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Car systems call for secure memory

Posted: 16 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:non-volative memory in automotive? car secure memory? car processors safety control?

As cars move closer to becoming highly-integrated transportation, information and entertainment systems, their semiconductor content continues to grow. Today's cars comprise several dozen processors, a wide range of sensors, and various types of control, safety, comfort and communications systems. All these systems have one thing in common!the need for non-volatile storage.

Automotive system memory comes in many different forms!ranging from just a few hundred bits to store IDs and sensor calibration data up to several megabytes!to hold complex programs in firmware. Different systems have different requirements for the non-volatile memory (NVM) they use, but all are looking for a secure storage that is inexpensive, reliable, secure and easily implemented.

Firmware storage
Today's cars are truly personal automotive systems, comprising many electronic subsystems that depend on MCUs. Examples of MCU-enabled systems include braking systems, electronic stability, cruise control, motor control, power management and dashboard instrumentation. The software that these processors require totals millions of lines of code, all of which must be stored in NVM. The current technique for this code storage is in flash memory embedded with the MCU.

One option is to use one-time programmable (OTP) memory, such as mask-configurable ROM, to store code. A lack of field programmability, however, precludes the possibility of upgrading the code at a later time, either to correct a problem or to enhance the feature set of the system. Field programmability also lets MCU manufacturers extend the life of their automotive products to support new car models.

MCU vendors have turned to embedded flash memory for firmware storage, since it is field-programmable. However, embedded flash with its floating-gate technology adds significant manufacturing cost to the MCU chip, as much as 30-50 percent. Flash also has security issues with respect to the firmware, since the flash contents, representing IP of the MCU vendor, can be read by voltage contrast or other scanning techniques.

Field-programmable OTP memory!if it is compact!may be used in place of flash memory for program storage by designing the memory to be "few times programmable" at the system level. MCU vendors can include one or more uncommitted sectors in the OTP memory along with the sectors storing existing program code. To upgrade one of the program-code modules, the upgraded module is programmed into an unused memory sector and control logic is switched to point to the updated module. This technique can also be used for other automotive systems, such as sensor calibration and digital rights management (DRM) encryption keys.

Programmable firmware storage for embedded MCU is an ideal area for inexpensive and reliable embedded NVM. The memory must be very inexpensive, ideally not adding more cost to the chip fabrication. In addition, it must be highly reliable and able to work with the demanding automotive operating temperature range associated with these systems.

Manufacturers can upgrade the MCU's program module by downloading the upgrade into the originally unprogrammed sector.

Sensor calibration
The car uses many inexpensive sensors for various functions!engine control, driver assistance and safety, and convenience subsystems that monitor critical parameters. Very often, these analog sensors are configured as piezoresistive bridge networks that the automotive system uses to convert physical parameters into electrical signals to measure pressure, temperature or humidity.

The physical parameter to electric signal conversion is usually low-level, nonlinear and very dependent on temperature. You need to do some sort of signal conditioning to amplify and correct the random part-to-part variations in sensitivity, offset and nonlinearity of the sensor signal. You also have to make it non-temperature dependent to create a linear and accurate signal for the system. An additional problem is that parameters such as offset or sensitivity are different from one system to another, requiring individual field-calibration for each sensor and conditioning circuit.

Several automotive semiconductor manufacturers offer chips for amplifying, calibrating and temperature compensating bridge sensors. They normally use MCUs to digitally control one or more DACs to program a set of sensor calibration coefficients into an EEPROM (embedded or external) to create a look-up table. However, this type of memory is not suitable for an automotive environment. High operating temperatures can cause data loss or corruption, which could cause the sensor module to become uncalibrated. This can result in a signal that the system "thinks" is correct. In safety-critical systems such as those used for braking or steering, this loss of calibration is unacceptable. These systems use some sort of error checking for the EEPROM's contents and correction mechanism, which adds cost and complexity to the system.

This is an ideal area for inexpensive and reliable embedded NVM, embedded with the signal amplifying and conditioning chip for calibrating and trimming the analog sensors whose inputs the MCU uses. To keep system cost down, the memory must be very inexpensive!ideally not adding any additional cost to the chip fabrication. In addition to high reliability and low cost, the NVM used for analog signal calibration in automotive systems must be able to work with the high automotive operating temperatures associated with these systems.

A typical programmable automotive sensor conditioner uses EEPROM to store sensor calibration coefficients.

IP security
As in-car entertainment systems have increased, so has the need for non-volatile memory to support DRM for the automotive entertainment systems used to exchange audio and video content with external sources. This memory must be inexpensive and more importantly, highly secure, since this content represents valuable IP of its developers. Using encryption and decryption keys is the most common way to guarantee that digital content is only received by authorized equipment, with correct values known only by authorized devices. Key storage must be inexpensive and key security very well protected!this requires highly secure embedded NVM. The keys may have to be updated, eliminating the use of ROM or embedded fuses. Flash is an option but their suspect security and added cost are leading manufacturers of entertainment systems, automotive and otherwise, to look into other types of embedded NVM.

As the need for content exchange between the car and an outside source (such as a DVD player in the house) develops, so will a mechanism for short-range wireless transmission of this content. Wireless USB is one proposed standard. Wireless USB communications requires two operations that need secure communication keys!association and security. Association is a one-time process of establishing a trusted relationship between two wireless devices. This ensures that devices are authorized to communicate, and is designed to prevent unauthorized or accidental connections between two unrelated devices. Security for wireless content communications refers to the encryption mechanism that protects data in transit. For cost and security purposes, wireless transmitters and receivers need embedded, secure, low-cost NVM for implementing circuitry for association and security operations.

Finally, electronic identification storage such as the Vehicle ID number, and IDs for cellphones and other automotive communication systems, is another area where secure NVM is needed. These applications need inexpensive, low-bit-count OTP NVM that is easy to initially program and very difficult to alter. Soon, automobiles will no doubt incorporate RFID tags on many of their components, allowing wireless detection of IDs that are also implemented with embedded OTP NVM.

- Charles Ng
VP of Worldwide Sales and Marketing
Kilopass Technology Inc.

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