only serve to increase cost and confusion among consumers. The most cost-effective approach, which also reduces subscriber confusion, is to merge the two USB data pipes together onto a single connector (Fig. 1).
Every designer who has designed with high-speed signals knows that while you might get away with the FS link operating sufficiently, the HS link will never work. That’s because the FS traces act as a stub and antenna to the HS transmission lines, causing a severe degradation in signal quality and a closing of the signal eye. Such a configuration also assumes that both the HS and FS USB outputs can support some kind of tri-state mode to achieve this design (i.e., the HS signals are tri-stated while FS is in operation, and vice versa). This is something that most USB devices don’t support today—such is also the case in traditional applications such as PCs. With PCs, multiple connectors are the norm, so there’s never a need to merge multiple USB signals onto a single connector.
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