Analysis of vehicle MCU, principle and characteristics of MCU in automotive electronics, factors that should be considered when selecting automotive MCU

In various systems of automotive electronics, vehicle MCUs (vehicle microcontrollers) are often used as the core of operational control, and the dependence of automobiles on electronic systems also stimulates the rapid growth of the automotive microcontroller market. The use of electronic systems in automobiles is becoming more and more complex, and automotive MCUs are playing an increasingly important role.

As a large-scale mechatronics equipment, automotive electronics is becoming more and more important in the overall cost of automobiles. At present, the average cost of automotive electronics in developed countries in Europe and America is more than 350 US dollars, covering everything from body control, power transmission, body safety to in-car entertainment.

Microcontrollers (MCUs) are the core of the internal processing and processing of automotive electronic systems, and are also used in dozens of sub-systems such as suspension, airbags, gating and sound. Since the car as a high-speed vehicle carries the safety guarantee for the user's life, and the car often works in a very harsh environment, the reliability requirement for the internal electronic device is much higher than that of the general electronic product. Therefore, the structural difference between the MCU used in automotive electronics and the general product is not very large, and the general MCU product cannot be selected because the reliability cannot meet the requirements of the manufacturer. This is also the market for automotive electronics and the general electronic product market. One of the differences.

Technical characteristics requirements:

With the ever-increasing demands of today's automotive applications, the systems that need to be integrated are becoming more and more complex, making the demand for high-end 32-bit MCUs in automotive electronic systems increasing. Such MCUs for vehicles are often placed in a high-heat, dusty, shock, and electronically disturbed operating environment, so the tolerance requirements are much higher than general-purpose MCUs. In addition, in automotive applications, automotive MCUs must be connected to multiple automotive electronic control units (ECUs), the most common of which are CAN and LIN.

CAN and LIN are the most common body system bus interfaces. Therefore, in addition to high reliability and resistance to harsh environments, automotive electronic MCUs must also support the above bus interfaces.

CAN: CAN is divided into high-speed CAN and low-speed CAN. The transmission rate of high-speed CAN can reach 1 Mbps. It is suitable for applications that emphasize real-time response such as ABS and EMS. Low-speed CAN can reach 125 Kbps, suitable for lower speed body parts. control. In addition, the CAN controller can be divided into the old 1.x, the standard 2.0A and the extended 2.0B. The newer specifications are naturally better, and the 2.0B can be divided into passive and Active (acTIve) type.

LIN: LIN is a lower speed and lower cost communication scheme than CAN. It adopts the concept of one master node and multiple slave nodes (up to 16 nodes), up to 20 kbps data transmission rate, and the length of bus cable can be up to Expand to 40 meters. It is ideal for Climate Control, Mirrors, Door Modules, Seats, Smart Switches, Low-cost Sensors A distributed communication solution that is simpler than a simple system.

The CAN bus is the Controller Area Net. It is a field bus. It was originally designed by German BOSCH for vehicle monitoring and control. It is mainly used for various process detection and control. The CAN bus is divided into high-speed CAN and low-speed CAN. The former is mainly used for critical applications such as power and safety, such as engine control unit, automatic transmission control, ABS control, airbag control, etc.; the latter is usually for general body applications. Such as centralized locks, luggage locks, windows, and interior lighting. The CAN bus protocol is also evolving. From the earliest 1.x version to the current CAN2.0A and its extended version of CAN2.0B, CAN2.0B is divided into active (AcTIve) and passive (Passive). )formula.

Due to the different versions and classifications of the CAN bus protocol, there are also differences in the requirements for automotive MCUs. In addition to the protocol versions mentioned, the number of CAN bus controller buffers and receive filters also affects the choice of MCU. As shown in the figure, ST's CAN controller has five different types of pCAN, beCAN, bxCAN, FullCAN and cCAN for different application scenarios. Among them, beCAN and bxCAN are suitable for mid-to-high-end body function control and low-end gateway; FullCAN is suitable for engine management system; cCAN is suitable for high-end gateway and power transmission control.

Number of buffers and receive filters for ST different CAN controllers

The LIN (Local Interconnect Network) bus is a new low-speed serial bus with simple structure, flexible configuration and low cost. It is mainly used as an auxiliary network or sub-network of high-speed bus such as CAN. In areas where bandwidth requirements are low, functions are simple, and real-time requirements are low, such as control of body appliances, the use of LIN bus can effectively simplify network wiring harnesses, reduce costs, and improve network communication efficiency and reliability. As shown, LIN is mainly suitable for Air-CondiTIoning Control, Door Modules, Seat Control, Smart Switches, Low-Cost Sensors. Such as distributed communication applications.

