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Touchscreen has noncontact sensing, fingerprint reading

Posted: 04 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Teardown? Motion Computing? LS800? Tablet PC?

Motion Computing Inc.'s second-generation Tablet PC, the LS800, represents an increased emphasis on portability compared with the previously launched LE1600. Billed as an "ultramobile slate," the LS800 is based on a 1.2GHz Pentium ULV (ultralow-voltage) processor with 512Mbytes of double-data-rate SDRAM and a 1.8-inch, 30Gbyte hard drive for system storage. The central interface is, of course, the LCD screenthis is a Tablet PC, after alland here Motion selected an 8.4-inch SVGA thin-film-transistor display with 800 x 600-pixel resolution.

The touchscreen is an important differentiating element of the design, since it provides for most of the user interface and input to the LS800. The touchscreen technology does not rely on traditional resistive or capacitive overlays (either of which can compromise display quality), but rather uses an X-Y matrix of traces on a PCB behind the panel to sense in noncontact fashion where the special touchscreen pen or stylus is located. Wacom Technology is behind the technical solution for the touchscreen system, in which the stylus itself is passive, requiring no battery and sending out no internally generated signals.

The system works by scanning signals into the X-Y matrix board, where traces form the equivalent of rectangular, planar coils. Signals sent into the planar-coil set are induced into a resonant-coil circuit (pickup) internal to the pointing stylus. When the periodic scan signals are stopped, the resonant excitation of the stylus coil now serves as a signal that can be picked up in the X-Y trace coil loops to determine position. The X-Y matrix coil, which exhibits the greatest level of induced voltage, corresponds to the physical location of the stylus. It all works quite well, and the pen can hover almost a full centimeter away from the glass surface and still accurately convey position over the display.

Aside from pen and X-Y PCB matrix, a small controller board is populated with a Wacom ASIC responsible for the mixed-signal sending and sensing magic. A Matsushita microcomputer rounds out the touchscreen solution.

Motion Computing's LS800 Tablet PC

Along with a unique pointing apparatus for sensing "where," the design incorporates technology for sensing "who." Based on a swipe-style fingerprint reader for biometric security, the LS800 allows training for a user's (or users') set of validated fingerprints as a means of controlling tablet access. The fingerprint reader is a sensor from AuthenTec, a device whose core technology is a CMOS-based 192 x 16-pixel array able to read out unique fingerprint patterns detected as the peaks and troughs of the finger's skin. The pattern detection rests on variations in pixel-to-pixel capacitance read at scan time. The finger provides a "plate" of the capacitor, whose height (and therefore contribution to interpixel capacitance) varies with ridges in the skin. While the smarts to perform precise pattern matching (authentication) are located elsewhere in the LS800, the AuthenTec sensor also contains a full USB 2.0 interface for easy integration with the host device.

An Infineon Trusted Compute Platform chip, the SLD9630, lends additional system security. The device stores keys, passwords and digital certificates that, in theory, identify the host platform as a trusted device and provide necessary keys for digital rights management, among other things. While some have voiced concerns that the initiatives behind the Infineon component and related devices from other vendors threaten user autonomy, it remains a sign of the times that increasing security threats mean increasing hardware responses.

Intel supplies the processing horsepower for the LS800 in the form of the 753 chipset, which includes the 1.2GHz Pentium M, an Intel graphics memory controller hub (82915GMS) and the I/O controller hub (82801FB). Intel also provides both key devices for the PRO/Wireless 2200BG 802.11 mini-PCI Wi-Fi solution internal to the design. For other connectivity, a Realtek Ethernet controller (RTL8101L) services the wired-LAN interface, while a CSR BlueCore4 Bluetooth solution on a separate daughtercard implements local wireless connectivity along with IRDA.

Additional support chips to the Intel solution include an audio codec from SigmaTel, SDIO/flash card interface from Winbond, Super I/O controller from Standard Microsystems and keyboard controller from Ene Technology. Why put a keyboard controller in a slate PC? Quite simply because keyboards are unlikely to ever fully disappear. Despite the product's quite-usable handwriting recognition, keyboards are still the input mechanism of choice for heavy-duty typing.

Characteristic of designs with intensive power-management needs, an array of global and local power supply devices manage energy for the varying requirements of the main system components, including compute-load-dependent voltages for the CPU. Maxim can be seen as a key beneficiary of the complexities of power management in the LS800, with other smaller-scale devices also visible.

By using the ULV chip set and the attention to power control, Motion achieves cooling without fans. A shared heat sink tied to the graphics-memory controller hub and the Pentium processor, combined with spreading over the internal shield plate, are sufficient for the design to go fanless.

Hardware costs are less than 40 percent of the LS800's $1,900 retail price tag.

As the interface changes to more screen-centric, the sensor technology has become more expensive, but more satisfying. Likewise, as the need for security grows, hardware has kept up to deliver biometrics and authentication for well under $10a smart and affordable addition that serves a need far beyond the particular slate-style PC discussed here.

- David Carey
President, Portelligent

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