Authored by:
Andy Gryc
QNX Software Systems
Ottawa, Ontario, Canada

Key Points
• Instrument clusters must display vehicle vital signs in an intuitive and immediately recognizable fashion.
• OpenGL ES taps a large pool of open-source graphics programming expertise and programming code.

Resources
Freescale Semiconductors, freescale.com
OpenGL Organization, opengl.org
QNX Software Systems, qnx.com 

To drive safely, a driver must remain focused on the task of driving. However, other objects can and do demand a driver’s attention. For example, one object in the vehicle cab consistently distracts a driver from the road: the instrument cluster. Safe driving means the driver spends as little time as possible looking at the cluster. Thus, the instrument cluster must display vehicle vital signs in an intuitive and immediately recognizable fashion. Many automakers are migrating to digital instrument clusters to reach that goal. At the same time they’re finding the panels cut costs and add some pizzazz to market appeal.

Traditional instrument clusters consist of plastic housings that contain indicator lights and mechanical gages. The instrumentation in early clusters were ruggedized electrical meter movements that reported gasoline level or engine temperature, or had friction drag instruments as used in the design of most speedometers. An enclosed flexible-steel cable ran the speedometer. It snaked from the transmission to the instrument panel and synchronized the gage to driveshaft rotation.

As automotive systems transitioned from mostly mechanical to ever more electronics, mechanical instrument clusters gave way to other kinds of displays. For example, many gage instruments today operate from stepper motors controlled by embedded processors receiving data from automotive sensors. However, the instrument cluster is poised to undergo another radical shift with the removal of all moving hardware. The all-digital instrument cluster replaces mechanical gages with virtual ones drawn on an LCD display, driven by a microprocessor and graphics controller.

Once available only in high-end luxury automobiles, digital instrument clusters are beginning to appear in mid and low-end vehicles. Many factors are driving this migration: Automakers can deploy the same hardware in multiple vehicle lines simply by reskinning the graphics. A skin is the term applied to the graphic-design layout and style. By comparison, manufacturers must retool to change the appearance of fixed-function gages.

Cars with complex hybrid or electric drivetrains can have multiple drive modes; a digital cluster can dynamically change the information displayed as the car shifts from one mode to another. Safety wise, digital clusters can reduce driver distraction and promote better driving by displaying only information the driver needs.

Attractive graphics also give the vehicle brand sex appeal more easily than static gages. Compared to a static-mechanical display, a digital display packs more functions in a given space without adding much cost on a per-unit basis.

Displays for digital instrument clusters must meet several criteria. Because the cluster fits in the space between the steering wheel and windshield, the display must be as wide as possible yet short. Most designs currently use displays 1,280 pixels wide × 480 pixels tall. The display must be visible in sunlight. To aid visibility, displays use bright, high-contrast graphics along with a matte, nonreflective finish. Placing the display deep in the dashboard keeps sunlight from washing out the display.

Digital instrument cluster displays need a color depth of at least 16 bits/pixel to render gage needles drawn diagonally. In some cases, the display may need to support up to 24 bits/pixel to have a smooth look or to render graduated blends in the background image. Attractive gages also need antialiasing to produce a smooth transition from one pixel to the next. This is typically handled by the graphics controller.

The CPU power needed for a digital-instrument cluster depends on the sophistication of the human-machine interface (HMI) and on whether the system uses a graphicsprocessing unit (GPU). Several automotive processors are well suited for this area, such as Freescale’s 5121e and i.MX35 and Fujitsu’s MB86R01 (“Jade”) and MB86298 (“Ruby”) processors. The Fujitsu processors also have special features for instrument cluster designs such as graphics- validation units for safety monitoring and dedicated graphical-display icons called sprites for indicator lights.

Software for digital instrument clusters is generally more sophisticated than that used to control an analog gage. With a simple analog instrument cluster, the processor needs to get measurements off the vehicle CAN or MOST bus, directly measure some values through a/d channels, drive stepper motors and indicator lamps, and possibly control an LED or LCD driver-information display. A digital cluster replaces the small, single-line LED with a full graphics display that needs correspondingly more horsepower and software complexity.

Using a standard graphical framework, such as OpenGL ES for 3D or OpenVG for 2D, isn’t mandatory, but does simplify design choices for graphical toolkits. In most cases, toolkits like those from Adobe, Altia, Elektrobit, and Tilcon are ported to talk to a standardized application program interface or API. An API such as OpenGL ES or OpenVG lets developers use these toolkits without having to port the toolkit software to a nonstandard API.

OpenGL ES is a well-defined subset of OpenGL, the most widely used 3D graphics API in the computer industry. As a result, automotive-development teams that use OpenGL ES can tap a large pool of graphics programming expertise and source code as well as a wealth of documentation online and in print. Despite its small footprint, the API supports advanced features such as alpha blending and Gouraud shading. Alpha blending controls the transparency between two or more colors while Gouraud shading simulates the differing effects of light and color across the surface of an object. Both effects are used to produce a more realistic looking computer-generated image.

These processors also perform texture mapping, modeling, transforms, lighting, and many other graphic operations. As a vendor-neutral, multiplatform API, OpenGL ES lets developers reuse 3D code in new projects or across an entire product family. An OpenGL ES application can, without code modifications, run on multiple graphics chips and operating systems; it can also migrate from a low-cost system that uses software rendering to a moreexpensive system that uses a 3D acceleration chip to improve frame rate or resolution.

The capability to merge content from multiple sources can be helpful as well. For example, the system designer may decide to use a full-featured graphical environment like Adobe Flash Lite for the background image “skins,” but use a simple application based on OpenGL ES for the needle display. The system designer needs to choose software and hardware that can combine the two layers, either through hardware layering features in the graphics controller, or through a software compositing system such as OpenKode.

It is imperative that the instrument cluster operate correctly at all times. If the gage readings falter, drivers may break laws, damage vehicles, and endanger occupants. A reliable real-time operating system (RTOS) is a must. High availability monitoring processes that watch for software failures and take corrective actions help ensure fail-safe operation. The RTOS and cluster application software must also boot quickly, bringing up the gage display within 1 or 2 sec of ignition crank.

The qualities described so far create a digital-instrument cluster that works at least as well as a standard analog cluster. However, the flexibility gained by a completely addressable display gives automotive OEMs and Tier 1 suppliers more room to innovate. For instance, a digital cluster could dynamically display road conditions, such as speed limits, road ice, or surrounding vehicles. It could let users customize the display with selectable color schemes or wall papers as well as reconfigure it for day/night or English/Metric measures. Readings within a normal range can be deemphasized or dimmed, reducing driver distractions. Readings outside a normal range are made brighter or changed in color, quickly alerting the driver to the malfunction.

The flexibility of the display opens up new realms of information not easily displayed by analog meters. For example, a display of car performance can emphasize rpm at shift points and show energy consumption in fuel-saving mode. Navigation features may be incorporated directly into the dashboard, as can displays from a back-up camera, GPS navigation, Internet applications for current weather, nearest gas stations, and so forth. Some of these features are already in current production vehicles, while others are still on the drawing board.

A growing amount of digital content is making its way into the car — everything from satellite traffic monitoring to music from personal media devices. Digital- instrument clusters, with their ability to display the right information at the right time, help drivers enjoy the benefits of this content without being overwhelmed and thereby distracted. This flexibility, combined with the potential to reduce costs and differentiate products, explains why many automakers and Tier 1 suppliers are developing digital instrument clusters now for future vehicles.