In order to integrate different data buses together, the gateway must provide bandwidth and response time.
For electronic system design engineers, the automotive industry is entering an encouraging and challenging period. Applications such as infotainment, remote sensing testing, security, and control require several networking standards. In order to select the most suitable and powerful bus that users need for most applications, design engineers are faced with heavy tasks.
The automotive electronics market is no different from data communications, telecommunications, and consumer electronics. There are many networking protocols available, each with its own advantages and disadvantages. One protocol cannot meet the needs of all automotive applications.
Automotive networks can generally be divided into three categories:
Body control: high bandwidth, high reliability and data integrity are required;
Infotainment: requires high bandwidth and real-time processing capabilities for audio and video;
Safety: Traditional hydraulics and sensors are being replaced by wire-driven driving and braking methods.
In order to store and process data from these networks, it is necessary to use gateways to interconnect the networks and process data from vehicle-mounted embedded networks. A typical gateway consists of several automotive network interfaces (CAN, MOST and Flexray), embedded microcontrollers and peripheral functions.
CAN (Control Area Network) is particularly suitable for body control due to its low cost and high transmission reliability. A typical car contains several CAN network functions, such as engine management, instrument control, and body control. Its maximum data rate is 1Mbps, so its bandwidth does not support the transmission of video and audio data. However, CAN is very cheap and fault-tolerant, so it is an important network protocol. According to ABI Research's "Vehicle Network Research Report" (the fourth quarter of 2004), approximately 528 million automotive CAN nodes will be installed by 2010.
MOST (Multimedia Transmission Protocol) meets the needs of video and audio functions. It supports synchronous and asynchronous data transmission at a rate of 24Mbps on plastic optical fiber. Devices such as DVD players, overhead players, GPS devices, and displays all require MOST transceivers. The MOST Alliance has defined MOST interconnection standards and software application interface standards. MOST is a flexible and highly reliable network standard.
At present, the brakes and steering wheel are controlled by hydraulic mechanical methods. In the future, cars will replace hydraulic machinery and use FlexRay-based controllers to achieve inline control, including inline brakes and inline steering wheels. This wire-based network is fast and fault-tolerant. FlexRay supports synchronous and asynchronous data transmission at a rate of approximately 10Mbps, which ensures stable data transmission, fault tolerance, and response time to messages, and provides redundancy measures in a dual-channel mode.
Ethernet
In addition to the above vehicle network standards, a typical gateway includes several other interfaces. Ethernet is a widely used network standard suitable for diagnosis and as a service interface. Its hardware cost is low and application software is available everywhere. Because this interface is used for diagnosis, it does not have fault tolerance and noise immunity. To process data from CAN, MOST and Flexray networks, an embedded processor is required. The processor splits, aggregates, and converts data. The Ethernet interface requires an embedded processor to run the TCPIP stack. The off-chip memory can store program codes. In order to store temporary data from the embedded network, additional memory may need to be added.
Design factor
When designing an automotive gateway, a system design engineer may need to make many decisions, including:
Which networks need to be bridged?
What bridge topology is used?
Is DMA (direct memory access) required?
How big is the data buffer?
What bus is required for internal data exchange?
What should the bus width be?
What arbitration mechanism is needed?
How much processing power is required?
The above problems depend on the system and application you design. However, there are some common issues that must be addressed. Obviously, CAN, MOST and Flexray are different protocols. Their payload, data rate and requirements for real-time processing are different. The gateway must be able to effectively process all incoming and outgoing data from these interfaces.
Adopt programmable logic devices to solve the bandwidth challenges facing in-vehicle gateways
A universal system bus must be selected to transmit the data in the gateway. This bus is usually a synchronous bus that works in the automotive network at different frequencies. The system generally matches the clock frequency and bandwidth of the embedded processor, so that data is efficiently transferred between the processor and the network interface. The system bus line must meet the bandwidth requirements of all interfaces, and the cumulative maximum bandwidth of each network protocol is a basic approximation.
