CAN Typical Design Challenges

Design Challenges

Different CAN networks have different performance needs. To deal with the demands of each type of CAN network, very different hardware and software system design approaches must be employed. Because Freescale recognizes the challenges that face designers of automotive CAN devices and systems, we are able to provide different hardware options to address these challenges.

CAN also has several different physical layers that can be used, each specifying certain characteristics of the CAN network, such as electrical voltage levels, signaling schemes, wiring impedance, maximum baud rates, and more. Over the course of the last decade, two major physical layer designs have emerged as those used in most CAN applications. They both communicate using a differential voltage on a pair of wires and are commonly referred to as a high-speed CAN (ISO 11898-2, SAE J2284) and low-speed CAN (ISO 11898-3).

High-speed CAN networks are implemented with two wires, allowing transfer rates up to 1 Mbps. Low-speed/fault-tolerant CAN networks also use two wires. They can communicate with devices at rates up to 125 Kbps and offer the ability for CAN data traffic to continue in the event of a wiring fault. Another very different CAN physical layer (SAE J2411) uses only a single wire. Single-wire CAN interfaces can communicate with devices at rates up to 33.3 Kpbs (88.3 Kpbs in high-speed mode).

Below is a chart displaying typical applications for low-speed, high-speed and single-wire CAN

Automotive CAN networks can be divided into two distinct categories based on the nature of the traffic on the network—body control networks and powertrain networks. Body control networks communicate with passenger comfort and convenience systems and deal with a wide range of message identifiers that appear in no particular order or frequency. Freescale's Scalable CAN (msCAN) architecture is well suited for applications where messaging can be very sporadic and unpredictable. Messages received by msCAN are placed in a single first-in, first-out (FIFO) storage structure. The FIFO maintains the order of received messages, allowing many messages with identical identifiers to be received in rapid succession without overflowing the receive buffers.

Powertrain networks service engine and transmission control. They deal with a low range of message identifiers, but unlike body control networks, they are predictable and appear very regularly and in rapid succession. Freescale's FlexCAN™ module (CAN version 2.0 B-compliant) is well suited for these applications where messages are very regular and predictable. The hardware module is based on the traditional mailbox, or "full-CAN," hardware architecture that provides at least 16 message buffers. When messages are received, a hardware filter match will drop each message into one of the “mailboxes” (receive buffers).

To address the need for multiple types of CAN physical layers, Freescale offers a range of CAN physical layer devices designed to meet or exceed the performance standards set by ISO and SAE. However, a simple physical layer device is not always enough, if, for instance, a local switch or sensor needs to quickly wake up the CAN module from sleep state to active running state. Freescale System Basis Chip (SBC) products provide a highly integrated solution that combines the CAN physical layers with voltage regulators, independent watchdog timer and the local wake-up circuitry all in one package. SBCs allow greater design flexibility using fewer components, reducing assembly costs while increasing system reliability.