Check your mirrors: Using MPUs to optimize LIN-controlled Exterior Mirrors

The mirrors on new car models are bursting with embedded comfort functions: side directional indicators, the door's outside lights with bulbs and/or LEDs, a defroster, the hinging movement and the X and Y adjustment as well as memory functions to just name a few.

Due to these features, controlling a state-of-the-art rearview mirror requires that a significant number of wires must be crammed into a narrow mirror joint. A mechatronic approach using standard LIN communication is useful in order to minimize the wiring harness, depending on the car manufacturer’s requirements.

Mirror-mirror on the car
An additional mirror feature, electro-chromatic (EC) mirror glass control, has recently been adopted, and according to analyst reports, dimming mirror glasses are one of the fastest-growing comfort electronic features with an average of double-digit growth rates for the next years. For 2012, the volume demands in the US as well as in the European Commission are forecast to be more than 5 million units each.

The EC feature is required at night to dim headlight glare from cars behind the driver. Electro-chromatic mirrors automatically darken, preventing the driver from being dazzled by the reflected light. The EC glass is a capacitive-resistive equivalent load and its control and protection requires a sophisticated strategy, as this technology is very sensitive in reference to the maximum ratings required for the voltage and current density applied to the glass. The voltage range of the EC characteristic, between transparency and maximum dipping, lies between zero and a small voltage just above 1V, and the current profile of EC glass used today is around a few hundredths of a mA with a higher current dip while the glass transparency changes.

Additionally, EC glasses require a "fast discharge" in order to implement a prompt brightening of the mirror. In conjunction with the improved quiescent current behavior of the new control electronics, the system is an extremely interesting solution to save fuel by using future upcoming EC glass technologies which are characterized by lower current demand than the known ones. Today, this is mandatory to comply with challenging CO2 requirements defined by legislation.

Two control approaches
The control of mirror electronics inside today’s car doors is dominated by two different topologies. In one, a decentralized approach is applied - driving all loads located in the door by an electronic module mounted locally inside all the doors. Communication with the body computer or the gateway is normally realized by using a HS-CAN physical layer (figure 1).


In the second topology option are those with a centralized control electronically driving all the loads located inside the doors. This approach is characterized by a high number of wires connecting the “satellite loads” with the central ECU, which normally is located under the hood. Which is the right solution to minimize the overall system cost? There are a lot of individual variables depending on the OEM, which makes a decision for one of the competing topologies almost impossible. Each approach has advantages/disadvantages in reference to cost, efficiency, car manufacturer strategy, and design capability. Some of the main parameters are the wiring harness, reliability of connectors, mounting locations and functionality of the actuators and sensors that must be controlled, degree of equipment of the different functions, and finally the limitation of available space.

In the end, the decision for the most appropriate topology is a shared process between the car manufacturer, the module supplier, and the semiconductor supplier with the goal of achieving the most optimized system cost solution. In some cases, even a mixed topology can be the best fitting approach, especially for high end mirrors that are characterized by a wiring harness of more than 20 wires. In this case, the mirror joint can be an argument against a central approach, as there is simply not enough space.

A solution is a mechatronic mirror. In this case, the electronics are mounted inside the mirror on top of the actuators and are controlled by a physical layer, in which normally the LIN is the choice solution. By using such an approach, the number of wires is minimized by a factor of 7 – just three wires - VS, GND, and LIN - are needed.       

Balancing topologies
Which semiconductor process technology and electronic concept topology (figure 2) is the most viable and effective for such a mechatronic mirror? It is clearly a balancing act since two contradictory needs are identified: High-voltage/medium power requirements to comply with the control of the higher power mirror loads, on the one side, and on the other, the ultra-low-power/high-speed requirements plus the possibility for reprogrammable memory on board for the data processing part.




We have to look beyond past examples of implementation - the highly integrated L9813 SoC (System on Chip) mirror driver, for example, where complex monolithic devices that expend time- and resource-consuming efforts might integrate a microprocessor core plus reprogrammable memory with multiple power stages on the same silicon chip. Today’s trend for low to medium volume applications uses mixed technology topologies to achieve the best cost-optimized solution (figure 3).




For the mechatronic mirror system, a chipset based on a two separate chip approach (for example, STMicroelectronics’ L99MM70 + STM8A) represents a starting point when a system-in-package device is later needed. By adopting a suitable package, for which a feasibility analysis is conducted, an SiP may later be implemented. With the chip-set starting configuration, all the requirements for scalability, flexibility and space are fulfilled by splitting up the signal processing and power part to dedicated standard devices using mainstream packages. Due to the use of volume technologies, process efficiency, quantity effects, and reduced development efforts, an additional positive impact to the system cost is achieved.

Take care with IP selection
Particular attention to the application required that new IP be constructed to ensure easy control and diagnosis of the upcoming new EC-Glass. The IP had to be compatible with the established technologies on the market. As a matter of course, all necessary protection strategies to protect the EC-Glass, especially the voltage limitation, are implemented and easy to use. The dimming level data is provided from the interior mirror via LIN protocol to the exterior mirror, where the target value for the dimming is realized by a comparing measurement of two light sensors. In the mechatronic mirror module, the LIN signal is transferred to an SPI signal and the L99MM70 controls the EC glass voltage with a granularity of 6bit, which gives a resolution of 25mV considering a maximum voltage of 1.2 resp. 1.5V that may be applied to the electro-chromatic layer. 

Decrease optical inspection needs
Besides the control via the SPI interface, diagnosis functionality is implemented that indicates a ‘voltage not reached’ or a ‘voltage too high’ flag. This feature is an important argument for EC electronics inside the mirror compared to a hard wired and paralleled use of two outside mirrors controlled together with the interior mirror, which is widely spread in the market today. At the car manufacturing line end, time-consuming optical inspection can be eliminated while still ensuring a well-mounted and functional load by  simply sending a control and diagnosis request via the LIN. A fail or not expected behavior will be reported in the test protocol, saving cost and testing time. In addition to the easier and more reliable diagnosis and control, the influence of reduced contacts and wiring harness resistance in the voltage regulation loop optimizes the accuracy of the voltage applied to the EC glass. As described above, this load is very sensitive to violation of the maximum voltage ratings, and proper control of the voltage is key for reliability and long lifetime of the electro-chromatic material.  

With the new chip-set, L99MM70 + STM8A, STMicroelectronics has released a new system solution to the automotive market to save quiescent current and overall system costs, making driving safer and more comfortable.

 Giovanni Torrisi joined STMicroelectronics in 1995. As an IC designer, he worked in the field of power electronics (designing electronic ignitions and AC/AC converters for Halogen Lamps); in these areas, he holds several patents. In 2002 he joined the technical marketing team and now leads a group, covering USA and Japan areas, responsible for automotive body products for ST.
 
Manuel Gärtner joined STMicroelectronics in 1999. Prior to that, he worked as a research engineer for the Technical University of Berlin and the Fraunhofer Institute for Silicon Technology. At ST, his combined expertise in smart power, motor control and door electronics, as well as team leadership, led to his current responsibilities as Senior Technical Marketing Manager for ASSP/ASIC Products.