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The success of a mobile handset design depends on much more than just its appearance, battery-life, and reliability. Viable architectures must also have the ability to adopt new usage models and still meet, if not exceed, accepted industrial benchmarks. The convergence of a cell phone and other consumer electronic devices, such as PDA, PMP, and digital cameras, is progressing rapidly as seen with recent introduction of such handsets as the Sony Ericsson Walkman W950i and Nokia Nseries. System architects must cram more features into mobile handset architectures in an attempt to increase the chance of their product's success.
Many of these "convergence" handsets, however, often fail miserably in terms of providing a positive consumer experience. The downfall of highly integrated handsets can be attributed to many different factors, but poor usability is usually the most likely reason why some handsets do not attract as many consumers as they might have otherwise. Usability does not only imply the ease-of-use of a particular product, but also how "well" each feature performs in comparison to other products on the market that provide similar functionalities.
The quality of the integrated features is often overlooked. One example is the infamously low-quality digital cameras found in many cell phones. System designers need to think of each feature as more than just a feature box to be checked-off on the system specification. For the integration of the feature to be worth the effort, attention must be paid to the consumer experience that the integrated feature is supposed to provide.
In the consumer electronics world, multimedia handsets that support music/video playback and have integrated cameras are quickly moving from being classified as high-end to mid-end and even sometimes low-end handsets. Multimedia handsets will continually evolve to support larger mass storages over time, especially with the cost of flash memories declining at an amazing rate. Having more available mass storage enables handsets to support higher bit-rate media content as well as higher mega-pixel camera modules. In today's market, 1GB of memory can be considered as a single "increment unit" in mass storage density, and multimedia handsets are expected to support 1GB+ of storage space. This trend is validated with the recently released SD2.0 specification, where the maximum SD card size defined is at 32GB. Apart from SD/MMC cards, other popular mass storage devices include embedded NAND and HDD.
It is extremely important, then, for a multimedia handset to provide a fast external connection for transferring large amounts of media content such as music, movies, and pictures. This connection is most commonly established by a USB link with a PC. While the majority of mobile handsets on the market today still only support USB1.1 (Full-Speed USB at 12Mbps), it is quickly becoming a necessary requirement to support the faster USB2.0 link (High-Speed USB at 480Mbps). Table 1 presents an overview of the two specifications, as well as some example transfer times calculated with the raw USB signaling rate (discounting SW overhead). Figure 1 shows a typical handset baseband processor architecture. Note that such an architecture only integrates the FSUSB SIE (Serial Interface Engine) and that an external FSUSB transceiver is still required.
Figure 1. Mobile Handset with Processor Integrated FSUSB
Data presented in Table 1 reflects the raw signaling rates of FSUSB and HSUSB, and as illustrated, their difference is reflected as an order of magnitude (HSUSB is 40 times faster than FSUSB). It should be noted that these numbers do not account for any software overhead and hardware limitations; thus, the effective sustained throughput in real-world systems is expected to be noticeably lower.
At first glance, one may assume that implementing HSUSB will automatically increase the bandwidth to effectively resolve the slow transfer speed between mass storage and PC. The fact is that the realistic data throughput in an USB link is also strongly dependent on the system architecture, which determines the amount of software/hardware overhead and limitations.
Figure 2. Mobile Handset with External HSUSB Controller
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