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Introduction to FlexRay

FlexRay is a fast, deterministic and fault-tolerant bus system for automotive use, based on the experience of DaimlerChrysler with the development of prototype applications and the byteflight communication system developed by BMW. Byteflight was developed by BMW especially for use in passive safety systems (airbags). In order to also fulfill the requirements of active safety systems, byflight was further developed by the FlexRay consortium in particular in relation to time-determinism and fault tolerance. Today, the car-makers BMW, DaimlerChrysler, General Motors, Ford, Volkswagen as well as the companies Bosch, Motorola and Philips Semi-conductors are represented in the FlexRay consortium.

The data exchange between the numerous number of control devices, sensors and actuators in automobiles is nowadays mainly carried out via CAN networks. However, the introduction of the new x-by-wire systems results in increased requirements especially with regards to error tolerance and time-determinism of message transmission. FlexRay fulfills these increased requirements by message transmission in fixed time slots and by fault-tolerant and redundant message transmission on two channels.

Access to the transmission medium
FlexRay works according to the principle of TDMA (Time Division Multiple Access), where the components or messages have allocated fixed time slots in which they have exclusive access to the bus. The time slots are repeated in a fixed cycle. The time at which a message is on the bus can be exactly predicted and bus access is therefore deterministic.

However, the fixed allocation of the bus bandwidth to the components or messages by means of fixed time slots has the disadvantage that the bandwidth is not fully exploited. For this reason FlexRay subdivides the cycle into a static and a dynamic segment. The fixed time slots are situated in the static segment at the beginning of a bus cycle. In the dynamic segment the time slots are assigned dynamically. Exclusive bus access is only enabled for a short time in each case (so-called "mini-slots"). The time slot is only extended by the required time if a bus access occurs within a mini-slot. Bandwidth is therefore only used up when it is actually needed.

Data rate of FlexRay
FlexRay communicates via two physically separated lines with a data rate of 10 Mbit/s each. The two lines are mainly used for redundant and therefore fault-tolerant message transmission but can also tranmit different messages, in which case the data throughput is doubled.

FlexRay can also be operated with lower data rates. With data rates < 1 Mbit/s, a classic bus structure (such as with CAN) is permitted. With data rates > 1 Mbit/s, the components are connected by point-to-point connections with active star couplers.

Synchronization of nodes
In order to implement synchronous functions and optimize the bandwidth by means of small distances between two messages, the distributed components in the communication network require a common time base (global time). For clock synchronization, syn-chronization messages are transmitted in the static segment of the cycle. With the aid of a special algorithm, the local clock-time of a component is corrected in such a way that all local clocks run synchronously to a global clock.

Structure of a FlexRay network node
A FlexRay network node consists of a host processor, FlexRay controller and bus guardian. The host processor supplies and processes the data, which are transmitted via the FlexRay controller.

The bus guardian monitors access to the bus. The host processor informs the bus guardian which time slots the FlexRay controller has allocated. The bus guardian then allows the FlexRay controller transmit access only in these time slots. Data can be received at any time.

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