The other half of the optical chain is a CMOS image sensor and image processor ASIC, seemingly designed by and made for Microsoft. A built-in window in the sensor package allows the reflected image to reach the device's 24x24 pixel array--low resolution by imaging standards but adequate for the intended purpose. Because the part has control and processing duties along with imaging--all on a budget of two AA batteries--a small array reduces power and likely improves the rate of frame processing possible at a given power consumption. A 32-Kbit (4 Kbyte) EEPROM from Atmel is used by the ASIC for embedded code.
Optoelectronics fill out another critical function of the mouse design in the scroll-wheel. A plastic molded "wagon-wheel" for the scrolling control forms the optical chopper used by the LED/phototransistor sensing of wheel motion. Other mouse inputs abound but as simple surface-mounted switches activated by the usual left/right buttons and special-function actions achieved by depressing and tilting of the scrollwheel. Given the lack of other logic in the mouse, controller activity for switches, scrollwheel chopper encoding and the USB interface itself are all resident on the main Microsoft ASIC.
Click on image to enlarge.
One last remaining bit of silicon and circuitry completes the total package. Achieving the always-welcome elimination of cords, the design uses a wireless link between mouse and host computer to de-clutter. At the host end, a small USB plug-in dongle receives all the movement data from the mouse but interestingly the silicon solution does not appear to be compliant with the Wireless USB standard set forth by the USB industry interest group. Nordic Semiconductor supplies the proprietary 2.4-GHz transceiver at both ends of the link, although with slight variations at each end. The mouse gets the company's NRF24L01 while the USB makes use of the NRF24LU1. Onboard EEPROM in the monolithic transceiver chips obviates the need for separate memory.
The mouse has most of the electronics on the larger of two printed circuit boards with scrollwheel and switches residing on a riser board attached by flex circuit. About a dozen molded plastic pieces are needed in total and small internal screws provide for fastening everything together.
Now past its 40th birthday, the mouse continues to evolve as the pointing weapon of choice for most computer users. Optics replaced the gunk-laden mechanical rollers of the past, and now refinements to the illumination source and image processing help you work with greater precision on a wider array of surfaces.
David Carey is president of Portelligent, a TechInsights company. The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.