Description And Operation: Communication: Description

The primary on-board communication network between microcontroller-based electronic control modules in this vehicle is the Controller Area Network (CAN) data bus system. A data bus network minimizes redundant wiring connections; and, at the same time, reduces wire harness complexity, sensor current loads and controller hardware by allowing each sensing device to be connected to only one Electronic Control Unit (ECU). Each ECU reads, then broadcasts its sensor data over the bus for use by all other ECUs requiring that data.

The CAN bus is a two-wire multiplex system. Multiplexing is any system that enables the transmission of multiple messages over a single channel or circuit. The CAN bus is used for communication between most vehicle ECUs. The Security Gateway Module (SGW) and Body Control Module (BCM) are connected to CAN-C and CAN-IHS that act as managing ECUs to these buses. The BCM is the Network Manager (NM) for the CAN-C and CAN-IHS that can be awakened by the bus and as well as it can wake up the bus if necessary while the ignition is off, and the SGW is the Non-Networking Manager. The SGW and BCM physically and electrically isolate the CAN buses from each other, coordinating the bidirectional transfer of messages between them that are protected by trust anchor functioning ECUs.

These CAN bus networks provide on-board communication between all of the ECUs that are connected to them. The CAN-C (Chassis), CAN-FD (Flex-Data), and Automotive Ethernet networks are the faster of the CAN systems providing near real-time communication (500 Kbps to 1Gbps). The CAN-IHS (Interior High Speed) system, while slower (125 Kbps) is used for communications between less critical ECUs. These networks comprise the electronics architecture known as Atlantis.

The added speed of the CAN data bus is many times faster than previous data bus systems. This added speed facilitates the addition of more electronic control modules or ECUs and the incorporation of many new electrical and electronic features in the vehicle.

All modules transmit and receive messages over one of these buses. Data exchange between the modules is achieved by serial transmission of encoded data messages (a form of transmission in which data bits are sent sequentially, one at a time, over a single line). Each module can both send and receive serial data simultaneously. Each data bit of a CAN Bus message is carried over the bus as a voltage differential between the two bus circuits which, when strung together, form a message. Each module uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time. Corruption of a single bit within a message will corrupt the entire message. Each message contains a Cyclic Redundancy Check (CRC) which specifies the message size exactly. If the message detected conflicts with the CRC the ECU receiving it will determine the message to be an error and consider that communication has not been possible. Diagnosis of this condition using a lab scope may reveal activity that appears to be Bus data messages even if no actual communication is possible. Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed. Some modules will use trust anchors that are being used to secure storage, memory, execution environment and authenticated diagnostics.

CAN-FD 

A rise in data traffic has contributed higher bus loads and rising demand in bandwidth on the CAN buses. CAN-FD (Flex-Data) bus can transmit several times more information than that of the CAN-C bus without raising the bus load. The CAN-C transmits at a speed of 1Mbit/s and capacity of 8 bytes, while CAN FD increases the transmitting speed capability to 10Mbit/s rate and capacity to 64 bytes, while decreasing the overall bus load. The CAN-FD ECUs are compatible with CAN-C ECUs; and capable of communicating, even after the CAN-C ECUs have been put asleep via partial networking.

AUTOMOTIVE ETHERNET 

The Automotive Ethernet network is being used for communications between critical time sensitive and video related ECUs. Only two ECUs are ever connected to a Automotive Ethernet network. CAN messaging from multiple ECUs can be mirrored/reflected using an Ethernet switch, that are internal to the Security GateWay (SGW) Module, and Radio at the Diagnostic Port/Data Link Connector (DLC). The Ethernet switch joins multiple physical connections and can independently forward messages from one CAN to another CAN. The Automotive Ethernet voltages represent symbols that an ECU sender encodes based on a desired bit stream and an ECU receiver uses the resulting symbol stream to decode. There are several methods used for the encoding and decoding the different voltages generated. Information can be transferred bidirectionally on Automotive Ethernet at 100Mbit/s. Two interconnected ECUs can send and receive simultaneously (full duplex). As a sender, an ECU adds its own differential voltage to the two wires; while as a receiver, it subtracts its own voltage from the applied total voltage. The result of the subtraction corresponds to the voltage that was sent by the opposite ECU. This mechanism is a component of the echo cancellation method that is used in other Ethernet systems. For differential voltages to be added or subtracted, the two ECUs must know when a new symbol begins. This means that both ECUs must be synchronized to the symbol stream. This is done with the help of a Master ECU and Slave ECU. The Master ECU generates a continuous symbol stream to which the Slave ECU is synchronized.