Dan Lunecki, Silicon Labs
The demand for sophisticated automotive body electronics continues to increase at a record pace, driven by consumer expectations and OEMs striving for differentiation and governmental compliance. The system design to production time has dropped dramatically over the past five years, and these body electronics systems often require tight integration with the rest of the vehicle while reducing cost, weight and energy consumption. AEC-Q100-qualified 8-bit mixed-signal microcontrollers (MCUs) are well suited for many small-function vehicle applications operating downstream and stand-alone from the main body control module (BCM).
Some designers may wonder: “Why would anybody use an 8-bit MCU today when 32-bit MCUs based on ARM Cortex technology are readily available in the market?” The brief answer is that when the system function is not computationally intensive, an 8-bit MCU generally offers several advantages that make it an optimal choice:
• Cost-effective solution (including system BOM cost reduction)
• Small form factor (easy integration with sensors)
• Low power (instant-on and always-on functions)
• Small code size for flash efficiency (1 byte versus 4 bytes)
• Well-developed OEM automotive system software for CAN and LIN interactions.
These advantages make 8-bit mixed-signal MCUs a good choice for a wide range of body electronics applications such as anti-pinch window lifters, networked door controls, power seat controls, LED lighting control, occupancy sensing, heating ventilation and A/C (HVAC) systems, windshield wiper and rain systems, steering angle measurement, and network interfaces and peripheral functions for a variety of electronic modules.
Anti-pinch window gate lifters, for example, are frequently controlled by 8-bit MCUs that handle interlocking and a pinch signature algorithm that simplifies automotive window obstruction detection and eliminates the need for a separate interlock system. CAN and LIN bus controls are frequently used in these systems.
Mixed-signal 8-bit MCUs can be used to control vehicle door and integrated side mirror functions by providing a crystal-less CAN bus and LIN bus communication gate between the BCM and the door functions. Additionally, 8-bit MCUs are ideal for power seat control where three or more motors are employed to optimize driver seat safety and comfort. Basic seat functions include motor control with mechanical switches while higher-end seat systems include driver/motor position memory store/recall as well as seat surface heating and cooling systems.
In steering applications, 8-bit MCUs can provide steering angle position from an anisotropic magneto resistance (AMR) sensor input via a high-speed CAN bus. In this analog-intensive application, calibration measurements are provided, and the full range of the MCU’s analog-to-digital converter (ADC) is maximized with a variable attenuation feature for each sensor.
The windshield as an application space
Automotive windshield systems are rapidly evolving from three basic functions: wipers, washers and defrost. Highly integrated 8-bit MCUs help modernize these functions with features such as CAN, LIN or other serial networking to reduce wiring harness weight and cost. They also enable more efficient motor control for wiper and fluid pump motors, which is important for vehicle electrification not only in hybrid electric vehicles (HEVs) and EVs, but also in internal combustion engine (ICE) vehicles in which stop/start functionality, electric power steering and electric A/C compressors are being deployed to increase ICE engine fuel economy.
The rearview mirror and windshield electronics have become increasingly complex systems. Mirrors often include compass, outside temperature, garage door openers, OnStar technology, reading lights, auto-dimming and backup video display control to enable the rear camera image when backing up. Within the rearview mirror mount (or separately behind it on the glass), more advanced functions can include rain sensing, glass dew point/condensation for auto-wipers, ambient light sensing for auto-headlight on/off/high-beam control, and forward-facing camera image modules for advanced driver assistance (ADAS) functions. ADAS functions today include collision and pedestrian avoidance, lane departure warning and road sign recognition.
Windshield electronics with a variety of features are increasingly common. Many include infrared (IR) proximity sensors that measure motion associated with rain on the glass regardless of the ambient lighting range (total darkness to full sun). CMOS-based temperature and relative humidity (RH) sensors provide RH and ambient temperature data. Controlled by an 8-bit mixed signal MCU, the combined windshield and mirror system provides the interface, control and data analysis for rain detection, outside temperature, glass condensation, compass sensors, automated HVAC windshield defrost and ADAS functions (see Figure 1 for an example of the components typically used in windshield and rearview mirror electronics systems).
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Figure 1. Rearview Mirror and Windshield Electronics System Architecture
By integrating a wide range of precision analog peripherals and communication protocols such as controller area network (CAN) and local interconnect network (LIN), 8-bit mixed-signal MCUs can help minimize the need for additional discrete components such as a voltage reference source, regulator and resonator in the bill of materials (BOM) of a body electronics application (see Figure 2). A reduction in component count results in a smaller system footprint as well, further improving reliability since more interconnections on the PCB usually equate to more reliability problems.
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Figure 2. Traditional System Bill of Materials vs. Mixed-signal, Integrated MCU
With clock speeds typically ranging from 25 to 100 MHz, 8-bit MCUs offer ample performance for most body electronics applications. They also deliver high levels of integration within very small footprints required by many space-constrained automotive applications such window lifts and door locks.
A practical example of an 8-bit automotive controller solution used in body electronics is the C8051F58x 8-bit MCU family from Silicon Labs. Qualified and tested to the AEC-Q100 specification with high-temperature operation up to 125°C, this auto-grade MCU family features on-chip peripherals that are not typically integrated on other 8-bit MCU alternatives.
