Garmin GPSMap 76S: Sensory overload

Starting with the GPS receiver, the 76S makes use of a unique, if somewhat bulky, quadrifilar/helical microstrip antenna to receive faint signals from the 27-satellite GPS constellation. This antenna feeds a dedicated receiver chip designed by and manufactured for Garmin. The receiver processes the multiple satellite radio signal streams needed for position fix and converts them into coordinates can then be matched to accumulated geographic data. While a detailed explanation of the GPS system is beyond scope of this column, suffice it to say that clever analog design is critical to pack so much capability into a single RF semiconductor device measuring less than 2mm square.

Interesting features beyond GPS lie with the additional sensors in the 76S.

A clever barometric pressure module manufactured by Intersema (Switzerland) provides the basis for altitude data. Two devices are contained within the Intersema MS5534A module, the first a micromachined piezo-resistive bridge. The bridge is patterned on a thin film drumhead whose small pressure-induced deflections alter the bridge characteristics. The second chip of the module is an A/D converter that translates the bridge variations into machine-readable 16-bit format reflecting barometric pressure and thus elevation.

An equally clever two-chip assembly from Honeywell provides directional data. Based in this case on a magneto-resistive bridge, the Honeywell HMC1022 package uses two separate chips"one bridge for East- West and the other for North-South. As the two chips experience variations in magnetic field, the bridge characteristics are again altered and fed to Cirrus Logic #CS5523- AS 16-bit A/D converter. The output of the Cirrus part" as with the Garmin receiver chip and Intersema module" is managed in a Garmin-branded DSP ASIC from TI that forms the digital heart of the 76S.

The story is pretty simple"while digital devices provide sensory integration and system smarts, it's a mix of MEMs technology, magneto-resistive devices, and radio-on-a-chip technologies that make the location-savvy 76S a reality. Analog content gives the 76S the ability to not only answer "where" but also "which way" and "how high."

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