This presentation is the result of my team in the course "Embedded Systems" at the University of Massachusetts, Amherst. It presents the findings of the paper "Timing analysis of the FlexRay communication protocol", a communication network with automotive uses.

1. 1
TIMING ANALYSIS OF THE FLEXRAY
COMMUNICATION PROTOCOL
presented by:
CHENG SUN // YINGLAI YANG // YUNNING LI
TRAIAN POP, PAUL
POP, PETRU ELES, ZEBO
PENG, ALEXANDRU
ANDREI

2. 2
Developments in automotive
 Comfort:
• x-by-wire (drive-by-wire, brake-by-wire, …)
• Lane control
 Safety:
• Automatic emergency brake
• Pedestrian recognition
 Future:
• Self-driving cars
CHENG SUN // YINGLAI YANG // YUNNING LI

3. 3
Realized by distributed systems
CHENG SUN // YINGLAI YANG // YUNNING LI

4. 4
New solution: FlexRay
 We need fast, deterministic (safe) communication
protocol: Development started at BMW and continued by
a consortium resulted in FlexRay protocol.
 CAN, LIN vs. FlexRay
Bus LIN CAN FlexRay
Speed 40 kbit/s 1 Mbit/s 10 Mbit/s
Cost $ $$ $$$
Wires 1 2 2 or 4
Typical
Applications
Body
Electronics
Powertrain Safety-critical
apps
CHENG SUN // YINGLAI YANG // YUNNING LI

5. 5
The FlexRay Protocol
 The overview of the system
CHENG SUN // YINGLAI YANG // YUNNING LI

6. 6
The FlexRay Protocol
 Detailed information of cycles
CHENG SUN // YINGLAI YANG // YUNNING LI

7. 7
Internal architecture of a node
 CPU: writes
messages into...
 CHI: reserves a
buffer...
 Controller:
reads messages
from...
 Black Chunk:
~Static
 Grey Chunk:
~Dynamic
CHENG SUN // YINGLAI YANG // YUNNING LI

8. 8
Internal architecture of a node
The number
of slots in
ST is fixed,…
The size of
each slot is
fixed,…
CHENG SUN // YINGLAI YANG // YUNNING LI

9. 9
Delays in dynamic segment
 (1) Delays by higher priority messages.
Here mg and mf
share the same
Frame ID,
but ...
CHENG SUN // YINGLAI YANG // YUNNING LI

10. 10
(1)Delays by higher priority messages
 Here mg is delayed by mf .
CHENG SUN // YINGLAI YANG // YUNNING LI

11. 11
Delays in the dynamic segment
 (2)Delays by earlier (lower frame) messages and without
enough slots left.
Such as mh,
static segment;
Frame ID=5;
Assuming that the
length of mh takes
2 slots
CHENG SUN // YINGLAI YANG // YUNNING LI

12. 12
(2)Delays by earlier (lower FrameID) messages
and without enough slots left.
 We can not
be too
stingy to
mh !
CHENG SUN // YINGLAI YANG // YUNNING LI

13. 13
(2)Delays by earlier (lower FrameID) messages
and without enough slots left.
 Finally, mh
finds an
available
position.
 Although…
 What if mg …
CHENG SUN // YINGLAI YANG // YUNNING LI

14. 14
Timing analysis
Question for safety issue:
How long can it take until a message reaches its recipient?
“The driver turns the steering wheel to the left. How long does it maximally
take until the car follows that movement?”
Value of interest:
Worst Response Time – Longest possible time from
creation of message to complete arrival.
CHENG SUN // YINGLAI YANG // YUNNING LI

15. 15
Quantization of delay-causes
Note:
Assume regular, periodic messages with period T (dynamic
messages)
Such a message will occur
𝑡+𝐽
𝑇
times in the time frame 𝑡.
E.g. ℎ𝑝 𝑡 =
𝑡+𝐽ℎ𝑝
𝑇ℎ𝑝
for higher priority messages
J is the jitter of the message → difference between best response time to worst response
time
CHENG SUN // YINGLAI YANG // YUNNING LI

16. 16
Definition of response time
Important for us:
Those are the only things that can change.
CHENG SUN // YINGLAI YANG // YUNNING LI

17. 17
Mathematical formulation for 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚
Make it an ILP problem:
Furthermore constraints are added to the ILP formulation
to satisfy the FlexRay protocol …
frame overfilled: 𝑦𝑖 = 1
frame can take message: 𝑦𝑖 = 0
CHENG SUN // YINGLAI YANG // YUNNING LI

18. 18
Messages are not sent multiple times per cycle
CHENG SUN // YINGLAI YANG // YUNNING LI

19. 19
FrameIDs are unique
Or: „You shall not use the same 𝑓𝑟𝑎𝑚𝑒𝐼𝐷 multiple times in
one cycle“
CHENG SUN // YINGLAI YANG // YUNNING LI

20. 20
Fixed assignment of frameID
Or: „A message can only be transmitted with the 𝑓𝑟𝑎𝑚𝑒𝐼𝐷
that was assigned to it“
CHENG SUN // YINGLAI YANG // YUNNING LI

21. 21
Messages need to fit completely
Or: „Any message can only be sent if it fits in the remaining
space.“
CHENG SUN // YINGLAI YANG // YUNNING LI

22. 22
Optimal solution for 𝑤 𝑚
′
After 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚 is known, we maximize the delay in the
cycle where the message 𝑚 is being sent (inserted in the
dynamic frame).
ILP is used again and the max. 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚 is being
enforced. The optimization goal is now to find the largest
possible 𝑤 𝑚
′
.
Same constraints as before (FlexRay compliance)
CHENG SUN // YINGLAI YANG // YUNNING LI

23. 23
Summary of optimal calculation
 ILP used
 „Optimization“ goal: Maximize 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚 and 𝑤 𝑚
′
 Constraints used to model FlexRay rules
However:
 ILP is very computationally complex. Takes extremely
long and can exceed available computational power
easily!
CHENG SUN // YINGLAI YANG // YUNNING LI

24. 24
Heuristic Solution for 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚 & 𝑤 𝑚
′
Simplification of calculation of 𝐵𝑢𝑠𝐶𝑦𝑐𝑙𝑒𝑠 𝑚:
Straightforward computation of 𝑤 𝑚
′
:
CHENG SUN // YINGLAI YANG // YUNNING LI
by adding pessimistic 𝑚𝑠-count to communication time:

25. 25
Experimental Setup
CHENG SUN // YINGLAI YANG // YUNNING LI
OO- is not used in production since it is optimistic about
WRT. It is only used to be able to calculate a common
reference value for OH and HH.

26. 26
Experimental Results
Where A presents OO, OH or HH, and n is the number of
messages in the analyzed application.
CHENG SUN // YINGLAI YANG // YUNNING LI

27. 27
Experimental Results
CHENG SUN // YINGLAI YANG // YUNNING LI
Fewer frameIDs/processor → constraints more important →
heuristics worse

28. 28
Conclusions
 The paper showed a way to analyze the worst response
time.
 It was shown how heuristics can be used to radically
reduce the calculation time of the WRT with a trade off
in additional pessimism of the result.
 Allows timing assertions for safety analysis.
 First paper to analyze dynamic FlexRay timing without
simplifying constraints.
 Good heuristics with very low calculation time.
CHENG SUN // YINGLAI YANG // YUNNING LI

29. 29 CHENG SUN // YINGLAI YANG // YUNNING LI
Thank you for your attention!

30. 30
Q&A
CHENG SUN // YINGLAI YANG // YUNNING LI