Today, two trends - one in automotive-communication-bus standardisation and the other in semiconductor technology - are enabling the development of smarter sensors and actuators to extend the utility of the vehicle's electronic systems through effective communication. The Local Interconnect Network (LIN) bus architecture, now in version 2.0, answers the need for a simple communication scheme for sensor/actuator interfaces that reduces costs and improves robustness through standardisation. The availability of the LIN standard also coincides with advances in mixed-signal semiconductor process technologies that can bring together all the functionalities typically needed for sensor and actuator interfaces onto a single integrated circuit. Together, the LIN standard and advanced mixedsignal processes provide an opportunity for auto manufacturers to introduce affordable new electronic systems and cost-reduce existing systems. They also improve maintenance and reliability while providing advanced convenience and safety features to a car's occupants.
Less satisfactory alternatives
Among the automotive-electronics bus standards available, LIN provides the best solution for the signalling needs of sensors and actuators. Several proprietary solutions exist that transmit signals digitally through simple schemes such as pulse-width modulation (PWM) or variable pulse-width (VPW). These schemes, which are based on various physical-layer (PHY) designs, frequently require a wire per sensor or actuator for communications, and are typically uni-directional outputs from the sensor to the ECU, or the ECU to the actuator. As a result, these network architectures offer no possibility for two-way communication and diagnostics - their utility is thus limited. Also, because the solutions are proprietary, they defeat the economies of scale and design reuse achieved throughout an industry by the implementation of an open standard. An alternative option would be to use an established communication standard, such as the Control Area Network (CAN) bus, to transmit signals between sensor/actuator interfaces and the ECU. However, CAN and similar communications typically require a microcontroller and supporting circuitry, which makes them more complex and costly than simple sensors and actuators require or can justify. CAN is also based on a two-wire bus, while the optimal solution would employ only a single wire for low-speed, low-cost communications.
A simple, open standard
While LIN was originally targeted for the vehicle's body electronics, it is also proving its value in new ways, one of these being in sensor/actuator interfaces. The LIN 2.0-specified data rate of up to 20.0kbps is sufficient for most sensors and actuators, and the LIN PHY and protocol controller can be integrated easily into mixed-signal integrated circuits for remote devices. The LIN 2.0 specification includes its protocol definition, which describes the physical and data-link layers; a configuration-language description that provides for system configuration and a common interface among network nodes, as well as serving as an input for development and analysis tools; and an application-program-interface (API) definition for software add-ons. Defined specifically for automotive applications by the LIN Consortium, the standard allows the implementation of a seamless chain of development and design tools, as well as enhancing the network's speed of development and reliability. LIN has a single-wire PHY implementation, reducing wiring and wiring-harness requirements and thus helping to save weight, space and cost. The standard provides for a single master with up to 16 independent slave nodes.Communications are schedule-triggered by the master, eliminating the need for arbitration among simultaneously reporting devices. The slave nodes are selfsynchronising and can use an on-chip RC oscillator instead of crystals or ceramic resonators, translating into significant cost-reduction. The LIN protocol has guaranteed latency times for signal transmission, making the system predictable - a factor that is essential for most sensor/actuator signals. The protocol is fairly simple and operates via an asynchronous serial interface (UART/SCI). As a result, silicon implementation is inexpensive, making LIN a very suitable bus solution for the mixed-signal process technologies typically used to manufacture signal-conditioning and output ICs for automotive sensors and actuators.
Enabling mixed-signal processes
The LIN standard is an important development for automotive sensor/actuator communications, but it becomes much more significant in conjunction with recent advances in mixed-signal semiconductor processes. Today, IC manufacturers who can leverage expertise in both high-speed CMOS digital and advanced analogue processes are making possible levels of system integration that were not even imaginable just a few years ago. Representative types of advanced mixed-signal processes that can be used for automotive sensor/actuator applications are linear BiCMOS (LBC), high voltage CMOS, and Silicon-onInsulator (SoI). Many of these processes will allow for a monolithic System-on-Chip (SoC) implementation for the entire sensor/actuator electronics, including power, high-voltage, digital logic, memory and precision analogue functions. In cases where intelligence is needed on-chip, advanced mixed-signal processes make it possible to integrate a reasonable level of digital logic in addition to the LIN protocol controller. For instance, designs might include logic that can report the status of the sensor or actuator, making possible diagnostics that can be used not only for immediate maintenance, but also for the development of lifetime reliability databases. The next generation of mixed-signal process technology will allow small microcontrollers to be integrated along with analogue functions. SoC integration such as this will be invaluable for an application such as a «one-touch" window lifter, which needs to run an algorithm that keeps the rising glass from pinching fingers. And for applications that require more complex, higher speed communications, the same semiconductor capabilities that make it possible to integrate LIN communication also allow CAN capabilities to be integrated on mixed-signal devices.
