Controller area network (CAN) is an asynchronous multi-master (mulTI-master) serial communication protocol that can be used to connect various electronic control modules in automotive and industrial applications. Initially, CAN was designed for automotive applications that require advanced data integration capabilities and data rates above 1Mbps. In the future, the application range of CAN will continue to increase, so that any system or device that requires a stable, reliable, and low-cost network may become a CAN node.
CAN application challenges
CAN networks in automotive applications can be divided into two distinct types based on the nature of the traffic. The first is the body control network, whose function is to control the passenger's comfort system, so the network mainly handles a variety of message identifiers that appear out of order or at irregular frequencies (message idenTIfier). The other is the automobile powertrain network (Powertrain network), whose function is to transmit messages related to the engine and transmission control. These types of information to be processed are relatively simple, but the frequency of occurrence is very fast and very regular. However, due to the different types of information to be processed, the design of hardware and software systems of the two networks is also very different.
Like other important network protocols, CAN requires a physical layer device to perform communication functions. The physical layer specification is derived from the 7-layer model specified by ISO / OSI and is responsible for current and voltage control of the bus. Physical layer devices also need to handle transient voltages and errors on the signaling link, and correct errors as much as possible.
In the past 10 years, two major physical layer designs have come to the forefront and have become the basis for the physical layer design of most CAN applications. They are usually called high-speed and low-speed physical layers, and they perform communication functions on a pair of differential signal lines in the manner of voltage difference. When one of the differential signal lines is short-circuited or open-circuited, the low-speed physical layer architecture can become a single-wire architecture (reference ground level). Due to the need to perform this function, the low-speed architecture is too expensive for operations with bus speeds higher than 125 kpbs. This is also the fundamental reason why 125kpbs has become the division between low-speed CAN and high-speed CAN. Although both architectures use a voltage reference on a pair of wires, the termination method (terminaTIon method) of each architecture is different, and the product system is not compatible.
In addition, GM recently developed a new CAN physical layer. This physical layer uses only one wire and limits the speed performance to 33.33 kbps. This single-wire CAN physical layer is significantly different from the above two types and has not been widely accepted.
In fact, there are no physical layer requirements in the CAN standard specification. Other standards organizations have also developed various standards to help design engineers develop various CAN devices that are compatible with each other. The International Standards Organization (ISO) and the Association of Automotive Engineers (SAE) have developed various standards for the European and American markets to ensure interoperability between various physical layer devices and recommended design procedures. Please visit to understand ISO11519-2 (low-speed fault-tolerant CAN) and ISO 11898 (high-speed CAN); or visit to understand the standard specifications of SAE J2411 (single-wire CAN) and SAE J2284-125 / 250/500 (high-speed CAN).
Solutions for CAN applications
Freescale's 32-bit MCU uses TouCAN or FlexCAN hardware modules to communicate with the CAN bus. These modules are based on the traditional "mailbox" or "full CAN (full-CAN)" hardware architecture, with 16 message buffers. When a message is received, the corresponding hardware filter will load the message into one of these 16 "mailboxes" (receive buffer). This method is very suitable for the powertrain system, because the messages in the system are very regular and predictable, application developers can use the software to clear the mailbox fast enough? "Mailbox" so that new messages will not be overwritten Discard old unprocessed messages. However, if multiple messages enter too fast to be processed and emptied, data will be lost. This is why the mailbox architecture is not necessarily suitable for unpredictable, event-driven data The reason for the network.
As mentioned earlier, the body electronics network news is sporadically generated and has unpredictable properties, which makes Freescale's Scalable CAN (msCAN) architecture very suitable for these applications. Because MCU series such as HC08, HC12 and HCS12 are 8-bit and 16-bit controllers, they are the core of body electronic systems and devices, so msCAN modules are suitable for development with these MCU series. The CAN messages received by the msCAN module are placed in a first-in first-out (FIFO) storage structure. This structure maintains the order of the received messages, so many messages with the same identifier can be received quickly and orderly without having to Worried about the overflow problem of a single receive buffer.
In order to meet the needs of various types of CAN physical layer, Freescale provides a series of CAN physical layer devices to meet or exceed the performance standards set by ISO or SAE.
But a simple physical layer device is not enough. For example, all automotive modules need to be powered by a regulated power supply. Sometimes, a local switch or sensor needs to quickly activate the module from a sleep state to an operating state, and the voltage of the switch or sensor is the voltage of the car battery. This is the help and value that Freescale's System Basis Chip (SBC) brings to the automotive design table. SBC combines the voltage adjustment required by the CAN physical layer, an independent watchdog timer, and a local activation circuit to enable greater flexibility with fewer components. When these circuits can be manufactured using the same semiconductor process, it is necessary to integrate these functions into a single package, reducing the number of components required in the final stage of the design. This will reduce development costs, increase reliability, and increase design flexibility.
Design challenges for LIN applications
Local Interconnect Network (LIN) is a UART-based single-master (node) multi-slave (node) network architecture, which was first developed for networking applications of automotive sensors and actuators. It provides a low-cost network connection for motors, switches, sensors and lights. LIN can not only connect independent sensors and actuators, but its master node can also connect between LIN networks and higher-level networks such as CAN.
However, it is not easy to integrate LIN networks into the automotive environment. At present, the vast majority of applications suitable for LIN use discrete, point-to-point harness systems, without taking into account the chips, circuits and components on the load side. Therefore, whether the load is a lamp or a motor, or a sensor, it is usually connected to a messy wiring harness through a simple connector. As a result, the remaining board space is very limited, and it is difficult to integrate the components required by LIN. For example, in electric rearview mirrors, suppliers may place up to three motors, heating elements, chrome-plated glass and multiple car lights, so there is not enough space left to meet other potential requirements.
For automotive electronics manufacturers, the ability to manufacture control modules is another major challenge. It is not only a problem encountered by LIN developers, but also a problem that motor, sensor and brake manufacturers often face.
Finally, the intelligent LIN network system provides developers with new options. Now, the front end of the system can complete the control of loads such as motors, lights and solenoids. Because it provides unprecedented control and system-level information, LIN networks can easily overrule diagnostic data. However, the problem is, in order to control and diagnose these loads, how to design a small volume semiconductor chip to fit a very narrow application space.
It is also important to reduce the energy radiation of the LIN network, because when the long-distance cable bus transmits information, it will radiate energy to other surrounding devices like an antenna. As a single-wire bus, LIN can switch between ground level and battery voltage. This large voltage switch will also cause a lot of electromagnetic radiation, so the design of the physical layer device needs to be more careful.
Solutions for LIN applications
As an open standard UART protocol, LIN enables Freescale to develop a complete, highly integrated product line of electromechanical components. These components are integrated in a very compact package for a specific slave application (slave applicaTIon) all the required semiconductor devices and connectors, providing LIN slave node (slave node) developers with great benefits.
High integration solves many customers' design challenges. One of the manufacturing methods of electromechanical devices is to place the MCU, physical layer and load hand-ling semiconductor device on an insulated metal substrate (IMS) (the substrate can be used as a small PCB) This method can obtain excellent thermal performance. The IMS provides unmatched heat dissipation performance for load processing, communications and logic semiconductors.
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