Overcoming UTMI interface limitations in USB-enabled handsets

With this in mind, handset processor vendors are beginning to integrate USB into the base band and applications processors. Today most processors integrate full-speed (FS) USB (12 Mbps), which is sufficient for small data transfer such as address book contact synchronization. With the addition of features such as MP3 players and high-resolution digital cameras, FS USB no longer provides sufficient throughput. Consumers are spoiled by the high-speed (HS) USB (480Mbps) transfers they enjoy with their dedicated PMPs and DSCs and are quickly disappointed by the experience of transferring MP3s and pictures to PCs using only their handsets' full-speed USB connection.

An example of the difference can be seen by comparing two commonly available handheld devices — the first supporting high-speed and the second supporting full-speed USB — transferring a fixed amount of data from a host PC to each device, and recording the time it takes for the transfer to complete. It takes a high-speed USB device approximately 33 seconds to transfer 105 MB of data from the host PC to the handheld device. The full-speed USB device takes almost 13 minutes to perform the same transfer! With cutting-edge flash-based handheld devices currently supporting 8GB of data storage, it could take over 17 hours to update files using full-speed, versus 44 minutes using high-speed USB. Hard disk drive (HDD) based handheld devices support up to 80GB, requiring an increase of 10 times to 170 hours using full-speed versus 7.3 hours (440 minutes) using high-speed USB. While users may not want to upload or download this amount of data every time the handheld device is connected to the PC, the example serves to illustrate the disparate consumer experience when using full-speed versus high-speed USB.

Handset processor vendors are reacting to HS USB's pull and are integrating HS USB directly on chip. However, they are integrating USB only as far as the Serial Interface Engine (SIE). While FS USB can be realized in digital-only circuitry, HS USB achieves a much higher data rate (40x) and therefore requires an analog PHY. As handset processor vendors continue to cut costs by moving down the technology geometry curve, they run into problems integrating the analog PHY because analog circuitry does not scale as well as digital with each shrinking technology node. Processor vendors are therefore choosing to omit the PHY and exposing standard interfaces such as UTMI (USB 2.0 Transceiver Macrocell Interface) and ULPI (UTMI+ Low Pin Interface) for use with standard USB PHYs.

The UTMI interface consists of between 22 and 26 pins, depending on whether an 8- or 16-bit data bus is implemented. This translates to a requirement for 22 or 26 processor GPIOs when considering all data and control signals. Although newer generations of processors are increasing the number of available GPIOs, the requirements on GPIOs to support an ever increasing feature set far outweighs the number of GPIOs available. This forces designers to pick and choose the features they need to add, or come up with creative ways to reuse these limited GPIOs.