Advanced automotive electronics and the memories that drive them.
From an automotive electronics designer's perspective, one question is: What type of memories should be used for these systems?
The most popular memories presently in use are: SRAMs, DRAMs, and non-volatile memories. In the following sections, we will look at their function and applications in automotive electronics.
SRAMs are static randomly accessible memory devices that hold data without the need to refresh their contents while the power is being applied. SRAMs are also faster than other types of RAMs. Asynchronous SRAM (ASRAM) is one of the earlier forms of SRAMs. They are used with microprocessors and embedded processors for code and data storage. Asynchronous SRAMs are still useful in many applications today, such as cell phones and industrial controls.
As microprocessor speeds increased, Synchronous SRAMs (SSRAM) were invented to take the place of ASRAMs. Today SSRAMs are used widely, thanks to the popularity of personal computers.
AS their name suggests, DR&Ms are comprised of storage elements which require the memory to be refreshed while it is being powered. However, because of its compact size as compared to the SRAM, DRAM can be manufactured with very high density. DRAMs are, in general, less speedy than SRAMs, and are used in systems where extreme performance is not the primary requirement. DRAMs are also used in other mobile, automotive, and personal electronics where density and overall memory capacity is a primary concern. They are also used as video memory for graphics systems.
Just like SRAMs, early DRAMs employed an asynchronous interface because that was the state-of-the-art for the earlier forms of microprocessors and microcontrollers. Asynchronons DRAM is still widely used in slower performance applications and in legacy systems. DRAMs evolved to employ the synchronous interface as microprocessors and memory busses became faster. Synchronous DRAMs (SDRAM), because of their higher performance, are commonly supported by new generations of microprocessors and microcontrollers.
For low power applications, some DRAMs are designed such that they can retain their contents with reduced power consumption. This mode is commonly called self-refresh mode.
Non-volatile memory (Flash and EEPROM]
Non-volatile memory are distinct from SRAMs and DRAMs in that they do not need to be powered to retain their data. Non-volatile memories are perfect for storing configuration data or user data which should not be lost when power is removed. It is not surprising that non-volatile memory (NVM) is used in cell phones, PDAs, VCRs, DVDs, TVs, PCs, and numerous other applications.
Popular NVM in use today are: EEPROM and Flash. Both types are composed of storage cells which stores electrical charges representing the data on a floating gate.
Automotive Memory Applications
Fully integrated electronic displays, such as the units used by the I-Drive (tm), are increasingly used in a variety of new automotive platforms. These displays are used to control many different functions such as GPS, Audio Controls, and as entertainment displays. Behind every display is a video graphics controller, and the memory most commonly used for video memory is DRAM. Memories are also used in the two leading automotive satellite receiver systems--XM Radio and Sirius Radio. Each of these systems receives broadcast compressed digital audio. The digital bit streams must be decompressed to provide the audio and channel description information. Scratch pad memory is needed to support computations during de-compression. In an automotive environment, it is also necessary to prevent interruptions in reception due to signal dropouts when a car goes under bridges and other obstructions, so buffer memory is used to store several seconds of signals.
DRAMs are used for more than satellite radio systems. They are also used in automotive DVD and CD players. All DVD players in production today use DRAM for decompressing the audio and video streams (such as MP3 and MPEG3 formats). Similarly DRAMs are used in DVD and CD systems in the automotive market.
DRAMs are also used for the anti-skip feature for these disk drives, which prevents skipping caused by potholes in the road. To prevent skipping, data and pointers are buffered in DRAM so that the CD and DVD playback is seamless. This feature used to be performed, to some extent, by mechanical damping components. With more electronics being placed in the dash, there is less room available for the often larger mechanical damping systems. Mechanical damping systems also add costs associated with materials, weight, and more complex production steps.
The electronics inside engine control and braking systems have been increasing for years. The first electronic fuel injection system was actually put in limited production using vacuum tubes! Fuel injection systems have certainly come a long way since then. With today's microprocessors and algorithms, engine control systems are available which eliminate the need for frequent tune-ups. These control systems also help increase fuel efficiency. However, as algorithms become more complex, the amount of memory required increases. The operating temperature requirement of most engine control modules is +125[degrees]C, which requires SRAMs that are especially designed and manufactured for this operating environment.
Virtually every major electronic system in a car contains some type of non-volatile memory. EEPROM is a non-volatile memory that is used widely in these applications. These memories are used for storing identification inn codes, user settings and device history. For example, a remote control system for a door lock must store the secret codes inside the car and inside the key remote. This code is retrieved from memory transmitted wirelessly each time the key remote is pressed. Another example is in the radio settings for channel selections. Within a digitally tuned radio, these settings can also he stored in a nonvolatile memory such as an EEPROM. Many user convenience features such as driver's seat position and steering wheel tilt-angles are all good applications for non-volatile memories.
The next generation of cars will introduce drive-by-wire systems, which promise to reduce vehicle weight and increase fuel efficiency. In these systems, brake pedal input parameters are combined with steering wheel parameters to calculate the direction control for the front wheels. Faster and higher capacity memories are needed to support the microprocessors and software algorithms used to perform these calculations.
What differentiates automotive memory from standard commercial memory components? One basic key is the design. Automotive memory must be designed to meet extreme temperature and reliability requirements. While the Automotive Electronics Council (AEC) has created a series of standards to qualify electronic components for automotive applications, the real quality and reliability factors are designed into the parts. Electronic components must be able to handle temperatures from 40[degrees]C to +85[degrees]C, +105[degrees]C, and even +125[degrees]C.
It is understandable that the quality of semiconductors used in automotive applications is one of the foremost considerations. For this reason the Automotive Electronics Council (AEC), under the guidance of the Chrysler/Ford/General Motors Supplier Quality Requirements Task Force, developed Quality System Requirements QS-9000 Semiconductor Supplement. Memories for automotive applications are generally designed and manufactured to meet these standards.
As in the case of the older mechanical carburetor, which were eventually replaced by electronically controlled fuel injection systems, other mechanical components of a car are being increasingly replaced by electronics that promise to be lighter, less expensive, and have improved performance. Cost-effective memory, designed and manufactured for automotive environments, is a key component in realizing these improvements.
John Chiang is an Applications Engineering Manager at ISSI. Lyn Zastrow, Automotive Business Unit Manager also contributed to the article. ISSI is located at 2231 Lawson Lane, Santa Clara, CA 95054; (408) 588-0800; Fax: (408) 588-0805; www.issi.com.
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