CAN bus protocol (1)

Nowadays, cars are no longer just a dead vehicle. The invention of computer also gives cars a functional heart. Embedded computer core is applied to automobiles. How can we standardize this core task? This requires the CAN bus protocol to help. Now let's take a look at this knowledge.

1. Basic concepts of CAN bus protocol:

(1), message: The information on the bus is sent in different formats, but the length is limited. When the bus is open, any connected unit can start to send a new packet ｡

(2) Information routing: In the CAN system, a CAN node does not use any information about the system structure. Here there are some important concepts: system flexibility-nodes can change any software or hardware without requiring all nodes and their application layers, is connected to the CAN network. Packet communication-the content of a packet is named by its identifier ID. The ID does not indicate the purpose of the packet, but describes the meaning of the data, so that all nodes in the Network may use packet filtering to determine whether to activate the data. Group-all nodes can accept packets due to packet filtering, and is activated by the same message at the same time. Data compatibility-in the CAN network, the message CAN be accepted by all nodes or no node at the same time. Therefore, the data compatibility of the system is achieved through grouping and error processing ｡

(3) Bit Rate: the data transmission rate of CAN is different in different systems, and a fixed rate in one system ｡

(4) Priority: During bus access, the identifier defines the static priority of a packet ｡

(5) Remote Data Request: by sending a remote frame, the node that needs the data can request another node to send the corresponding data frame, the data frame and the corresponding remote frame are named with the same identifier ID ｡

(6) Multi-master station: when the bus is open, any unit can start to send packets. Units with the highest priority will win access to the bus ｡

(7) Arbitration: when the bus is open, any unit can start to send packets. If two or more units start to send packets at the same time, the bus access conflict will be governed by the one-by-one arbitration rule, using the identifier ID, this arbitration rule can eliminate any loss of information and time. If a data frame with the same identifier is sent at the same time as a remote frame, the data frame takes precedence over the remote frame, during the arbitration period, each transmitter compares the send bit level with the level detected on the bus. If it is the same, the Unit can continue sending. When sending a recessive level, when the dominant electrical frequency is detected on the bus, the unit is withdrawn from arbitration and no subsequent transmission is performed ｡

(8). Security: in order to achieve the highest possible data transmission security, the CAN bus protocol provides a powerful measure for error detection, calibration, and self-inspection in each CAN node. detection measures include: sending self-check, cyclic Redundancy check, bit filling, and packet format check ｡

(9) Error mark and recovery time: damaged packets are marked by the faulty node. Such packets are invalid and automatically resend. If no new error occurs, recovery from an error detected to the next packet can take up to 29 digits ｡

(10) fault definition: CAN nodes CAN identify permanent faults and temporary disturbances, and CAN automatically close faulty nodes ｡

(11) connection: the CAN serial communication link is a bus that CAN be connected to many units. Theoretically, the number of units is infinite. In fact, the total number of units is limited by the delay time and the electrical load of the bus ｡

(12) response: all receivers check the compatibility of received packets, answer a compatible packet, and mark an incompatible packet ｡