Image Processing Reference
In-Depth Information
Active star
Bus
Active cascaded stars
Active stars with bus extension
FIGURE .
Topologies.
The existing FlexRay CCs support communication bit rates of up to Mbps on two channels
over an electrical physical layer. The physical layer is connected to the controller via a transceiver
component.
This physical layer supports bus topologies, star topologies, cascaded star topologies, and bus stubs
connected to star couplers as shown in Figure .. ..This multitude of topologies allows a maximum
of scalability and flexibility of electronic architectures in automotive applications.
Beside transformation of bit streams between the CC and the physical layer, the transceiver com-
ponent also provides a set of very specific services for an automotive network. The major services
are alarm handling and wakeup control. Alarm signals are a very powerful mechanism for diverse
information exchange between a sender controller and receiver controllers. A sender transmits an
alarm symbol on the bus parallel to alarm information in a frame. A receiver ECU receives the alarm
information in the frame like normal data. Additionally, the CC receives the alarm symbol on the
physical layer and indicates this symbol to the ECU. hus the ECU has two highly independent indi-
cators for an alarm to act on. This scheme can be used for the validation of critical signals like an
air-bag fire command.
The second type of service provided by the symbol mechanism is the wakeup function. A wakeup
service is required in automotive applications where electronic components have a sleep mode, in
which power consumption is extremely reduced. he wakeup service restarts normal operation in all
sleeping ECU components. In a network, the wakeup service uses a special signal that is transmitted
over the network. In FlexRay, this function relies on the ability of a transceiver component to identify
a wakeup symbol and to signal this event to the CC and the remaining components of the ECU to
wake these components up.
16.3.5 Bus Guardian
The media access strategy completely relies on the cooperative behavior of every CC in a network.
The protocol mechanisms inside a controller ensure this behavior to a considerable high level of con-
fidence. However, for safety-relevant applications, the controller internal mechanisms do not provide
a sufficiently high level of safety. An additional and independent component is required to ensure that
no controller can disturb the media access mechanism of the network. his additional component is
called bus guardian (BG).
In FlexRay, the BG is a component with an independent clock and is constructed such that an
error in the controller cannot influence the guardian and vice versa. The BG is configured with its
