The importance of a bridge architecture in tablets

The introduction of the Apple iPad and its initial successful adoption in the market has triggered the introduction of a whole new class of similar consumer devices. These devices, popularly known as tablets or Mobile Internet Devices (MIDs), come in various screen sizes ranging from 5” to 10”, each trying to provide the perfect user experience.

The tablet market segment was conceived for a richer multimedia experience. Today, the most popular use cases of such tablets are:

Mobile Internet device for Data Manipulation, spreadsheets, e-mail, browsing, etc.

Large screen portable media player (capable of playing HD video, sometimes up to 1080p)

E-book reader

Electronic Magazine / newspaper content delivery channel

Personal Gaming Device

Photograph viewing device

Portable Digital TV

It is evident that the key to the success of such devices is the ability to deliver rich multimedia content efficiently and reliably. However, most tablets fail to meet these expectations in terms of the time it takes to download data and the associated power consumption.

USB transfers, as known as side loading, still remain the most preferred method of transferring large data files to these tablets as they are reliable and free.  3G connectivity and Wi-Fi are not preferred for heavy file transfers because they are not reliable for long periods of connectivity, and the days of flat rate plans for unlimited data transfers are numbered. Telecom carriers are fast realizing that big money lies in data services and are starting to charge more for higher data bandwidth users. 

The most important characteristic of file transfers after reliability is how long they take.  For example, if it took 15 minutes to upload an HD movie to a tablet, many users might hesitate to use that feature often. This might eliminate a major incentive for consumers to purchase a tablet. In spite of this, most tablets have very poor side-loading speeds. As most tablets do not charge while syncing with a PC, slow side loading also means faster battery drain and shorter operating life.

Eliminating data transfer bottlenecks for faster side-loading

Typically in tablet designs, the HS-USB 2.0 and Storage controllers are integrated into the System Processor (SoC). In these SoCs, the side-loaded data is often buffered in the SDRAM (see Figure 1a).

Today’s Architecture

However, this puts the side-loading process in contention for the already busy SDRAM. The SDRAM is already being constantly accessed by the graphics engine, MP3 decoder block, and the microcontroller for code and data.  Because of these bus contentions, one can imagine the “internal processor pipe” between the ~30MBps USB pipe and the ~25MBps storage pipe being the bottleneck at ~4MBps bandwidth (see Figure 1b).  This is the primary reason for slow side-loading transfer speeds and thereby a poor user experience.

Consider a 3-port “West Bridge” device (see Figure 2).  A West Bridge is analogous to the South Bridge in the PC-world and serves as a bridge between the processor, storage, and USB port.

Including such a bridge in the system architecture of an embedded system eliminates many side-loading issue. A typical West Bridge has a processor port, storage port and USB port. It also supports DMA transfers between the USB and Storage to avoid placing any load on the processor. Thus, the West Bridge behaves like a fat pipe between the USB port and the Storage (see Figure 3). With such a bridge, it is possible to achieve up to 25MBps transfer speeds between USB and storage.  Such devices are already available in the market from multiple vendors.

Bridge architecture provides optimized data transfer and processor offload.

Processor off-loading
The bridge architecture not only offloads the main SoC processor while side loading, but also can also behave as a Master controller on its storage port and take care of all the various connectivity protocols.  For example, if the Storage port supports SDIO extensions and SPI protocols, this controller further offloads functions from the main SoC like Storage, Wi-Fi, and Bluetooth control, allowing the processor to provide better User Interface responsive.