Ceramic resonators for high-speed USB

With an increasing number of consumers carrying and transferring large multimedia files such as MP3 audio and MPEG video into their portable devices, leading consumer electronics manufacturers are rushing to design high-speed USB (HS-USB) into their products. Any designer looking at adding HS-USB will immediately run into the common hurdles of optimizing size, power, and cost.

Because of the difficulty of integrating the PHY onto small process technologies, few application processors and baseband processors currently integrate HS-USB, so designers must add a separate HS-USB controller to their designs. One large component that always accompanies a HS-USB peripheral controller is a crystal, which can be replaced with small-footprint price-comparable ceramic resonators that make designing HS-USB into space-constrained applications a less daunting task.

Multimedia and handsets
Cell phones today are equipped with more multimedia capability than ever. Our favorite phones boast cameras up to the 2 to 3-Mpixel range, making them very suitable for capturing photos and videos on the go (no more missing a baby's first steps or first words).

Phones are also doubling as portable media players and consumers can now download music via services like Verizon's Vcast on the road and take them home to their computers. To support these bandwidth-intensive multimedia features, many phones come with the ability to add 2 to 8 Gbytes of expandable storage in the form of flash-based mass storage like Secure Digital or disk-based mass storage like CE-ATA.

This enables users to accumulate a considerable amount of multimedia before offloading onto their computers. New photos, videos, and music are created or downloaded, while older files are moved onto the computer for archiving. With standalone digital still cameras and portable media players, consumers are already accustomed to fast transfers via high-speed HS-USB (480 Mbits/s).

Most cell phones today, however, support full-speed USB (FS-USB) at 12 Mbits/s, which only provides enough throughput for simple synchronization of address book and calendar. To compare the difference between FS-USB and HS-USB in terms of the end-user experience, consider how long it takes to transfer roughly100 Mbits of music, which is 25 songs at 4 Mbits per MP3 file.

FS-USB takes 13 min to transfer 25 songs, while HS-USB takes a mere 33 s. The impact of transfer speeds on the end user experience is clear and with the growing usage model of cell phones acting as both camera and media player, cell-phone designers are responding quickly by adding HS-USB into existing phones.

Integration challenges
There are few application processors and baseband processors on the market that fully integrate HS-USB. Some baseband vendors have no plans to ever fully integrate HS-USB.

With phones packing more features in the same space or less, these processors are quickly adopting smaller process technology. This presents distinct challenges when integrating the HS-USB PHY because high-speed analog signaling does not scale easily.

As a result, the fastest route to adding HS-USB phones today is by either adding a UTMI transceiver or ULPI transceiver to basebands that integrate the HS-USB SIE, or offloading HS-USB control completely to a USB 2.0 HS-USB peripheral controller. Both of these approaches are available on the market today, but one caveat to adding such devices to a cell-phone design is that a 6-, 12-, or 24-MHz crystal is required.

Size constraints
Despite the very different architectures and usage models (see Fig. 1), open any HS-USB device and you will often find one commonality: a clunky crystal measuring 11.35 x 4.65 x 3.5 mm

Given the tight space requirements in cell phones, such a large crystal makes adding HS-USB a challenging task. Fortunately, at least one ceramic resonator solution on the market today measures 3.2 x 1.3 x 1.0 mm, which provides a 90% board area savings and 70% reduction in thickness over the traditional quartz crystal.

While USB is ubiquitous, HS-USB devices may have different tolerances for oscillators. Therefore, it is important to evaluate whether your HS-USB design can take advantage of these small resonators.

Designing with a ceramic resonator
For most HS-USB devices, the basic crystal requirements are driven by the USB specification for bit-rate stability. The clock input is the primary contro