Free up that phone: Use a West Bridge architecture to multitask effectively

The emergence of these new features has also created a growing need to connect a phone to a PC. A USB connection now provides a multitude of functions, including syncing with a PC desktop, transferring media, charging a battery, and transmitting modem data. While the expanded functionality of a USB connection brings many user benefits, one of the largest drawbacks of many phones today is the difficulty or inability to perform multiple functions on a phone while it is connected to a PC.

For many phones, a PC connection does not provide the ideal user experience. Upon connection to a PC, users are commonly required to select the actual operating mode from a menu on the phone, which typically includes the different possible connection scenarios, such as mass storage, modem, or sync. Asking the user to select the connection mode in this way adds complexity to the user experience. A user would rather the device just work when connected to a PC.

In addition, many times selecting a particular connection mode disables other phone modes. Imagine the frustration of a user who connects to the PC to update his or her media collection and is suddenly unable to make or receive phone calls. Finally, switching modes while connected to a PC can also require the user to unplug and plug the phone from the PC, and in some cases, even reboot the phone.

Clearly, the added functionality of phones today is posing a new challenge for phones. To understand where these limitations come from, it is useful to examine the typical architecture of a mobile phone.

A common configuration in mobile phones is to have a single digital baseband processor performing most of the phone functions, with separate analog RF and power management components connected to the baseband processor. The USB controller and software are commonly also integrated within the baseband processor. The baseband processor handles all of the communication between the PC and phone, as well as running applications and phone functions. Figure 1 illustrates this architecture.

The first limitation that may arise with this architecture is the availability of USB endpoints within the processor. USB endpoints, which reside on the device, are required to establish a series of unidirectional connections from the device to the host. The more endpoints available on the device, the more pipes or logical channels that may be set up for communication between the device and host. Common protocols such as mass storage or CDC for modem usage typically require two to three endpoints each. Therefore, for a device such as a phone connected to a PC, multiple groups of pipes would be required to support simultaneous usage of different functions. Table 1 lists some of the common configurations desired in a phone and the number of endpoints required to support each.

It's evident that for a phone to support multiple functions while connected to a PC, upwards of 12 or more device endpoints may be required. Baseband processors and standard USB controllers, many of which were developed at a time when USB was used for only one function, often do not have enough endpoints to support all desired functions. If sufficient endpoints are not available, then the device will be required to re-enumerate and re-configure the endpoints for different use. For a user, this may involve unplugging and plugging in their device, or have the device reboot and re-establish a USB connection in order to switch phone modes.

The second limitation that may arise is the limited availability of processor power. In the phone architecture discussed above, the baseband processor is handling all of the various functions of the phone, including USB. Oftentimes, if the phone is configured to perform some level of multitasking, the performance of each function will suffer. Data transfers may be interrupted if a phone call is received, or data transfer performance may degrade significantly if the phone is performing other functions while transferring data. The phone processor is not able to adequately support various multi-tasking scenarios.

One resolution to these limitations is to use a bridge architecture. Similar to the North and South bridge architecture in the PC industry, a bridge chip can offload processor load, and provide support for peripherals not natively supported by processors. In the embedded world, a West Bridge architecture has been developed to address the need to quickly add support for new peripherals and communication protocols. The West Bridge aims to offload processing from the main processor while maximizing the performance of attached peripherals. A device such as Cypress Semiconductor's West Bridge Astoria is a controller incorporating a Hi-Speed USB controller along with mass storage support. In a West Bridge architecture, not only is all of the USB communication handled by West Bridge, but a direct path to mass storage is also offered, thus offloading both USB and mass storage control from the baseband processor.

While standard USB controllers may incorporate four to six endpoints, a West Bridge device offers up to 16 endpoints for communication with a host device. Several configurations may be allocated across the endpoints, and standard Windows drivers may be used for communication. From a user perspective, this enables all desired phone configurations to be available and presented for use on the PC without the need to select a certain mode on a phone or reboot the device. A multi-functional phone may be presented as multiple devices, enabling a multitasked experience for a user.

In addition to merely enabling multitasking on a phone, it is necessary to also provide the best user experience possible. With a standard phone architecture, if multitasking is even offered, performance of functions may diminish when several are done simultaneously. By offloading key functions from the main processor, a West Bridge architecture enables the best performance and user experience when performing multiple functions. To illustrate, a simple test was performed with both a phone utilizing a West Bridge architecture and a phone utilizing a single processor architecture with Hi-Speed USB support integrated within the main processor. Each phone was connected to a PC, and a large media file was transferred via USB. While the file was transferring, playback of a different media file was started on each device. The performance of the USB transfer was captured using USB monitoring software, and is shown in Figure 3.

The resulting test data illustrates a few important points. The first most apparent observation is that the USB data transfer performance of a phone without a bridge degraded significantly during video playback, dropping by over 60% on the standard architecture phone. Contrast this to the phone with bridge architecture where USB transfer performance stayed consistent. The bridge-enabled phone was also able to perform multiple functions without a loss in performance of both USB transfer speed and in media playback quality.

The second observation is that the overall side-loading performance of the bridge-enabled device is much higher than that of the comparison phone. While both phones were Hi-Speed USB enabled, the West Bridge enabled phone was able to transfer media at over 15MB/s, while the comparison phone peaked at just under 3MB/s. By offloading the USB protocol from the processor, and by offering a direct transfer path from USB to storage, maximum USB side-load performance is achieved. From a user perspective, this means shorter wait times and a better experience when syncing files with a PC. A West Bridge architecture would enable the transfer of a 1GB file in under 1 minute whereas the standard phone would take close to 6 minutes to transfer the same file.

In this day and age of podcasts and TV episodes being offered on cell phones, users are syncing data with their PCs more and more often, which is placing a greater importance on high performance data transfers.  The advent of smartphones and media-centric phones has opened the door to many new functions for a phone, many of which a user may want to perform simultaneously. With the limitations discussed, it is clear that previous phone architectures were not designed with multi-tasking in mind. Today, it is simply unacceptable for a user not to be able to use their phone when connected to a PC.  With the new consumer demands on phone usage changing, new phone architectures, such as a West Bridge configuration, are becoming increasingly more important.

About the Author

Timothy Kung is a product manager in the Data Communications Division at Cypress Semiconductor.  He has a BASc (Bachelors of Applied Science) from the University of Waterloo, in Waterloo, Canada.