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Toyota Prius delivers dashboard display system

Posted: 19 Apr 2007     Print Version  Bookmark and Share

Keywords:EEPROM; 16-bit microcontroller 

While much of the Toyota Prius's status information and vehicle control functions comes by way of the central touch-screen panel, critical information is delivered from a dedicated display system visible directly behind the steering wheel. The dash module (DM) provides most of the output-only information such as speed, fuel-level, selected gear, and odometer/mileage by way of a digital readout and icon-based vacuum fluorescent display (VFD) panel. Probably supplied by Futaba or Noritake, the VFD technology has been around for quite some time, and Toyota has elected to shun the analogue gauges used years ago and more recently back in fashion in favor of the numeric readout.

My first Casio calculator circa 1977 (yes, I took that apart too) used VFDs for their segment-based, high-contrast, high visibility qualities. Readability and contrast are even more key in the automobile where harsh ambient sunlight might threaten legibility. To further improve visibility the driver actually sees a double-reflected image of the VFD rather than a direct view of the panel. Two mirrors on the underside of the dash direct the output of the VFD to the optical path of the driver, keeping stray light off of the VFD module itself and improving perceived viewing quality.

While not particularly high-current, the VFD does require high voltage, and an Oki Semiconductor #MSC1162A 40-bit VFD display driver translates output of the lone Fujitsu #MB90583C 16-bit microcontroller to the appropriate levels for achieving fluorescence from the driven electrodes of sealed vacuum chamber of the panel.

A 5-volt regulator, mystery Toshiba chip (clock?), and serial EEPROM are the primary remaining ICs on the DM board. Along with display control, the Fujitsu microprocessor forms the communications interface back to the gear-position sensor and engine control module. It is unclear whether speed and mileage information is calculated and stored within the DM itself or whether the DM serves strictly as a "dumb" display of information determined elsewhere in the Prius.

Two white modules connect into the DM as well, and here again a bit of "Huh?" factor set in. When opened up, the modules revealed an oil-damped V-shaped pendulum inside with the ends of the pendulum arms holding a magnet which swings overtop a Hall-effect sensor. My first thought was that these may form the yaw sensors mentioned earlier in the context of the skid control module, but the slow damping would tend to rule that out. Perhaps a more likely use given their construction, tie-in to the DM circuits, and their orthogonal arrangement when mounted is a tilt sensor for compensating displayed fuel level. When parked at an angle in one or both the axes of the car, the sensors might be used to recalibrate displayed fuel level so that accurate tank readings are possible even when gas in the tank is piled up away from the in-tank level gauge. Again, with the complexity of the car it all gets a bit hard to sort out sometimes.

David Carey
President of Portelligent

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