EMC Poses Challenge for Automotive Electronics

May 1, 2010 12:00 PM
Sam Davis, Editor In Chief

With respect to the number of units in service, automotive electronic systems are now the most pervasive of today's electronic applications. And, continuing demand for new automotive functionality is leading to the addition of even more electronic systems. Also, there is a trend to replace automotive mechanical and hydraulic systems with “by-wire” configurations of sensors, actuators and microprocessors. This leads to the conclusion that today's automobiles are now merely distributed, embedded computer environments on wheels. Compared with an embedded computer in a stationary, environmentally protected area, today's automobiles have to survive a wide temperature range, humidity, bumpy roads, and an external environment that can generate broadband electrical noise. There is one similarity: software is a major ingredient in all embedded computers, although automotive software requires more fail-safe functions.

Software and electronics now make up more than 30 percent of the cost of a typical modern car. The vehicle's software controls its engine, maps its transmission shift points, and interacts with the components of the powertrain, climate control, infotainment systems, anti-lock braking, engine control, active suspensions, and vehicle dynamics. So much so that some modern cars now total up to 100 million lines of software code running on dozens of microprocessors. In many cases, that amount of code could possibly represent an investment of millions of dollars for a new car design. As an example, the GPS navigation system alone can employ a million lines of code. And, as more electronic systems are added the total lines of code could double or triple in a few years.

Although automobiles employ many different types of microprocessors, virtually all use the basic stored-program (Von Neumann) architecture shown in Fig. 1. These microprocessors store their programmed lines of software code instructions, as well as data, in read-write, random access memory (RAM). The microprocessor may have some internal RAM, as well as external RAM. In a typical simplified sequence, the microprocessor fetches an instruction from RAM and decodes it. If necessary, it fetches data from RAM, executes the instruction, and stores the results in RAM. Then, the process repeats itself with the next instruction. Program execution involves continuous interaction between instructions and data stored in RAM and typical internal microprocessor units that include:

• Program counter that indicates where the computer is in its instruction sequence.

• Instruction Decoder that interprets and implements the instruction.

  Internal registers that hold values of internal operations, such as the address of the instruction being executed and the data being processed.

• ALU (arithmetic and logic unit) that performs arithmetic and logic operations.

• Data I/O (input/output) going into and out of the microprocessor.

This movement of instructions and data must move flawlessly through the microprocessor units, RAM and external sources. Any unintended interruption can upset software control of the microprocessor and may affect automobile operation.

There can be cases where the internal microprocessor units and external RAM are subjected to sudden, random noise pulses that cause software to misbehave and affect automotive functions. This could be a one-time occurrence that goes away. If the affected software controls climate control or infotainment, the result would probably be inconsequential. However, if an affected microprocessor subsystem impacts software that controls operation of the car, then the results could be damaging to the car and its occupants. These one-event problems are very difficult to troubleshoot and cure.

These issues mean that EMC (electromagnetic compatibility) tests are an essential element of automotive electronics design. Designers must consider EMC issues at all stages of the process, from designing printed circuit boards, implementing modules, and final car layout.

Therefore, measures must be taken to:

• Protect against unwanted electromagnetic emissions by other on-board electronic systems, such as high-speed microprocessors.
• Protect on-board electronics from the automotive environment that can cause transients or interference from the switching of heavy or inductive loads, such as lamps and motors.
• Protect or harden on-board electronics against EMI produced by sources external to the automobile.

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