Why One-Time Programmable Anti-Fuse Non-Volatile Memory Provides a Better Alternative to Flash in Automobile Electronics David Hsu

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Today, the automobile is distinguished as much by their electronics content in them as the mechanical features they offer. "The value of automotive electronic products accounts for roughly one quarter of an automobiles total cost," according to market research firm Databeans, Inc. "In luxury cars, (electronics) can account for more than 30 percent of the cost. This proportion continues to rise, and it is expected that the content value of electronic products in a number of luxury cars may soon reach half of the cars' total cost." A large percent of the electronics contained an automobile is made up of microcontrollers (MCUs).

Microcontrollers (MCUs) are what provide the intelligence managing this electronics, with vehicles containing over a hundred to provide control of engine, transmission power, auto body, cabin environment, lamp, security, and audio entertainment (see figure 1). With the growing awareness for the need for security, car manufacturers are looking for new alternative to flash and EEPROM for secure, low-cost, more temperature tolerant and often field-programmable non-volatile memory (NVM) for the microcontrollers in their automotive electronic systems. One such alternative is one-time programmable anti-fuse NVM in applications where storage contents changes infrequently over the life of the automobile.

The major difference between flash and anti-fuse NVM is the storage mechanism. Flash relies on a floating gate to trap electrons that represent data. Anti-fuse employs a far simpler mechanism. It uses the gate oxide of a MOS transistor as the storage media. In default, the gate oxide is capacitive, thus open or '0'. By assert high voltage, causing permanent breakdown, it is converted to resistive, allowing current to flow to the substrate or drain, thus representing '1'. Anti-fuse implies that it is the opposite of fuse. With fuse, the default state is '1', and is converted to '0' by blowing the fuse open with large current. The alteration to the transistor is completely invisible to conventional methods used to determine the stored contents illicitly, thus providing far greater security than that afforded by flash.

Figure 1. Automotive Applications Containing Microcontrollers
2008 Department of Chemical Engineering, Faculty of Engineering and Applied Science, Queen's University Kingston, Ontario, Canada

In any automotive applications depicted in the figure, each MCU is controlled by firmware (the control program or code) that must be stored in a secure and highly reliable NVM embedded on the same chip as the MCU. Programs for all the MCUs in today's cars run into millions of lines of code per vehicle, resulting in a huge demand for memory to contain this software.

Firmware storage on an MCU has historically been provided by EEPROMs. In recent years, floating gate flash memory has replaced EEPROM for this function. MCUs in automotive applications demand programmable memory since the code for individual CPUs varies depending on the automobile model it serves. However, once the code is loaded into embedded memory it changes infrequently throughout the life of the vehicle.

Its ease of use and field-programmability has made flash the embedded NVM of choice for automotive applications. However, embedded Flash also adds as much as 50 percent more cost to a standard logic CMOS MCU. The additional cost comes from the extra manufacturing steps needed to add the floating gate for each memory bit. A second drawback to flash is its susceptibility to tampering. Table 1 shows a comparison of the two technologies.

Table 1. Comparison of Flash and One-Time Programmable Anti-Fuse NVM

As seen in the table, one-time programmable memory provides a better alternative to flash for all applications that do not require a great deal of re-programmability. However, for a large number of applications where data does not change often during the life of an automobile, anti-fuse OTP is a good alternative. For example in an application requiring security key storage, OTP can provide limited re-programmability by over-provisioning the amount of memory. The amount of over-provisioning will be determined by the number of times the security key is likely to be changed. Anti-fuse OTP can also be programmed during final test thus providing the ability to write unique code for each MCU on a wafer.

Analog calibration and trimming for the many analog/mixed signal circuits populating the large number of sensors found in automotive electronics is another example where anti-fuse OTP can provide an ideal solution. Automobiles use these inexpensive sensors for engine control, driver assistance and safety, and convenience subsystems. These sensors monitor critical parameters such as air bag readiness, tire pressure and engine temperature, manifold pressure, light intensity, battery and electric subsystem voltages, car positioning for braking and turn control, along with various climate control and other comfort factors.

Traditional trimming technologies include laser trimming of thin-film resistors, laser cutting of metal or polysilicon fuses, Zener diode zapping based on avalanche diode breakdown to adjust resistance, opening poly fuses with a current, and storing bits in an EPROM or EEPROM to control a DAC to adjust currents or voltages. All of these methods are effective to varying degrees, but each has its own drawbacks when used to adjust analog circuitry on a standard mixed-signal CMOS process. Inexpensive and reliable anti-fuse NVM embedded on the MCU chip overcomes these drawbacks.

The amount of storage the sensor needs for calibration is small, typically just a few tens of bits per sensor. Thus, an embedded, low-cost, field-programmable anti-fuse NVM technology that adds negligible cost to a processor chip is ideal for this purpose.

While firmware storage and analog sensor calibration and trimming are the most important uses for NVM in automotive systems, there are other applications for which NVM is well suited. With the explosion of various types of in-car entertainment systems, the need for Digital Rights Management (DRM) for the audio and video content these systems use and exchange with external sources becomes increasing important. The most common way to make sure digital content is received by only authorized equipment is with encryption and decryption keys, where only authorized devices know the correct key values. Key storage must be inexpensive and the security of these keys inviolate-this requires a very secure embeddable NVM.

Identification numbers for car phones and other automotive communication systems can also be stored in secure NVM, as could the vehicle ID number (VIN) itself. This would make it more difficult for someone to obfuscate or change a car's VIN number. The unique capability of anti-fuse OTP to be completely invisible to conventional hacking technique used to read on-chip NVM illicitly make it ideal for secure storage.

The deployment of both re-programmable and anti-fuse OTP non-volatile memory is on the rise as the semiconductor content in automobiles continues to grow. The need for secure and inexpensive data and code storage is here and new memory technologies continue to become available to meet the growing automotive demand.

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About the Author

David Hsu
Kilopass Technology Inc.

David Hsu is Senior Field Marketing and Applications Manager at Kilopass Technology Inc. He graduated from The Ohio State University and received his Master’s degree in Electrical Engineering from Purdue University. Hsu started his career in Siemens Components in the telecommunication division; later at Datapath Systems’ read channel program and as principal engineer at LSI Logic HyperPHy SerDes group from1998 to 2006. Between 2006-2009, he worked at TSMC in the role of customer support and in IP/Library quality management.