Analysis of vehicle MCU

Among various automotive electronic systems, it is often necessary to use automotive MCU (Automotive microcontroller) as the core of operation control, and the dependence of automotive on electronic systems also stimulates the rapid growth of automotive microcontroller market. The application of electronic system in automobile is more and more complex, and vehicle MCU also plays a more and more important role.

As a large mechatronics equipment, the proportion of automotive electronics in the overall cost of automobile is becoming larger and larger. At present, the average cost of automotive electronics in European and American developed countries is more than 350 US dollars, which covers all aspects from body control' target='_blank' style='color:#ff0000;'>body control, power transmission, body safety to in car entertainment.

Microcontroller (MCU), as the core of internal operation and processing of automotive electronic system, is also distributed in dozens of sub systems such as suspension, airbag, door control and audio. As a high-speed vehicle, automobile carries the guarantee of users' life safety. At the same time, automobile often works in a very bad environment. Its reliability requirements for internal electronic equipment are much higher than general electronic products. Therefore, although the structural difference between MCU used in automotive electronics and general products is not great, general MCU products cannot be selected because their reliability can not meet the requirements of manufacturers, which is also one of the differences between automotive electronics and general electronics market.

Principle of MCU in automotive electronics

Technical characteristic requirements:

With the higher and higher requirements for application functions in today's automobile, the systems to be integrated are becoming more and more complex, which makes the demand for high-order 32-bit MCU in automobile electronic system increasing. This kind of vehicle MCU is often placed in the operating environment with high heat, dust, severe vibration and serious electronic interference, so the requirements for tolerance are much higher than those of general-purpose MCU. In addition, in the automotive application environment, the vehicle MCU must be connected with multiple vehicle electronic control devices (ECUs), among which the most common transmission interfaces are can and Lin.

Can and Lin are the most common body system bus interfaces. Therefore, automotive electronic MCU not only has high requirements in reliability and resistance to harsh environment, but also can 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, which is suitable for ABS, EMS and other applications that emphasize real-time response; Low speed can can reach 125 Kbps, which is suitable for low-speed vehicle body parts control. In addition, the type of CAN controller can be divided into old 1. X, standard 2.0A and extended 2.0B. The newer the specification, the better the performance. Among them, 2.0B can be divided into passive type and 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 extended to 40 meters at most. It is very suitable as a distributed communication solution for simpler systems such as climate control, mirrors, door modules, seats, smart switches and low-cost sensors.

Can bus, namely controller area net, is a kind of field bus. It was originally designed for vehicle monitoring and control by Bosch Company in Germany. It is mainly used for various process detection and control. Can bus is divided into high-speed can and low-speed can. The former is mainly used for key applications such as power and safety, such as engine control unit, automatic transmission control, ABS control, airbag control, etc; The latter is usually aimed at general body applications, such as centralized control lock, trunk lock, window, interior light, etc. The protocol of CAN bus is also evolving from the earliest version 1. X to the current can2.0a and its extended version CAN2.0B, in which CAN2.0B is divided into active and passive.

Due to the different versions and classifications of CAN bus protocol, the requirements for vehicle MCU are also different. In addition to the protocol version mentioned, the number of cache and receive filters of CAN bus controller also affects the selection of MCU. As shown in the figure, the CAN controller of ST has five different types for different application scenarios: PCAN, beCan, bxcan, fullcan and ccan. Among them, beCan and bxcan are suitable for medium and high-end body function control and low-end gateway; Fullcan is suitable for engine management system; Can is suitable for high-end gateway and powertrain control.

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 buses such as can. In the occasions with low bandwidth requirements, simple function and low real-time requirements, such as the control of body electrical appliances, the use of LIN bus can effectively simplify the network harness, reduce the cost and improve the efficiency and reliability of network communication. As shown in the figure, Lin is mainly suitable for distributed communication applications such as air-conditioning control, door modules, seat control, smart switches and low-cost sensors.

Application fields of Lin

Gateway Controller 

The function of the gateway controller in the vehicle is the communication hub of different networks in the electronic system in the vehicle, so that each unit distributed in the vehicle body can communicate. Gateways generally include bus transceivers, regulators, and low-cost, high-efficiency microcontrollers that support a variety of network protocols; It also widely supports vehicle electronic communication interfaces such as low-speed and high-speed can, Lin, ISO-9141 and J1850. The design of gateway controller is flexible. Generally, manufacturers will customize it according to their own needs. For different applications, it can be integrated into equipment such as body control' target='_blank' style='color:#ff0000;'>body control unit or instrument components, or it can appear as an independent module.

The role of embedded flash memory

MCU embedded memory can provide guarantee to meet the needs of industrial computer system, improve the stability, help to achieve lower cost and increase the flexibility of work processing. Therefore, it has become the current design trend to provide embedded memory on MCU and even integrate DSP units.

The embedded memory of vehicle MCU includes ROM, EEPROM, ram and flash. As the memory for microcontroller program and data storage, nor flash can make MCU have higher flexibility, and has gradually become the mainstream of current design. Due to the embedded memory, MCU does not need to be connected in series with external components at high speed, so it is not easy to produce the problem of signal interference, reduce the complexity of wiring and improve the stability. In addition, embedded memory eliminates external components, effectively reduces PCB size and gives product design greater flexibility. In terms of data security, the data protection mechanism of MCU embedded memory can achieve high reliability and ensure that the data is not stolen.

DSP improves design flexibility

Digital signal processing (DSP) technology is the technical basis of today's high-tech digital industry. From MP3 player to hear the high-tech applications of aerospace, DSP technology is everywhere and growing rapidly. In the design of automotive electronic system, in addition to embedding memory in MCU mentioned above, adding MAC function of DSP to MCU can also effectively improve the flexibility of data processing. DSP belongs to the software function category of the system, so it can be flexibly improved and upgraded according to the needs of manufacturers or customers. In addition, the combination of DSP and processor (arm, PowerPC, etc.) can realize multi task division of processing. For example, the key control functions can be completed by the processor, and the DSP can be dedicated to the operation, which can reduce the system power consumption and improve the processing efficiency.

