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© Carl Hanser Verlag, München www.hanser-automotive.de Nicht zur Verfügung im Intranet- und Internet-Angeboten sowie elektronischen Verteilern 20lA UTOMOTIVE 2008l SPECIAL EDITION FLEXRAY Creating FlexRay COM Stack Configurations A FlexRay controller typically offers, or addresses via DMA access, several kilobytes of buffer memory. AUTOSAR defines a comprehensive set of layers to manage FlexRay communication in an ECU, from high-level signal management in the COM and handling of large data packets in the Transport Protocol layer down to the FlexRay Interface Layer for management of real-time interfaces and the hardware abstraction in the FlexRay Driver layer. But how can we efficiently allocate the FlexRay buffer memory of an ECU to the communication frames transmitted and received by an ECU? How does this affect the CPU load of this ECU, considering that each access to a communication buffer creates some work for the CPU? The Allocation Challenge The challenge itself is illustrated by an example: Assume a “telephone service” shop where 10 telephone booths are available for customer use and we are responsible for assigning customers to phones. Customers can “transmit” by making phone calls there, or “receive” by taking incoming calls there. How can we ensure that the use of the booths is optimized? Since we know our customers, we try to plan ahead: We have to exclusively reserve a booth for each of our “very important” customers. Whenever they require a phone, for ECUs in Complex Networks we need to have a phone booth available for them. Some customers are known for their short phone calls, usually no more than a few minutes. With regard to this additional information or “constraints”, we are able to develop efficient strategies for allocating phones to the customers. This illustrates the challenge that needs to be solved for complex real-time networks: We have a limited number of buffers (phone booths) and a large number of communication frames (customers), transmitted or received by the FlexRay communication controller (the shop) of our ECU. The buffers have to be allocated to the frames in a way so
© Carl Hanser Verlag, München www.hanser-automotive.de Nicht zur Verfügung im Intranet- und Internet-Angeboten sowie elektronischen Verteilern that no information is lost (this happens if no buffer is free upon reception) or unduly delayed (this happens if no buffer is available when a transmission has to be made). Optimization Goal of AUTOSAR An additional constraint is related to how AUTOSAR handles the FlexRay communication: The interface handling the access to the FlexRay buffers, called the FlexRay Interface Layer (FrIf), is a software layer executed at certain pre-defined times and with certain pre-defined actions (“jobs”) such as packing and unpacking data. It is like the manager at the phone booth shop: he has some overhead getting up to assign a customer to a booth. If there are many booths and many customers, it would be more efficient to do several assignments in one step. These actions correspond to task activation in an ECU. If the FrIf has to be activated for each job (e.g. transmission or reception of a PDU), a lot of CPU time will be spent just on FrIf activations. It reduces the overhead significantly if we can group these jobs. A FlexRay communication controller offers dozens or hundreds of buffers, depending on the hardware and, in some cases, on the length of the messages. But several dozens or several hundred FlexRay frames may have to be transmitted and received on this node. This is not a fictional scenario: Some ECUs such as gateways need to send or receive many of the frames on a network, and a nontrivial FlexRay system will usually contain hundreds of different frames. For many of these frames (“static frames”), the frequency, phase and order are exactly known from the Flex- Ray communication schedule, while for others (“dynamic frames”), this is not necessarily so. Even with a copious amount of communication buffers, the authors found that it is not a trivial task to find proper allocations for the buffers when implementing AUTOSAR configurations for ECUs sending and receiving significant SPECIAL EDITION FLEXRAYl AUTOMOTIVE 2008l21 amounts of data in large FlexRay networks. As explained above, the challenge is to define the buffer allocation in a way that buffers are efficiently used, all frames can be received and transmitted in time, and the CPU load generated by the FlexRay Interface layer activations is kept to a reasonable minimum. Fig.1:The validity span of the buffers, shown for a period of the green and the blue PDU. When using 2 buffers for the frames in slot 2, there are just 4 FRIF interrupts necessary. © automotive Several Possibilities for Allocation The purpose of the buffer allocation is to assign message buffers to slots and channels, select the buffer direction (“transmit” or “receive”), and define cycle counter filtering: Like for CAN, a FlexRay communication buffer can be programmed to receive only frames with defined identifiers or identifier ranges, e.g. “receive frame ID 3” or “receive frame IDs in the range 0x10..0x1F”. FlexRay communication is organized in (strictly) 64 communication cycles, and the filtering can refer to these cycles too, e.g. “receive frame 3 in any communication cycle” or, alternatively, “receive frame 3 only in communication cycle 20”. To find a buffer for every frame sent or received by a node can be a rather simple task when support for the FlexRay Interface layer is not considered: In this case, one buffer Mobile drives Automotive. Today‘s consumers are digital natives who live their life connected via blogs, communities, and online games. They expect anything, anywhere, anytime, on any device. Get mobile in your car. www.tietoenator.com TietoEnator is a professional service company providing IT, R&D and consulting services.
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