Time Triggered Communication on CAN: "TTCAN"

The communication in the classic CAN network is event triggered; peak loads may occur when the transmission of several messages is requested at the same time. CAN's non-destructive arbitration mechanism guarantees the sequential transmission of all messages according to their identifier priority. For hard real-time systems, a scheduling analysis of the whole system has to be done to ensure that all transmission deadlines are met even at peak bus loads.

Static Cyclic Scheduling

Some real-time operating systems (RTOS) are based on static cyclic scheduling of all tasks in the application system (control unit). They build a schedule of time slots and place each task in at least one slot. Tasks of high priority appear in more than one slot. All activity in one slot, including interrupt handling, must complete before the beginning of the next slot. If such an RTOS is considered for a distributed application system consisting of control units linked by a CAN network, system integration and composability are served when the communication on the CAN network also follows a synchronized schedule. When the nodes are synchronized, any message can be transmitted at a specific time slot, without competing with other messages for the bus. Thus the loss of arbitration is avoided, the latency time becomes predictable.

Synchronization Interface in existing CAN Controllers

ISO 11898-1 specifies an optional interface for CAN controllers in support of time triggered communication: Each node needs to provide a time base, which is provided either by an internal or by an external clock. Any message received or transmitted invokes a capture of the time base taken at the respective message's reference point, which is the sample point of the SOF bit. After successful message completion, the capture value is provided to the control unit for at least one message and it is readable until the next message is completed. It has to be possible to generate at least one programmable event trigger from the above mentioned time base. The trigger should be freely programmable by the control unit. The communication schedule's time slots are divided by trigger events. The automatic retransmission of messages that could not be transmitted successfully must be disabled. This optional interface is implemented in some existing CAN controllers.

Time Triggered Communication in Controller Area Network

The international standard ISO 11898-4 specifies "Time Triggered Communication on CAN" (TTCAN) as an extension to the well known CAN protocol. It introduces time triggered communication and a system-wide global network time with high precision. The time triggered communication is built upon the unchanged standard CAN protocol. This allows a software implementation of the time triggered function of TTCAN, based on existing CAN ICs. The high-precision global time however requires a hardware implementation. A hardware implementation also offers additional functions for the synchronization between TTCAN network and external time references as well as the synchronization between the several TTCAN networks. TTCAN is fully compatible with the existing CAN controllers, both in the data link layer as well as in the physical layer, it uses the same bus line and bus transceivers. Dedicated bus guardians are not needed in TTCAN nodes, bus conflicts between nodes are prevented by CAN's non-destructive bitwise arbitration mechanism and by CAN's fault confinement (error-passive, bus-off). Existing CAN controllers can receive every message in a TTCAN network, TTCAN controllers can operate in existing CAN networks. TTCAN controllers can be seen as existing CAN controllers enhanced with a Frame Synchronization Entity.

Migration from CAN to TTCAN

The introduction of a new protocol and a new bus line in automotive applications takes a major effort in the regard of the development of new protocol controller and transceiver ICs, new planning and analysis tools, and in acquiring the know-how for system integration. TTCAN can be implemented on the same physical layer as any existing CAN network, with the maximum bit rate of 1 Mbps. No new transceivers or bus lines are needed. The bit rate cannot be increased, neither for CAN nor for TTCAN.

TTCAN is downward compatible with CAN: TTCAN nodes can be configured to operate as existing CAN nodes, existing CAN nodes can operate as listen-only receivers in a TTCAN network. A CAN node transmitting in a TTCAN network would disturb the communication schedule. Starting with a CAN network, all CAN nodes can be replaced step-by-step with TTCAN nodes before switching the communication and the application programs to TTCAN, so a gradual migration from a CAN network to a TTCAN network is possible. All messages in a TTCAN network are transmitted in the CAN format, the wide range of existing analysis tools for CAN bus traffic can also be used for TTCAN. New tools are needed for the temporal configuration of the message transmission schedule.