LIN application area

Gateway controller

The gateway function of the in-vehicle gateway (Gateway) is the communication hub of different networks in the in-vehicle electronic system, so that the units distributed in the vehicle body can communicate. Gateways typically include bus transceivers, Regulators, and low-cost, high-performance microcontrollers that support multiple network protocols; and support for low-speed and high-speed automotive communications such as CAN, LIN, ISO-9141, and J1850 interface. The gateway controller is designed to be flexible, and the general manufacturer will customize it according to its own needs. For different applications, it can be integrated into devices such as body control units or instrument clusters, or it can be used as a stand-alone module.

The role of embedded flash

The embedded memory of the MCU can guarantee the requirements of the industrial computer system, the stability can be improved, and the lower cost and the flexibility of the work processing can be realized. Therefore, providing embedded memory on the MCU, and even integrating the DSP unit, has become the current design trend.

The embedded memory of the vehicle MCU includes ROM, EEPROM, RAM and Flash. Among them, the memory of the microcontroller program and the data storage of the NOR Flash can make the MCU have higher flexibility, and has gradually become the mainstream of the current design. Since the MCU does not need to be connected in series with external components due to the embedded memory, it is not easy to cause signal interference, which reduces the complexity of the wiring and improves the stability. In addition, the embedded memory eliminates the need for external components, and can also effectively reduce the PCB size and give the product greater flexibility. In terms of data security, the data protection mechanism of the MCU embedded memory can achieve high reliability and ensure that the data is protected from theft.

DSP enhances design flexibility

Digital signal processing (DSP) technology is the technical foundation of today's high-tech digital industry. From the MP3 to hear high-tech applications such as aerospace, DSP technology is ubiquitous and growing rapidly. In the design of automotive electronic systems, in addition to the above mentioned embedded memory in the MCU, adding the MAC function of the DSP to the MCU can also effectively improve the flexibility of data processing. DSP is a software functional area of ​​the system, so it can flexibly perform functional improvements and upgrades according to the needs of the manufacturer or customer. In addition, DSP and processor (ARM, PowerPC, etc.) can be combined to achieve multi-task division processing, for example, the key control functions can be completed by the processor, and DSP can be dedicated to the operation of the operation, which can reduce system power consumption. And improve processing efficiency.

DSPs are typically used to process large numbers of digital signals, codecs, and communication data analysis. In automotive electronic systems, such as on-board auxiliary road condition warning safety systems, the DSP can be used to process and identify complex road condition information and provide real-time advice and warnings to drivers in a timely manner.

16-bit MCU with MAC unit (take ST10 as an example)

16-bit vehicle MCU application scenario

1, high processing performance

To improve processing performance, MCUs must take the FR81S CPU core of Fujitsu's next-generation MCU from its core and software and hardware system architecture as an example. Its working performance reaches 1.3MIPS/MHz, which is 30% higher than the previous generation FR60 core. Performance; with built-in single-precision floating-point arithmetic unit (FPU), it can meet the requirements of image processing systems and systems that require floating-point operation (such as brake control). In addition, hardware FPU support simplifies software programs and improves computing performance.

2, a large number of network node processing capabilities

There are a large number of built-in ECUs in the CAN network in today's cars. Their size has been increasing with the number of nodes, so the car MCU must support more message buffers. The previous generation of 32-bit CAN microcontrollers can provide up to 32 built-in message buffers, but now it is not enough. With the new generation of Fujitsu MCUs, it can support up to 64 built-in message buffers, and support CAN 2.0A/B specification and high transmission rate of 1Mbps.

3, extensive interface support capabilities

The periphery of the car MCU connection is quite diverse, and the connected interface may be UART, frequency synchronous serial, LIN-UART and I2C, so it must have flexible interface connection capability. In order to meet this demand, Fujitsu uses the built-in multi-function serial interface as a serial communication interface, and switches the above various interfaces through software to flexibly support the communication specifications of external components and improve the freedom of system design. The new family of MCUs also offers six channels of LIN-UART for communication with more control units; the MB91725 series is easier to integrate with various functions due to multiple channels and A/D converters with timer function. .

In automotive applications, microcontrollers (MCUs) provide vital performance. With the reduction of prices and the increase in consolidation, MCUs are gradually becoming commercialized. But for different MCUs, there are still big differences, so how to choose the right car MCU to reduce costs without affecting the required performance is also very important.

