FlexRay Primer

Courtesy of, and in alliance with, our DG Partner Star Cooperation's Ebersp├Ącher

Star Coorperation


What is 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 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.

The 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.

FlexRay Physical

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.

FlexRay Communications Cycle

Figure 2: Typical FlexRay Communication Cycle

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

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.

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.

FlexRay ECU (example)

Figure 3: Possible FlexRay ECU