DSP is generally used to process a large number of digital signals, encoding and decoding, and communication data analysis. In automotive electronic systems, such as on-board auxiliary road condition warning safety system, DSP can be used to process and identify complex road condition information and provide real-time suggestions and warnings for drivers in time.

Characteristics of vehicle MCU:

1. High processing performance

To improve the processing performance of MCU, we must start from its core and software and hardware system architecture. Taking the fr81s CPU core of Fujitsu's new generation MCU as an example, its working performance reaches 1.3mips/mhz, which is 30% higher than the previous generation FR60 core; With built-in single precision floating-point unit (FPU), it can meet the requirements of image processing system and those systems that need floating-point operation function (such as brake control). In addition, hardware FPU support can simplify software programs and improve computing performance.

2. Processing capacity of a large number of network nodes

There are a large number of built-in ECUs in the can network of cars today, and their scale continues to expand with the increase of the number of nodes. Therefore, the vehicle MCU must support more message buffers. The previous generation of 32-bit can microcontroller can provide up to 32 built-in message buffers, but now it is not enough to use. For the new generation of Fujitsu MCU, it can support up to 64 built-in message buffers, support can 2.0a/b specification and provide 1Mbps high transmission rate.

3. Extensive interface support capability

The periphery of vehicle MCU is quite diverse, and the connected interfaces may be UART, frequency synchronous serial, lin-uart and I2C. Therefore, it must have flexible interface connection capability. In order to meet this demand, Fujitsu uses the built-in multifunctional serial interface as the serial communication interface, and switches the above interfaces through software, so as to flexibly support the communication specification of external components and improve the freedom of system design. The new series MCU also provides 6 channels of lin-uart, so that it can communicate with more control units; Among them, mb91725 series is easier to integrate various functions due to multiple channels with timer function and a / D converter.

What factors should be considered in selecting automobile MCU?

In automotive applications, microcontroller (MCU) provides crucial performance. With the decrease of price and the increase of consolidation, MCU is also gradually commercialized. However, there are still great differences for different MCU, so how to select the appropriate automobile MCU to reduce the cost without affecting the required performance has become particularly important.

Microcontroller (MCU) provides crucial performance in a wide range of automotive applications, from motor control to infotainment system and body control' target='_blank' style='color:#ff0000;'>body control. With the decline of price and the increase of consolidation, microcontrollers are becoming more and more popular, which means that MCU is more and more regarded as a commodity. Despite this commercialization trend, automotive system design engineers still believe that different controllers will have great differences, including various levels of integration and power requirements. Selecting MCU can usually reduce the material cost (BOM), so as to effectively reduce the price of electronic control unit (ECU).

When selecting an automotive MCU, the design engineer can consider the following important factors to balance the cost pressure with the specific performance features required for the application.

1. Low voltage detection

One of the fault risks of MCU operation is that the power supply voltage or MCU internal voltage may fall below the required level at the critical point. Obviously, if the operating voltage cannot be guaranteed and exceeds the recommended power supply voltage, this will cause a fault.

Traditional systems use an external voltage monitoring IC to check the voltage. However, this function can be integrated into the MCU through an internal block that monitors both the internal voltage of the MCU and the voltage level of the external power supply. As shown in Figure 1, MCU will reset automatically when the voltage drops below the preset threshold. The threshold level can be selected from a set of preset values (7). This method can remove external components from BOM, so as to reduce the cost.

2. Watchdog timer

Another important feature to consider is the watchdog timer (WDT), which helps to recover from failure situations such as "runaway microprocessor" or "processor in cluttered condition". Once the module detects that the MCU is in a non response state, it will reset the MCU. In the past, embedded systems used external IC to perform this function, but multiple Watchdog Timers can be integrated in MCU. For example, a timer can work as a separate 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 of MCU, or for a long software cycle to prevent runaway conditions. Another timer can be based on a faster peripheral clock. Theoretically, when the timer may feed back too fast due to some error conditions, the watchdog timer will support the window function and reset the MCU.

3. Dedicated NV memory

Like the watchdog timer, EEPROM has always been an external device of MCU. However, it is also possible to turn such a storage device into an internal device by using a dedicated ROM. Improving stability and adopting error correction mechanism can further enhance the built-in EEPROM.

The advanced way to integrate EEPROM internally is to use flash memory with dual operation function. One part of the flash memory repository can be read, while the other part can be programmed to implement EEPROM through a single flash module. Another method is to implement two flash memory modules, but this method will cost more than double operation flash memory.

4. Vehicle grounding

Due to the positioning mode of the electronic control unit, the electrical connection in the automotive environment is indeed very long. The automotive system contains many ECUs and other devices that draw relatively large current. Therefore, in addition to the parasitic noise generated by the ECU itself, the electrical grounding level is often not ideal and may drift within a certain range. MCU design based on such grounding conditions will improve robustness and fault safety level. Advanced MCU is often designed for standardized VIL according to automobile conditions. Since "floating" helps to prevent errors, the quality of ECU is improved.

5. Vbat level direct input

Some ECUs in automotive systems can process I / O signals around battery level voltage. For semiconductors based on CMOS design, the I / O signal is the maximum value of VCC level, generally 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 to allow 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 when PCB layout IC with a large number of pins. The peripheral components on PCB cannot always be ideally positioned according to the pin distribution of MCU. Sometimes, it is useful if the MCU has the built-in flexibility to relocate its internal modules to another set of pins.

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