As automobiles continue to grow in complexity—becoming true transportation systems—their semiconductor content also substantially increases. Traditional and new uses for non-volatile memory (NVM) are forcing car manufacturers to look at different ways of obtaining low-cost, secure and often field- programmable NVM for their electronic subsystems.

Much of the silicon content in cars comprises microcontrollers (MCUs), often several dozen per vehicle, that control various aspects of the car's operating systems as well as safety equipment, comfort controls, location devices and entertainment. Figure 1 shows the many electronics-rich systems that comprise today's automobile.

Figure 1 A modern car contains several dozen subsystems that use MCUs with embedded non-volatile memory for firmware storage and other purposes. (courtesy Xilinx)

A good example of the increasing complexity of an automotive MCU application is engine control. With gasoline prices and pollution awareness at high levels, efficient electronic engine control has become an important component of automotive engineering. Working with various sensors, the controller determines the correct gasoline and air combination to optimize gas mileage and minimize emissions. Many factors go into this decision, including temperature, humidity, vehicle weight, speed, engine age, and gasoline quality. MCU-based systems dynamically deal with all these contributing factors, adjusting engine parameters in real time. Also contributing to engine efficiency is a processor to determine shift points for electronically controlled (vs. hydraulic/mechanical) automatic transmissions. Under development for engine-operation optimization is variable valve control on a cylinder-by-cylinder basis. For example, such a subsystem could compensate for a sticky valve or out-of-spec fuel injector.

An important ongoing development for future automotive applications is "drive by wire," in which electronic systems replace hydraulic and mechanical systems for operations such as steering and braking. Electric systems offer higher reliability, more efficiency, better safety, less weight, and can have more functionality than can mechanical systems. Better efficiency and less weight translate to better fuel economy.

All of a car's processors use firmware that must be stored in a secure and highly reliable fashion—this requires inexpensive, non-volatile memory (NVM), ideally embedded on the same chip as the MCU which uses its stored code. Code storage for today's cars runs into millions of lines of code per vehicle, resulting in a huge demand for memory to store this software.

The most common way of implementing embedded firmware storage on an MCU is with field-programmable floating-gate Flash memory, which has replaced the older EEPROM technology. Embedded ROM storage, while inexpensive and reliable, must be programmed when the MCU is processed. This eliminates ROM as a choice for code storage for MCUs that target several models of a single automobile brand, with different code for each model. ROM storage also does not support code upgrades if they become available during the lifetime of the vehicle.

Several semiconductor manufacturers offer a range of Flash-based MCUs for automotive applications. Flash has become the embedded NVM of choice because of its ease of use and field-programmability. However, embedded Flash also adds significant extra cost to an MCU—as much as 50% more—and the physics of floating-gate Flash technology makes it possible to read the memory's contents, which represent valuable software IP. Many automotive MCU applications can be one-time programmable (OTP) and a more secure storage medium would definitely be a plus.

	ROM	Floating Gate Flash	OTP (Std. CMOS Process)
Overview
Process	CMOS	Flash	CMOS
Process Generation	Leading	1-2 gen. lag	Leading
Scalable Process	Yes	No	Yes
Benefits Comparison
Programming	Mask	Field	Field
System Performance	High	Low	High
Memory Performance	High	Low	Medium
Wafer cost	100%	150%	100%
Power Consumption	Low	Medium	Medium
Scalability and Reliability	High	Medium	High
Design Security	No	No	Very High

Table 1 A comparison of ROM, Flash and Kilopass' XMP one-time programmable memory technology attributes and benefits

For reconfigurability, an OTP memory can still be used by including one or more uncommitted sectors in the OTP memory along with the sectors storing the security keys. To upgrade the MCU's firmware, you program the upgraded module into an unused memory sector and switch control logic to point to the updated module. This enables the OTP memory cell to support "few times programmable" at the system level. However, with the large memory footprint of today's MCUs, this is only effective for very dense OTP technologies.

Cars also use many 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. This is another place for inexpensive and reliable NVM that resides on the MCU chip, this time for calibrating and trimming the analog sensors whose inputs the MCU uses. Traditional trimming technologies include laser trimming of thin-film resistors, laser cutting of metal or polysilicon links (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.

The amount of storage the sensor needs for calibration is small, typically just a few tens of bits per sensor, so an embedded, low-cost, field-programmable 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 with using encryption and decryption keys, where the correct key values are known only by authorized devices. 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 (VID) itself. This would make it more difficult for someone to obfuscate or change a car's VID number.

The deployment of both reconfigurable and 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.