Datasheet
www.tektronix.com/oscilloscopes 29
Debugging Serial Buses in Embedded System Designs
FlexRay
Background
FlexRay is a relatively new automotive bus. As cars get
smarter and electronics find their way into more and more
automotive applications, manufacturers are finding that
existing automotive serial standards such as CAN and LIN
do not have the speed, reliability, or redundancy required to
address X-by-wire applications such as brake-by-wire or steer-
by-wire. Today, these functions are dominated by mechanical
and hydraulic systems. In the future they will be replaced by a
network of sensors and highly reliable electronics that will not
only lower the cost of the automobile, but also significantly
increase passenger safety due to intelligent electronic based
features such as anticipatory braking, collision avoidance,
adaptive cruise control, etc.
How It Works
FlexRay is a differential bus running over either a Shielded
Twisted Pair (STP) or an Un-shielded Twisted Pair (UTP) at
speeds up to 10 Mb/s, significantly faster than LIN’s 20 kb/s or
CAN’s 1 Mb/s rates. FlexRay uses a dual channel architecture
which has two major benefits. First, the two channels can
be configured to provide redundant communication in safety
critical applications such as X-by-wire to ensure the message
gets through. Second, the two channels can be configured to
send unique information on each at 10 Mb/s, giving an overall
bus transfer rate of 20 Mb/s in less safety-critical applications.
FlexRay uses a time triggered protocol that incorporates the
advantages of prior synchronous and asynchronous protocols
via communication cycles that include both static and dynamic
frames. Static frames are time slots of predetermined length
allocated for each device on the bus to communicate during
each cycle. Each device on the bus is also given a chance to
communicate during each cycle via a Dynamic frame which
can vary in length (and time). The FlexRay frame is made up
of three major segments; the header segment, the payload
segment, and the trailer segment. These segment each have
their own components as shown in Figure 40.
Figure 40. FlexRay frame structure.
Header Segment Payload Segment
reserved bit
payload preamble indicator
null frame indicator
sync frame indicator
startup frame indicator
Trailer Segment
CRCCRCCRC
Data 0
6 bits
FlexRay Frame 5 + (0 ... 254) + 3 bytes
Data 1Data 2Data n
Cycle
count
Payload
length
Header
CRC
Frame ID
24 bits7 bits
11111
11 bits 0 ... 254 bytes11 bits