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Cut CAN power consumption with partial networking

16 May 2016  | Fritz Burkhardt, Giovanni Torrisi

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Reducing energy consumption has become a major need for any new product design, including industrial and automotive systems that use the CAN bus. One way to reduce system energy consumption is to shut down elements not currently being used. To use this approach in CAN systems, however, requires re-imagining CAN controller architectures.

In recent years, significant efforts have been made within the industry towards reducing the energy footprints of many systems. For instance, in the automotive environment, efficient engine management and weight minimisation of the vehicle bear the most significant potential for savings. But apart from factors including engine efficiency, vehicle weight, and aerodynamic drag, the power efficiency of the electronic control units has great potential as a field of activity for engineers designing to save energy. As a result, developers are also focusing on electronic functions in order to exploit every opportunity to reduce consumption.

The use of low-power electronic control units has in the past been particularly important for parked vehicles, in order to achieve maximum standby times with existing battery capacities. But current drain has now become important for driving vehicles, as well, because the electrical energy must be delivered by the combustion engine and thus has a direct influence on fuel consumption. And not only is the automotive industry facing the challenge of reducing its energy footprint: Global warming and the need of reducing emissions ask for better energy efficiency across many application domains.

Analysing the electronics landscape in modern electronics quickly raises several questions. Are the functions offered by the different electronic control units (ECU) really required all the time and in every operating situation? Is the continuous current consumption of these modules really justified? Often, the answer is no! Not, for instance, for convenience functions in the car such as seat electronics, trailer control units, or tailgate control units that are seldom operated or only required at specific times. Additional examples include door control units, auxiliary heating, sunroofs, and rear-view cameras.

On the other hand, it must be possible to activate these control units at any time in order to avoid any functional or convenience impairment. Networking, i.e. communication between different ECUs, can facilitate the job of selectively turning on only those modules needing to be activated at any given time. With networking to provide selective (partial) activation of the ECUs belonging to a complex system, there is considerable potential for energy savings.

Assume, for instance, that an automotive ECU has an average current drain of 100—200mA and a car battery voltage of 14V. The potential savings amount to 1.4W to 2.8W for each idling control unit. Total energy savings for 10 nodes capable of partial networking therefore amount to an average of 15W without any negative impact on functions or convenience features. According to the established conversion formula, 40W of electrical power represent 1.0g of CO2 emissions per kilometer. Thus, the introduction of partial networking leads to potential emission reductions of 0.375g of CO2 per km.

But there are even more reasons why a partial network is an interesting approach. Consider, for instance, the charging of electric vehicles. Although charging requires a communication link to the supervising control unit, most of the control units connected to the bus are not required for this task and can thus be selectively powered down. The same is true for future application scenarios entailing data transmissions between a parked vehicle and mobile end devices.

These future use cases also result in increased requirements regarding the operating life of the components. This can be compensated to a certain extent by partial networking, resulting in reduced costs.

High speed CAN is the networking technology usually used in automotive and industrial applications where the bus runs from one end of an environment to another one. But there is a problem with selective activation when using the CAN bus. Although current CAN nodes already provide low-power modes (e. g. standby, sleep), they immediately wake up if any communication occurs on the bus. These low-power modes can thus only be used if all nodes connected to the bus are disabled simultaneously (so-called 'bus idle').

Simultaneous disabling of nodes is possible in the case for a parked vehicle (figure 1). When a CAN message is transmitted on the bus, all connected controllers are awakened by the respective transceiver. But in the case of a moving vehicle these sleep modes are not helpful because at least some of the nodes on the bus must be continually active.

Figure 1: Conventional CAN network during the transmission of a message to ECU.

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