1. Guaranteeing Message Latencies on Controller Area Network (CAN)
Garrett Chandler
27 March 2006
EE 699

2. Garrett Chandler Guaranteeing Message Latencies on CAN
Outline
Introduction
System Model
System Analysis
SAE Benchmark Case
Discussion and Conclusions
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

3. Garrett Chandler Guaranteeing Message Latencies on CAN
Introduction
Common Automotive Industry View
CAN good for most urgent data
CAN unable to provide guarantees for less urgent data
View Incorrect
CAN scheduling virtually identical to real-time process scheduling
Analysis can be applied almost without change to CAN latency problem
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

4. Garrett Chandler Guaranteeing Message Latencies on CAN
System Model: CAN
Typical CAN Stuff…
Assumptions
Fixed Set of Messages
Each w/
Unique Priority
Rate
Max Size
No RTR Messages
Soft Real-Time as Background Messages (low-priority IDs)
Hardware
Intel 82527
15 slots
1 in only, 14 in or out
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

5. System Model: Messaging
Messages queued by software task
Bounded time between event and queue entrance
Queuing Jitter: variability between subsequent queuings of message
Period: minimum time between same task invocations
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

6. System Model : Queuing Jitter
Jitter is the difference between the earliest and latest possible times a given message can be queued
Jitter important for analysis
Options exist to reduce queuing jitter
Tm = Period
Jm = Jitter
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

7. Garrett Chandler Guaranteeing Message Latencies on CAN
System Analysis
Analysis will bound worst-case latency of given message
Almost direct application of processor scheduling theory
Assumptions
Deadline of a message must not be more than the period of a message
Controller always sends highest priority message
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

8. System Analysis: Notation
Deadline, Dm – Maximum time between event and response
Period, Tm – Minimum time between events
Worst-Case Response Time, Rm – Longest time between message entering queue and the latest said message arrives at receiving nodes
Queuing Jitter, Jm - Variability between subsequent queuings of message
Queuing Delay, wm – Longest time a message may sit in a queue before being sent
Transmission Time, Cm – Time it takes to send given message on the bus
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

9. System Analysis: Response
Rm – Worst-case response time of message
Jm – Queuing Jitter
wm – Queuing Delay
Cm – Transmission Time
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

10. System Analysis: Transmission
Cm – Transmission Time
Accounts For
Frame Overhead
Data Contents
Stuff Bits
sm – size of message in bytes
tbit – bit time of the bus (1us at 1MHz)
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

11. System Analysis: Queuing
wm – Queuing Delay
Set hp(m) contains all messages in system of higher priority than m.
Bm is the longest time that m can be delayed by lower priority messages.
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

12. System Analysis: Delay
Bm – Arbitration Delay
Set lp(m) contains all messages in system of lower priority than m.
In case of unbounded number of lower-priority messages of indeterminate size, Bm is 130 bit-times.
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

13. System Analysis: Scheduling
Given equations assume nothing about system choice of message priorities
Work on processor scheduling shows that optimal ordering of priorities is deadline monotonic
Tasks with smallest difference in deadline and jitter should have highest priority
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

14. Garrett Chandler Guaranteeing Message Latencies on CAN
SAE Benchmark Case
SAE Example System
Battery, Vehicle Controller, Motor Controller, Instrument Panel, Driver Inputs, Brakes, Transmission
53 Messages according to Kopetz [6]
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

15. SAE Benchmark: Latencies
Messages Ordered by D – J
1 to 1 ID to Data Assignment
Bold = Sporadic Signal
X = failed to meet latency requirements
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

16. SAE Benchmark: Utilization
System can not be scheduled at 125 kbps
Message Utilization
Data bits / Bus bits
Bus Utilization
Total bits / Bus bits available
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

17. Garrett Chandler Guaranteeing Message Latencies on CAN
82527 Limitation
Limitation
Vehicle controller transmits more than 14 message IDs
Solution
Concatenate more data into single packet
Results
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN

18. Discussion and Conclusions
Scheduling analysis developed for fixed priority scheduling can be adapted for use with CAN.
Analysis applied to benchmark system.
Paper omits discussion and analysis of re-transmission of messages after a detected error.
27 March 2006
Garrett Chandler Guaranteeing Message Latencies on CAN