Microcontrollers (MCUs) deliver critical performance in a wider range of automotive applications, from motor control to infotainment systems and body control. As prices fall and consolidation increases, microcontrollers are becoming more popular, which means that MCUs are increasingly seen as commodities. Despite this commoditization trend, automotive system design engineers still believe that different controllers can vary widely, including various levels of integration and power requirements. Selecting an MCU can often reduce the material cost (BOM), effectively reducing the price of the electronic control unit (ECU) itself.

When choosing an automotive MCU, design engineers can consider the following important factors to achieve a balance between cost pressure and the specific performance characteristics required for the application.

Low pressure detection

One of the risk of failure when the MCU is operating is that the supply voltage or the internal voltage of the MCU may fall below the required level at the critical point. Obviously, if the operating voltage is not guaranteed and is outside the recommended supply voltage, this will cause a malfunction.

Conventional systems use an external voltage monitoring IC to check the voltage. However, this feature can be integrated into the MCU through an internal block that monitors both the internal voltage of the MCU and the external supply voltage level. As shown in Figure 1, the MCU automatically resets when the voltage drops below a preset threshold. The threshold level can be selected from a set of preset values ​​(7). This method can remove external components from the BOM, thereby reducing costs.

2. Watchdog timer

Another important feature to consider is the Watchdog Timer (WDT), which helps recover from fault conditions such as "out of control microprocessors" or "processors in messy conditions." The module resets the MCU as soon as it detects that the MCU is in an unresponsive state. In the past, embedded systems used external ICs to perform this function, but multiple watchdog timers could be integrated into the MCU. For example, a timer can operate as an independent clock external to the CPU operating system clock. This timer will be based on a slower CR clock, suitable for use as a hardware watchdog for the MCU, or for longer software loops to prevent runaway conditions. Another timer can be based on a faster peripheral clock. In theory, when the timer may be fed back too fast due to some error conditions, the watchdog timer will support the window function and the MCU will also be reset.

3. Dedicated NV memory

Like the watchdog timer, EEPROM has traditionally been an external component of the MCU. However, it is also possible to turn such a memory device into an internal device by using a dedicated ROM. Increased stability and error correction mechanisms further enhance the built-in EEPROM.

An advanced method of integrating EEPROM into the internal is to use a flash memory with dual operation capabilities. A portion of the flash memory bank can be read, while another portion of the library can be programmed to implement the EEPROM through a single flash memory module. Another way is to implement two flash modules, but the overhead of this method is greater than the overhead of dual-operation flash.

4. Car grounding

Due to the positioning of the electronic control unit, the electrical connections in the automotive environment are indeed very long. Automotive systems contain many ECUs and other devices that draw relatively large currents. Therefore, in addition to the spurious noise generated by the ECU itself, the electrical ground level is often unsatisfactory and may drift within a certain range.

MCU design based on such grounding conditions improves robustness and fail-safe levels. Advanced MCUs are often designed for standardized VIL based on vehicle conditions. Since "floating" helps prevent errors, it improves the quality of the ECU.

5. Vbat level direct input

Some ECUs in automotive systems can handle I/O signals around the battery level voltage. For CMOS-based semiconductors, the I/O signal is the maximum of the VCC level, typically in the range of 3V to 5V. Therefore, the converter is required to perform voltage level conversion. In some cases, voltage protection can be achieved, allowing high voltage signals to be directly connected through current limiting resistors.

6. Terminal function relocation

It is often challenging to keep the minimum number of layers as possible for PCB layout of ICs with a large number of pins. Peripheral components on the PCB cannot always be ideally positioned based on the pinout of the MCU. Sometimes it can be useful if the MCU has built-in flexibility to relocate its internal modules to another set of pins. This can be done through software settings. This ability can increase flexibility in the PCB layout process.

7. ADC auxiliary function

Analog-to-digital converters (ADCs) have long been a basic functional block of embedded systems. The ADC converts the signal from the analog domain to the digital domain, enabling access to information from the analog domain.

The ADC function block can be modified according to the specific application to distinguish the MCU based on the ADC function block. This enhancement can distinguish the entire MCU package. For example, the ADC module can support range comparators and pulse detection in hardware. These features are useful for applications such as stepper motor control in dashboards, power monitoring and sensor applications. The ADC can process the input signal from the stepper motor coil to perform zero point detection (ZPD). When processing tasks are completed in hardware, the CPU can use its MIPS elsewhere.

