Automotive Communications Demand A Robust Infrastructure

In the not so distant past, automobile wiring was there to power the ignition, head and taillights, some auxiliary circuits, and interior lighting. Most dashboard instruments including the speedometer, gear controls and indicators, locking mechanisms, and braking were entirely mechanical. To improve safety and efficiency, more electronics were utilized to improve engine performance and traction control, as well as provide active protection to occupants.

However, today’s modern automobiles sport satellite radio, rear-seat high-definition displays driven by DVD players, and complete infotainment systems that track the car’s position, provide instant access to digital music, and connect to the Internet via cellular connections. They use up to a hundred microprocessors that control every aspect of the automobile’s performance. Large amounts of data must be moved over inexpensive wire in a harsh environment, presenting many challenges.

Download this article in .PDF format
This file type includes high resolution graphics and schematics.

The Early Days Of Automotive Electronics

Using electronics in cars and trucks is not a new idea. Following the 1973 oil crisis, automobile manufacturers were tasked with improving engine efficiencies. They did so by improving ignition timing and spark generation through a device called an engine control unit (ECU).

Early air bags were electromechanically triggered. Impact sensors mounted on the front of the vehicle were made from tubes, springs, and metal balls. When a front impact occurred, the force would dislodge the ball and short circuit the airbag’s firing mechanism. To provide more control over deployment, microprocessors were incorporated along with accelerometers to detect both forward and side impacts.

Related Articles
• The Essentials Of Automotive Hands-Free Communications
• 2 MHz SEPIC/Boost Controller Targeted For Automotive Apps
• Fire The Chauffeur—The Car Can Drive Itself!

As additional electronics found their way into vehicles, more wiring was required to interconnect the functions. Developments in cables and connectors emerged to provide higher density and lighter interconnects without compromising signal integrity. However, by the 1980s it became apparent that increasing the number of wires to handle more signals was not scalable and a solution to both reduce the number of connections and the weight of the cable was needed.

Robert Bosch, GmbH (simply known as Bosch) began work on a serialized method to connect sensors and ECUs together to simplify the wiring by allowing multiple sensors to communicate on a single bus. The Society of Automotive Engineers adopted this work later and released it in 1986 as ISO-11898, which is commonly known as a controller area network (CAN) bus.

Consolidation Of Data Transport

The current ISO-11898 standard supports up to 30 nodes on a single bus with a data rate of 1 Mbit/s and a maximum length of 40 meters. Initially, CAN was used to communicate with all sensors necessary to monitor engine and transmission performance. It was about improving the efficiency of the drive train.

As vehicles evolved, manufacturers began looking for ways to reduce weight, and eliminating large wiring harnesses was a good place to start. Switch nodes added to the network replaced heavy copper wires that carried current to head lamps and tail lights. Routing a single large-gauge cable along with smaller signal wires that carry the CAN protocol reduces wiring and weight significantly.

With the limitation of only 30 nodes, or more importantly logical bus separation, large numbers of independent CAN buses were running around a vehicle. To again simplify the growing number of CAN buses, designers used concentrators to consolidate multiple CAN buses into a single high-speed connection that could carry the ISO-11898 state information and reconstruct it logically in the chassis computer.

A good candidate for this high-speed traffic is notably Ethernet. By using an “automotive” version of 802.3, designers could concentrate many CAN buses into a single backbone that runs the length of the vehicle.

New Arrivals Of Data Sources

At the same time CAN was being employed to simplify wiring and provide additional diagnostic and control functions, additional data sources were arriving. One very important source is the back-up camera. There’s a blind zone behind large vehicles where drivers cannot see what’s there. According to KidsAndCars.org, at least 50 children are victims of “back over” accidents resulting in two deaths each week in the United States.

In an effort to remove these blind zones, which can hide more than a single child, manufacturers are adding rear-looking cameras that engage when the vehicle is in reverse. This simple move now adds another high-speed cable and LCD where the driver can see it. If the car is already equipped with a navigation system, then the same display is used. If not, a small LCD typically is mounted behind the glass of the rear-view mirror. This additional camera will be a safety requirement on all cars sold in the U.S. starting around 2015.

Others sources (or users) of high-speed data include the “infotainment” center located in front near the driver, which controls and displays various sources of information and rear-seat video displays driven from digital sources such as DVDs. A unique requirement of Blu-ray and DVD playback is that the content must remain encrypted until it is displayed on the LCD, even in an automobile, to conform to the license agreements of the content holders. This implies the information cannot be analog, but must remain digital all the way to the display, adding signal integrity issues to the transport.

Modern automotive infotainment systems such as Ford’s SYNC add additional loads on data transport. They integrate audio entertainment management, mobile phone integration via Bluetooth or USB, audio entertainment control, climate control, and health monitoring. They also may provide navigational maps via GPS.

The trend is to provide larger displays to the driver to improve both the visual appearance of the data presented and to reduce the amount of time the driver’s eyes are off the road. This “information overload” condition can distract a driver, resulting in the potential for increased accidents. With larger clearer displays, information can be presented in ways that minimize the time it takes to understand what is being presented.

A solution to the increasing resolution and number of displays is to serialize the digital data and provide signal conditioning so low-cost automotive cables can be used to transport the digital video to the LCD. One issue that commonly occurs is the requirement for a reverse channel to send either touchscreen or manual switch control information back to the infotainment or navigational computer via the same wires.

Products such as Texas Instruments’ FPD Link III can serialize the video data and transport it to a high-resolution display, as well as provide an I²C back channel for control information, allowing designers to use a single twisted pair for both data and control (see the figure).