Sensor/actuator examples
Texas Instrument's TPIC1021 LIN-2.0 transceiver is the starting point of how high-level mixed-signal integration can help make LIN-compliant sensor and actuator communications more robust, yet still affordable. Based on TI's LBC4 linear BiCMOS process, the transceiver operates from the vehicle's battery voltage, so no external power supply is needed. Fault protection enables the device to handle -40 to +40V on the LIN bus, and on-chip electrostatic discharge (ESD) protection withstands spikes up to 17kV (International Electrotechnical Commission) and 12kV (human-body model). From this starting point, it is possible to integrate the additional components needed to interface to the sensor or actuator in the vehicle's electrical network and LIN network. Typical SoC functions include an automotive voltage regulator matched to the system requirements, analogue filtering of front-end input from sensor output, an analogue-to-digital converter (ADC), digital filtering and control, and a LIN-compliant protocol controller. An example of a fully integrated sensor interface based on LBC4 is shown in Figure 1. The high level of integration and circuit protection make the device well-suited to the space and cost constraints of harsh automotive environments.An example of an LBC4-based device designed specifically for use with motors is the TPIC10271 actuator interface. This device integrates a 3.3V voltage regulator/supervisor from battery source, a high-voltage interface to the user switches, a high-side FET (field effect transistor) driver for a position sensor or other type of sensor, two low-side FET drivers for the relays that control the motor, an op amp for feedback, protection circuitry and a LIN-compliant PHY (Figure 2). Outputs interface directly to a microcontroller for control algorithms such as anti-pinch supervision in window lifters. Like the TPIC1021 and other devices in TI's mixed-signal family, the TPIC10271 enters a «sleep" mode when not in use to conserve batattery power and features low electromagnetic emissions (EME) as well as high electromagnetic immunity (EMI).For other applications, the same mixed-signal process technology is capable of integrating the blocks in these two devices along with other functions, including low-dropout and switching-voltage regulators for single and multiple rails, different configurations of high- and low-side drivers, various op amps, digital logic, and communications interfaces such as the LIN one. Possible actuator interfaces include H-bridge, intelligent drivers for DC brush and three-phase DC brush motors, as well as relay drivers. These drivers are used for power seats and mirrors, door locks, windshield wipers and defrosters, window and antenna lifters, heating, ventilation and air-conditioning (HVAC), as well as a variety of other electronic systems that ensure user comfort and security.
System benefits
Changing the sensor/actuator signal and communication interface to LIN-compliant mixed-signal ICs brings several advantages at the system-level. The first is an improvement in a system's robustness and diagnostics. Fewer wires means less cost and a reduction in potential sources of failures. Moreover, because LIN allows for two-way communication, the master can request diagnostic information from the slave device, or the slave can provide failure information when there are issues with the system. In addition, LIN removes the need for proprietary interfaces and allows the development of components and software that use a common communication scheme based on a known, reliable standard. With LIN, it is possible to build a sensor or actuator that has only three wires - battery, ground and LIN - to reduce wiring and wiringharness requirements. Device housings can be smaller, allowing for improved sensor/actuator placement with less concern about where to run the wires. LIN and advanced mixed-signal processes serve to reduce system costs in a number of ways: fewer components; fewer items to inventory; smaller and simpler printed-circuit boards and housings for sensors and actuators; use of on-chip oscillators as clock-sources instead of crystals or resonators; and higher reliability. Some of these factors also lead to a reduction in weight and space consumption - considerations that are always at a premium in vehicle design. This advance is just the first step towards increasing the intelligence and capabilities of automotive sensors and actuators. The next generation of mixedsignal automotive ICs will integrate smaller microprocessors that provide programmable features and added flexibility - which are necessary for designers to address the automobile's needs of tomorrow. As vehicle sensors and actuators become smarter, the possibilities of distributed intelligence throughout an auto's system are limited only by the applications that vehicle designers can imagine.