8. LIN hardware assist function

The Local Interconnect Network (LIN) is a low-cost, low-speed communication technology that is widely used in body applications. The automatic frame header transmission and detection, communication test function, variable interrupt length generation, and checksum generation and verification in hardware can be realized through the LIN bus. Therefore, using the LIN bus helps to enhance MCU performance. This method helps save CPU MIPS when used elsewhere.

9. ZPD enhancement

For dashboard applications, the ECU uses zero point detection (ZPD) to determine when the pointer reaches the end point to stop the stepper motor. This feature requires the stepper motor controller (SMC) to read and evaluate the voltage signal (also known as "back electromotive force") in the motor coil for detection. Adding hardware support enhances the SMC for voltage evaluation, so that no external components are required to implement ZPD. In addition, most back EMF evaluations can also be performed using hardware mechanisms. (In this respect, the ADC range comparator and pulse detection functions mentioned above are useful.) In addition, this method requires only minimal CPU usage.

10. Position and revolution meter

It is advantageous to provide a four position and revolution meter (QPRC) function in the form of a hardware block. This allows users to implement jog-dial functionality in audio and navigation applications. This module controls the degree and direction of rotation and determines the speed of rotation. In theory, this can be achieved by using a standard input capture unit in the MCU. However, implementing dedicated hardware modules dedicated to these tasks allows the CPU to conserve resources, resulting in better task allocation and simplified software packages within the system.

The market for in-vehicle MCUs is mainly focused on the 8, 16 and 32-bit microcontrollers, which can be used for different performance scenarios according to the different needs of automotive electronics.

8-bit MCUs are mainly used in relatively simple systems such as fans, air conditioners, wipers, windows, junction boxes, seat controls, and door controls due to processing power limitations. The 16-bit MCU is generally used in mid-range equipment. The main applications are engine control, clutch control, chassis mechanism and suspension, electronic brakes, electronic power steering, and electronic turbine systems. The 32-bit MCU is mainly used in the automotive electronics field for pre-crash modules, adaptive cruise control (ACC), driver assistance systems, electronic stability programs and other safety functions, and complex X-by-wire transmission functions. Modules that require high intelligence, computing performance, and real-time performance, such as multimedia information systems (TelemaTIcs), security systems, and engine control.

Currently, the living space of 16-bit MCUs seems to be continually squeezed by 8-bit and 32-bit MCUs. 8-bit microcontrollers continue to increase the power of the processor core, with the increase in embedded memory capacity and the more flexible number of pins, coupled with mature technology to further reduce costs, making the 8-bit microcontroller suitable for the market The space is getting bigger and can cover up to some 16-bit MCU applications, and can also replace most 4-bit MCUs down. 32-bit MCUs have a market potential with increasing emphasis on intelligence, real-time and diversity. In addition to handling complex computing and control functions, 32-bit MCU products will also play the role of master processing center in automotive electronic systems. That is, centralized management of low- and medium-level electronic control units (ECUs) scattered throughout. These capabilities are not available to 16-bit MCUs.

The 16-bit MCU seems to be in a very embarrassing situation, but with the addition of higher-capacity memory and the DSP-MAC mentioned above, 16-bit products can still meet the needs of special application functions. Moreover, it has gained market acceptance in terms of component quality, performance, and cost, and there is still an appropriate market space. On the other hand, although 32-bit MCU products have been widely used in the general market, they are currently found in high-end automotive products. In most critical applications such as transmission and safety systems, 16-bit MCUs are still the mainstay. The main reason is that most of the 32-bit MCUs are still in the verification stage of automotive electronic parts specifications, and then need to pass various environmental tests of the vehicle manufacturers themselves, so it will take a while to become the mainstream of the market.

8-bit MCU: Mainly used in various subsystems of the car body, including fan control, air conditioning control, wiper, sunroof, window lift, low-end instrument panel, junction box, seat control, door control module, etc. control function.

16-bit MCU: Mainly used in powertrain systems such as engine control, gear and clutch control, and electronic turbine systems; also suitable for chassis mechanisms such as suspension systems, electronic power steering, torque dispersion control, and Electronic pump, electronic brakes, etc.

32-bit MCUs: Main applications include dashboard control, body control, multimedia information systems (TelemaTIcs), engine control, and emerging intelligent and real-time security systems and power systems such as Pre-Crash, Adaptive Safety functions such as cruise control (ACC), driver assistance system, electronic stability program, and complex X-by-wire transmission functions.