For example, the F58x automotive MCUs include a CAN 2.0B interface, a LIN 2.1 interface, a high-accuracy oscillator that allows full spec operation without an external crystal oscillator, a high-precision voltage reference, a 5 V regulator, a 12-bit ADC, and an innovative I/O scheme that can reduce manufacturing and testing costs.
Featuring up to 128 kB of flash and 50 MIPS processing power in a small package, the F58x MCUs offer a combination of performance, peripherals, memory and small size that enables automotive designers to address body electronics design challenges at minimal cost. Complex algorithms and computations can be performed at run-time rather than using look-up tables, which saves memory that can be used to further enhance application performance.
F58x MCUs provide internal oscillator accuracy of up to ±0.5% over the entire automotive Grade 1 operating temperature (-40°C to 125°C) and voltage range. By using the MCU’s on-chip ADC and temperature sensor, the designer can further improve the accuracy to ±0.25% across voltage and temperature. Off-chip resonators cost an additional $0.20 to perform the same function. This capability enables designers to operate high-speed CAN networks without the use of external timing components, reducing cost and improving system reliability.
The integrated 12-bit ADC offers outstanding performance and supports up to 32 channels and sampling rates up to 200 ksps. The ADC includes an internal precision voltage reference that has stability of approximately ±30 ppm/ºC. An external component performing the same function would add to the footprint and the cost of the design.
Another unique feature of the integrated ADC is variable attenuation, which enables designers to dynamically attenuate the input signal to match the voltage reference. This technique has two distinct advantages:
• First, all input signals greater than the voltage reference can take advantage of the full range of output codes. This means the signal will not be clipped, and the designer can take advantage of all output codes for the maximum amount of dynamic range.
• Second, part-to-part variation (i.e., calibration) in sensors can be eliminated, enabling designers to use lower cost sensors, calibrate them in-system and achieve the same performance as expensive precision sensors at a much lower system cost.
Communication is key
Dedicated automotive serial busses also can offer performance advantages to designers. For example, a CAN 2.0 engine that offers 32 discrete message objects can support heavy network traffic. By integrating a hardware-based LIN 2.1 controller (not LIN emulated in software), automotive designers can further improve network performance. The combination of an 8-byte message buffer, hardware synchronization and checksum generation – all performed in hardware – frees valuable CPU resources and enables more complex LIN topologies.
The need for CAN or LIN connectivity in an automotive body electronics applications also depends on how far downstream or remote the MCU function is in the in-vehicle network. For example, 8-bit MCUs have been designed into rearview mirror systems that do not use CAN or LIN busses. In some applications, when a hardware LIN connection is not available, a software-based LIN interface through UART provides a cost-effective solution.
Design flexibility is another key concern for automotive engineers. Traditionally, MCUs use a fixed multiplexing scheme that forces designers to choose which resource they are going to use for a particular pin. Many 8-bit mixed-signal MCUs feature a digital crossbar that functions like a programmable switch-fabric, allowing designers to route digital peripherals to available I/O pins. This technology greatly simplifies printed circuit board (PCB) design effort and allows resources to be multiplexed to effectively have more functions than I/O pins. For example, a designer could have two independent LIN busses and re-map the pins dynamically during run-time, reducing system cost and enhancing design flexibility.
Crossbar technology can also be used to reduce programming and calibration costs. Many designers must calibrate their systems using some test text fixture at the end of the PCB assembly. During this step, a special “calibration firmware” can be programmed in the device to communicate with a calibration fixture. MCU resources can be used in conjunction with the calibration fixture to accelerate and increase the accuracy of the final calibration. Once the system is calibrated, the parameters are stored in flash memory, and the application firmware is programmed into the remaining MCU flash memory.
Body electronics applications often incorporate a digital isolator to provide an isolation barrier between the CAN physical layer and the MCUs that are running on the bus, as shown in Figure 3. Frequently used for applications in electrically noisy environments, digital isolators help protect the MCUs from any effects of noise that commonly occur in automotive systems. Digital isolators are used to eliminate ground loops present in automotive CAN and LIN networks. In addition, isolated gate drivers are often added to motor control modules to isolate and eliminate unwanted electrical noise from the motors. Having a low noise floor is critical to the occupant infotainment experience and the performance of in-vehicle RF modules.
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Figure 3. Automotive System Example Using CAN, LIN and Digital Isolation
Automotive electronics engineers face more design options, more system performance requirements and more design complexity than ever before. Mixed-signal 8-bit MCUs can play an important role in simplifying the overall design effort, improving performance, reducing cost and addressing space constraints.
The ability to integrate noisy digital circuits with sensitive analog circuits without degrading performance is an enormous benefit to automotive system designers who work with both digital and analog components. Integrating analog-intensive, mixed-signal capabilities on 8-bit MCUs results in cost-effective system-on-chip devices with smaller footprints that enable reduced system cost and complexity. These benefits are particularly helpful for body electronics on modern vehicles that increasingly rely on MCUs to deliver greater intelligence and connectivity.