Two keys to success in Tablet PC design

Rising interest in consumer and user-generated mobile video – coupled with growing demand for higher resolution – is creating a challenge for tablet design. Devices will need to handle large amounts of data quickly in order to satisfy consumer expectations. Two keys to meeting this need are enhanced USB connectivity and off-loading data tasks from the CPU.

Video is clearly emerging as a critical application for mobile devices such as tablet PCs. According to networking giant Cisco Systems, 40% of the mobile data bandwidth consumed in 2010 carried video. Further, the company expects that to grow to more than two-thirds of all mobile traffic by 2014.

Much of this traffic is user-generated. YouTube alone is responsible for nearly one-fifth of mobile data traffic. Streaming of consumer or copyrighted content such as mobile television and movies are another source. In addition, users are increasingly downloading full movies for off-line playback onto their tablet PCs.

Increases in video resolution are compounding this growing demand for video data bandwidth. Traditional VGA (640x480) resolution is already giving way to 720p for mobile devices. Tablets, with their larger display panels, will soon need to deliver full 1080p high definition video.

A second critical need for tablet devices is enhanced connectivity. Externally, consumers want to be able to link with home PCs for media downloading and file synchronization. They also want the option of adding keyboards, pointing devices, flash drives, memory cards, and data modems externally. Internally, developers need to provide the applications processor with connections to data ports, multiple radio modems for compatibility with various cellular and data networks, internal flash memory mass storage, and potentially many other peripheral devices.

The importance of video in mobile devices requires that this connectivity have as high a performance as possible. A movie download from a home PC or a WiFi network to a tablet, for instance, requires the transfer of 2 GB for standard definition and as much as 4 GB for a high-definition movie. Consumers have limited patience, so the faster a tablet can complete this transfer the user satisfaction. Users would also prefer to be able to use the tablet for other purposes during the transfer.

USB to the rescue
One key to addressing the video and connectivity needs of tablet PCs is effective leveraging of the Universal Serial Bus (USB). For the external connection, a dual-mode controller would allow the tablet to connect to peripheral devices as a host or to a PC as a device. Internally, a USB hub can serve many of the applications processor’s connectivity needs. USB-based controller IP is widely available for flash disks, data cards, and radio modems, for instance, eliminating the need for the processor to provide separate connections or handle flash and modem interface protocols in software.

USB is particularly convenient as an interface to storage or peripherals handling video information. One of the approved data classes for USB transactions is the Media Transfer Protocol (MTP), which allows the USB link to support data transfers on a file basis. The traditional USB mass storage device class (MSC) works with fixed blocks, which may not be a match to the logical file size. Further, MSC provides access to storage on an undifferentiated bulk basis. In practice this means that the target device is under the host processor’s absolute control for as long as the device is mounted and the device cannot modify any of its contents without risking data corruption.

The MTP, on the other hand, works on a logical file basis. The USB host shares access to the target device’s file system, allowing the target to continue working with the files the host is not using. This allows a storage device to be involved in multiple operations, such as giving the user access to stored documents while the system is downloading a video file to memory. The MTP can also be run over TCP/IP and Bluetooth, allowing such file-oriented transfers to occur over wireless connections, as well.

USB without the PHY
One drawback of using conventional USB for is that the link is serial, which adds considerable latency in data transfers during command handshaking and as information moves between parallel and serial formats. A solution to his problem is the use of the UTMI+ Low Pin Interface (ULPI) (Figure 1) without a physical layer (PHY) component. The PHY-less ULPI permits two devices to connect through an 8- or 12-line parallel bus while still appearing as the USB at the software link layer.

Figure 1. The ULPI interface developed for USB allows a small
parallel bus connection between devices that behaves as
a USB connection at the software link layer.

Use of the parallel bus to connect devices eliminates the overhead of serialization and greatly speeds handshaking between devices. Further, because the signals use today’s low-voltage logic levels, ULPI eliminates the need for charge pumps or voltage converters to generate serial USB signals, reducing cost, parts count, and battery drain. These benefits of high performance, modest signal count, and simple connection to a wide range of peripheral device controllers are so compelling that ULPI is even beginning to appear on many embedded processors as an alternative to traditional wide local bus structures for peripheral connections.

While the USB in all its forms helps solve connectivity challenges in tablet design, though, the resulting data flow presents a significant performance burden. High speed USB allows transfers with a theoretical limit of 60 Mbytes/second but many PC host processors have only about half that bandwidth in and out of system memory. Further, the flash memory technology used in mobile devices typically only achieves system memory bandwidths on the order of 8-10 Mbytes/second.  Moreover, CPUs in mobile devices can introduce latency in transferring data between USB and mobile flash memory if they cannot provide a direct data path between the two.

The key to opening this bottleneck is to offload data-intensive tasks from the tablet processor. Providing a direct path from the USB controller to the data card controller and using MTP, for instance, would allow the downloading of a video file from the home PC to mass storage without host CPU intervention - a process known as side loading. This sideloading eliminates the need to first write to and then read from system memory during the file transfer. Sideloading thus both speeds the file transfer and frees the system processor to perform other tasks.