FlexRay Introduction

What is FlexRay

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

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 regard to error tolerance and time-determinism of message transmission. FlexRay fulfils these increased requirements by message transmission in fixed time slots and by fault-tolerant and redundant message transmission on two channels.

Physical Layer

FlexRay works according to the principle of TDMA, 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.

Figure 1: Possible FlexRay topology with 2 channels

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. [Table of contents]

Figure 2: Typical FlexRay Communication Cycle

Data rate

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 transmit different messages, in which case the data throughput is doubled. FlexRay can also be operated with lower data rates. [Table of contents]

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, synchronization 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. [Table of contents]

Structure of a FlexRay ECU

A FlexRay ECU consists of a host processor, the FlexRay Communication Controller (CC) and the Bus Guardian (BG). The host processor supplies and processes the data, which are transmitted via the FlexRay contoller.

The BG monitors access to the bus. The host processor informs the BG which time slots the FlexRay CC has allocated. The BG then allows the FlexRay CC to transmit data only in these time slots and enables the BD. Data can be received at any time. [Table of contents]

Figure 3: Possible FlexRay ECU


The FlexRay network topology is largely open. However, only the daisy-chain connection and active star are used in automotive applications.

The active star is an active node that receives messages on one branch and broadcasts them to the other branches, similar to a hub.

A daisy-chain connection is understood to be merely a FlexRay line looped through from one node to another. [Table of contents]

Wakeup, Startup

A wakeup can occur either via the FlexRay network or locally via switch-on of a node, which initializes it.

Unlike with the CAN network, communication does not begin directly after wakeup. In order to start up a FlexRay network, a startup process must first be run by so-called cold-start nodes (these are nodes that may initiate FlexRay network startup) with startup messages. [Table of contents]

Database format

With a CAN network, a *dbc file is usually used as a communication matrix. The FlexRay network with all of its nodes, including their communication, is described either in FIBEX or AUTOSAR.

The FIBEX format is a XML file describing complete vehicle networks. It is an open standard defined by ASAM e.V. The FIBEX format permits the complete description of FlexRay, CAN, MOST and LIN networks, including all mutual gateway connections in the vehicle, in a single file.

AUTOSAR also describes communication similar to the FIBEX standard, but supplements this with additional information on functions and behavior of the control units. Therefore, conversion from AUTOSAR to FIBEX is also possible. [Table of contents]


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