Keys to the New Car Sensor

By Scott Monroe    

Courtesy of EE Times
(05/23/2005 10:00 H EDT)  

As the automobile's electronic content continues to increase, the need for low-cost, reliable sensing systems becomes even more important. While there are many challenges to overcome to meet those needs, advances in interconnect architectures and mixed-signal processes have combined to greatly increase intelligence, lower cost and improve reliability-and more advances are on the way.

Today, most sensor systems are based on an architecture using an analog signal chain from the sensor back to the system's main electronic control unit (ECU). It is a challenge to maintain signal integrity on these systems in the very noisy environment of the automobile.

One solution has been to use simple proprietary schemes such as pulse-width modulation or variable pulse width to transmit the signal digitally over various physical layers. But there are a couple of disadvantages to such schemes. Typically, a separate wire is required for each signal, and the communication is normally a unidirectional output from the sensor to the main ECU. There is no possibility for two-way communication and diagnostics with the sensor subsystem.


Another solution has been to use a CAN bus to transmit the signal back to the main ECU; however, this typically requires a microcontroller and supporting circuitry, which adds considerable cost.

Two trends currently support the evolution of automotive sensor systems: the Local Interconnect Network (LIN) protocol and advances in mixed-signal semiconductor process technology.

While LIN was originally targeted for body electronics, it has been creatively used in new ways, such as for sensor interfaces. There are several LIN features that make it a suitable physical layer and protocol for sensor subsystems. It is a low-cost, bidirectional, single-wire physical-layer implementation that reduces wiring and wiring harness requirements. This is especially true if more than one sensor is included in the module and all the outputs can be multiplexed onto a single LIN bus.

Architecture
The LIN protocol is based on a master/slave architecture in which all bus communication is controlled and scheduled by the master node. This feature gives the interconnect guaranteed latency times for signal transmission, making the system predictable, which is absolutely necessary for most sensor signals. The LIN bus architecture is scalable to 16 nodes, and no arbitration is necessary, since all bus communication is scheduled by the master.

The slave nodes are self-synchronizing and can use on-chip RC oscillators instead of crystals or ceramic resonators, thereby providing significant cost reduction at the system level. The protocol is fairly simple and is standardized, and it operates on an asynchronous serial interface (UART/SCI). In addition, the cost of silicon implementation is fairly low, even in the mixed-signal process technologies typically used for sensor signal-interface ICs. LIN-based sensor subsystems will also provide cost reductions and improved robustness through standardization.

Processes
Mixed-signal semiconductor processes continue to advance and allow for much greater integration, especially in terms of digital integration and analog precision. Today there are several types of processes that can be used for automotive sensor applications, such as linear BiCMOS, high-voltage CMOS and silicon-on-insulator. Each of those process types has strengths and weaknesses, and the choice of one over the others will depend on the sensor type and application requirements.

The processes allow for monolithic system-on-chip implementations of entire sensor system electronics, including power, high-voltage operation, digital, memory, clock sources and precision analog.

Mixed-signal interface ICs
Some of Texas Instruments' custom mixed-signal ASIC sensor interface offerings are examples of products in which the sensor signal interface converges with a mixed-signal semiconductor process and LIN communication bus. The single-chip sensor interface IC integrates almost every component needed to interface to the sensor, automotive electrical network and LIN bus. Typical components on one of these devices include automotive voltage regulators matched to sensor and system requirements; an analog filtering front end, for direct connection to the sensor output; an analog-to-digital converter; digital filtering and control; a LIN-compliant protocol controller; and a LIN-compliant physical layer.


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