For example, a network has four CAN nodes (4 Mb / s), a MOST (24 Mb / s), a FlexRay (10 Mb / s) and an Ethernet (100 Mb / s), so the total bandwidth is 138 Mb / s s. It can consist of a 16-bit bus running at 10MHz. However, the total bandwidth does not meet all requirements. Every system bus cycle has its purpose. The addressing and encoding cycles consume available bandwidth. Target devices such as memory controllers or network interfaces sometimes insert wait states when collecting necessary data. Both cases require additional bandwidth—either to increase the speed or to increase the width of the system bus.
Another important consideration is payload and delay time. The CAN node transmits information in 8-bit data packets. Ethernet can transmit up to 1,500 bytes of data packets. Every transmission has overhead. The addressing and encoding cycles consume the maximum available bandwidth. In extreme cases, the transmission capacity of a large system depends on the maximum effective bandwidth of the bus. However, it is not appropriate to interface the CAN network with its 8-byte payload. The data buffer needs to fill enough CAN data packets into the system data packets. The second issue to consider is that for a system data packet with an input CAN rate of 1 Mb / s, buffering enough data may cause a large delay in CAN data. Another extreme case is to configure the transmission capacity of the system to transmit very low payloads of 1 to 4 bytes. Here, excessively high bandwidth is a waste of overhead cycles for addressing and encoding.
Both MOST and Flexray are oriented to important real-time applications. For example, the audio stream from the CD player on the MOST network is transmitted at a fixed rate. Audio and video require real-time transmission. The system bus must ensure that there is no noticeable delay from network data such as CD players. No one wants to see gaps or echoes on the audio player. The reason for the delay is that there is too much buffered data-the system transmission with too much data buffer and inappropriate size affects the delay and real-time performance. Choosing the system bus speed and data packet size will determine the amount of buffering required for each interface in the gateway. The size of the buffer must be determined to maintain the bandwidth and meet the real-time transmission performance requirements of the equipment attached to the vehicle network.
In an ideal architecture, most embedded processors are only used for processing. However, processor cycles are consumed by functions such as interrupt response and data movement. In order to allow the processor to run with the highest efficiency, data movement operations can be performed by dedicated hardware components. A DMA controller is an embedded hardware module used to transfer data between a gateway interface (such as a MOST interface) and a memory or other interface. After configuring the processor, the DMA controller transfers data in the background, while the embedded processor processes application data.
The gateway design engineer has several design choices here, the most important is whether a DMA controller is required. This depends on the embedded processor architecture. For applications that perform data transfer, the processor may have enough idle cycles. Similarly, in order to minimize the cycle required for data transmission, each cycle of the processor must be calculated, and the transmission capacity and the type of DMA transmission must also be considered.
As you can see, the architecture of the in-vehicle gateway needs to consider several performance factors. At least the gateway needs to meet the bandwidth and delay time requirements of the interface, and the solution must be user-customizable, low-cost, and robust to maintain competitiveness.
Problems that the gateway needs to solve
Application-specific standard parts (ASSP) can meet the above requirements, but the main problem is that their functions and characteristics are immutable. If a system design engineer adopts ASSP, there is little chance to distinguish its products from those of competitors. If you consider adding functions or making adjustments based on changes in standards during the design cycle, the design changes will be significant. Custom ASIC design usually takes 18-24 months and the cost of one-time tape-out is as high as several million dollars.
On the other hand, the use of programmable devices such as gate arrays (FPGAs) in the evolving vehicle network has obvious advantages. FPGAs enable system design engineers to design unique feature sets based on their applications, and can adjust the design to support changing standards and feature set requirements. Design engineers can embed as many processors and DMA controllers as required by the application. The system bus bandwidth and buffer volume can also be adjusted to the ideal value according to the application. This user customization allows design engineers to choose the ideal cost and performance point.
In addition, programmable logic solutions can be reconfigured in the field and provide users with channels to update new applications. Time to market for products based on programmable devices. Some FPGA manufacturers and partner companies provide intellectual property (IP) cores for automotive applications. By modifying RTL parameters, users can customize these IP cores for specific applications. This can optimize the cost according to the problem space. System design engineers can also use custom logic to increase the available IP cores to differentiate their products from competitors' products.
Evolving standards and data-intensive applications such as navigation and video display require more and more IP content to shorten the design cycle. Vehicle-based gateways based on programmable logic devices and proven IP cores are cost-effective. The flexibility is very high.
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