When car makers first started exploring drive-by-wire systems, the identified shortcomings of available vehicle protocols led to a search for a high-speed, fault-tolerant protocol. Initially, consideration focused on TTP/C (Time-Trigger Protocol) that was already used in aerospace and, more recently, on FlexRay and TTCAN. All of these protocols rely on a time-triggered approach using Time Division Multiple Access (TDMA). The jury is still out on whether any will see wide-spread usage in automotive. A new alternative, called FlexCAN, may provide an easy transition to network communications for in-vehicle safety critical systems.
For an ideal in-vehicle protocol, four items are required: deterministic behavior, speed, dependability and flexibility. TTP/C is the least flexible. This lack of flexibility led to the development of FlexRay. Both TTP/C and FlexRay have a similar speed capability, which is in the neighborhood of 12 Mbits/sec. In contrast, the predominant vehicle protocol, CAN, has a maximum data rate of 1 Mbits/sec but it has been extremely dependable and flexible. Efforts to add a TDMA structure to CAN (as in the TTCAN proposal) have compromised one of its best characteristics error recovery. As a result, TTCAN has attracted few potential users.
In contrast, FlexCAN keeps the best features of CAN and adds a time-triggered aspect that does not use TDMA. In addition, FlexCAN also adds some dependable features to CAN making it more suitable for safety-critical systems. By adding a reference time structure, many aspects of CAN are simplified ­ especially those pertaining to safety. The structure does not rely on clock synchronization because the granularity of the time-triggered mechanism does not require it. In FlexCAN, the information goes on two channels for replication, but could go on several channels simultaneously. Since most of today's microcontrollers (MCUs) have two CAN channels, FlexCAN can be implemented on available hardware.
Similar to the way that TCP/IP resides on top of Ethernet to provide reliable data transfer in the Internet architecture, FlexCAN is an additional layer on top of the standard CAN protocol to provide even more dependable services. From the protocol's perspective, this layer could be implemented in firmware and/or software. The physical layer and datalink layers (OSI Layers 1-2) are CAN so the protocol's maximum speed is still 1 Mbits/sec. This will not handle the higher-speed requirements that have driven the high-speed aspect of time-triggered protocols. However, for safety critical sub-networks, it could extend the useful life of the rugged CAN bus for many years to come.
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Going Up
FlexCAN adds layers to CAN building on a proven protocol, the way the internet relies on the Ethernet for layers one and two. |
CONTACT: Juan Pimentel, professor, Kettering UniversityTel: 810-394-7144; e-mail: jPimente@kettering.edu
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