Under the Hood: Optical wireless mouse: Rodentia Analogus

While the ubiquitous computer mouse may not scream leading-edge, the very necessity and volume of the product has quietly driven some fantastic technology developments in this often overlooked peripheral. Long-time PC users remember the days of hard-wired mice and lint-clogged roller-balls, but modern mouse design dictates cutting the cord and scrapping mechanical motion translation.

Wireless links between pointing device and PC along with optical sensor technologies are rapidly becoming the norm for the lowly mouse thanks to the virtues of high volume and cost pressures characteristic of the PC peripheral world.

But when it comes to high-volume computer accessories such as the Microsoft optical wireless mouse analyzed here, an almost schizophrenic emphasis on integration emerges. While much of the key functionality gets implemented in an ASIC, the balance of the system is implemented with small-scale— but very inexpensive—components. Along with the flexibility to tune aspects of the design, the discrete approach builds on time-tested low-cost technologies.

An optical sensor from STMicro is central to the mouse design, representing an amazingly sophisticated piece of advanced mixed-signal CMOS integration. In essence an LED illuminates the surface on which the mouse rests and a CMOS imager array combined with special processor circuitry "looks" at the surface to detect shifts corresponding to mouse movement. Think of it as a vision-system-on-a-chip. The #VV5353 device was developed specifically for Microsoft's line of IntelliMouse Optical mouse products. A 16Kbit EEPROM from Microchip provides the code store for the processing portion of the ST sensor.

To create the wireless connection, a frequency shift keying (FSK) FM radio operating in the 27MHz public band is implemented in widely available commodity components. Presumably, the FSK data streams are encoded to reflect directional changes and inputs from the control buttons on the mouse.

Back at the USB-connected "base station," a fairly simple receiver based around Samsung's S1T3361D01 FM radio subsystem receives and demodulates the FSK signals. In combination with a Freescale #MC68HC908JB8 microcontroller—which also supports the USB interface—directional changes and clicks from the mouse are translated to user inputs to the PC.

All in all, the system puts most of its development eggs in the STMicro sensor basket. The remaining radio pieces are built with a discrete-intensive solution to leverage commodity bits and pieces which keep costs down and design flexibility up.

David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)