Interconnects-Keys to the New Car Sensors

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.

Several advantages are gained at the system level by making an architectural change in the sensor system to LIN for the signal and communication interface and basing it on a mixed-signal integrated circuit. LIN allows for two-way communication over a single wire so that the master has the capability to request diagnostic information from the sensor, or the sensor can provide system-failure information when needed.

The LIN protocol and physical layer are open specifications and have been designed for automotive applications by the LIN Consortium. Recently, the Society of Automotive Engineers has added best practices for LIN with the J2602 specifications. Removing proprietary interfaces and protocols allows for reuse of the sensors and bases them on a known reliable and robust communication system.

It is possible to build a sensor module that has only three wires (battery, ground and LIN), even if there are multiple sensors inside the module. This reduction in wiring and wiring harness allows for a reduction in the sensor housing size, better sensor placement and less sensitivity to wire placement. Through the use of advanced mixed-signal processes for the sensor interface IC, system cost is reduced in several ways: fewer components, less inventory to maintain, a smaller and simpler pc board design, a smaller sensor housing and higher reliability. Also, on-chip RC oscillators remove the need for crystals or resonators as clock sources.

These current advances are just small steps in increasing the intelligence and capabilities of automotive sensor systems. The next generation of mixed-signal processes, such as LBC5 and LBC7, will allow for even more intelligence and capabilities to be integrated into the sensor subsystems. It is even conceivable that the next generation of sensor interface ICs will include small integrated microprocessors that give the sensor subsystems programmable features and added flexibility.

Scott Monroe (scott-m@ti.com), system architect for mixed-signal automotive at Texas Instruments Inc. (Dallas)

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