Distributed Embedded Real-time Systems

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IL2226 Embedded SystemsDesign: Lecture 7

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Distributed Embedded

Real-time Systems

Zhonghai Lu

KTH Royal Institute of Technology

Agenda

Real-time networking

Real-time communication and analysis: CAN

End-to-End WCRT analysis for CAN

Real-time communication: TTP

Application to automobile communicationnetworks

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Realtime systems

Correct systemoperations mean notonly results correctbut also completion ingood time.

Timeliness

Soft deadline Firm deadline

Hard deadline

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Embedded Realtime systems

Uniprocessor system Multiple tasks on one CPU with focus on scheduling policies and

schedulability analysis

Multiprocessor system Multiple tasks multiple CPUs, assuming task communication in

shared variables and synchronization via lock, semaphore etc.

Distributed system Tasks communicate via message passing (not shared variable)

on a bus or network shared communication media Interaction between computation and communication tasks

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Disturbed embedded systems

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Computation

Communication

Control

System architecture

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H. Kopetz. A Comparison of CAN and TTP. Annual Reviews in Control, 24:177188, 2000.

A distributed vehicle control system


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Real-time networking

The end-to-end communication delay must be bounded! All services at all layers must be time-bounded

Requires appropriate time-bounded protocols

Distributed systems Node distribution

For embedded systems Not like open networking systems, embedded systems are closed

systems with good knowledge of application requirements Reliability and cost-efficiency

Centralized arbitration vs. Distributed arbitration

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Protocol stack

The 7 OSI layers impose aconsiderable overhead forembedded implementations.

Many real-time networks are dedicated to a well-defined

application (no need forpresentation)

use single network domain (noneed for routing)

use short messages (no needto fragment/reassemble)

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Collapsed OSI model

Application accesses the Data Link directly

Other layers may be present but are not fullystacked

In industrial automation these networks arecalled field buses

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Data Link Layer

Addressing

Logical Link Control (LLC) Transmission error control

Medium Access Control (MAC) for sharedmedia

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Data link layer: Addressing

Direct addressing The sender and receiver(s) are explicitly identified in every

transaction, using physical address (MAC addresses inEthernet)

Indirect (source) addressing The message contents are explicitly identified (e.g. temperature

of sensor X).

Receivers that need the message retrieve it from the network

(as in CAN) Indirect (time-based) addressing

The message is identified by the time instant at which it istransmitted (as in TTP Time Triggered Protocol)

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Data link layer: LLC

Upper sub-layer of the data link layer, dealing with thereliable information transfer at this level

Typical services Data transfer schemes

Send with immediate acknowledge: Sender waits for acknowledge from receiver Send without acknowledge: No synchronization between data and receiver

Connection-oriented services: A connection must be established before anycommunication may take place

Specify error detection and action upon this. Typical actions are Forward error correction (FEC): Error correcting codes (more related to the

physical layer)

Automatic Repeat reQuest (ARQ): The receiver triggers a repeat request uponerror

Positive Acknowledgement and Retry (PAR): The sender resends if ACK is notreceived

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Data link layer: MAC

Medium Access Control (MAC) Lower sub-layer of the data link layer Determines the order of the network access by

contending nodes and, thus the network accessdelay

Crucial for the real-time behavior of networks

that use a shared medium

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MAC protocols

Master-slave

TDMA

Token circulation

CSMA CSMA-CD CSMA-CR

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MAC: Master-Slave

Access is granted by the Master node Nodes synchronized with the master Requires one control message per data message

Example: LIN, Bluetooth (within piconets)

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What if multiple masters?Master

Slave Slave Slave

1

2

LIN: Local Interconnect

Network

LIN is a broadcast serial network protocol used forcommunication between components in vehicles. Itaims for cost-efficiency to complement CAN. It comprises one master and typically up to 16 slaves.

All messages are initiated by the master with at most one slave replying toa given message identifier. The master node can also act as a slave byreplying to its own messages.

Because all communications are initiated by the master it is not necessaryto implement a collision detection.

The master is typically a microcontroller, whereas the slavesare implemented as ASICs.

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MAC: TDMA

Time-Division Multiple Access Access granted in a dedicated time-slot

Time slots are pre-defined in a cyclic framework

Requires global clock synchronization

High data efficiency

Typically uses static table-based scheduling

Example: TTP/C, TT-CAN, PROFINET

March 1, 2014 17PROFINET is a standard for industrial automation using a computer network

Node A Node B Node C Node C

MAC: Token circulation

Access is granted by the possession of a token

Order of access enforced by token circulation

Real-time operation requires bounded token

holding time Example: FDDI, PROFIBUS

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PROFIBUS (Process Field Bus) is a standard forfieldbus communication in automation technology.

Fiber Distributed Data Interface (FDDI) is a data-transmission standard for in a LAN:

Node AT

Node B Node C

Node D Node E

Token

circulation


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MAC: CSMA

Carrier-Sense Multiple Access

Set of protocols based on sensing businactivity before transmitting (asynchronousbus access)

Collisions may occur.

Upon collision, nodes back off and retry later,according to some specific rule (this ruledetermines to a large extent the real-timefeatures of the protocols)

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CSMA - CD

Carrier-Sense Multiple Access with Collision Detection Used in shared Ethernet (hub instead of switch)

Collisions are destructive and are detected within collision window

Upon collision, the retry interval is random and the randomization

window is doubled for each retry until 1024 slots Non-deterministic (e.g., chained collisions)

Not suitable for a real-time network. However,

The physical Ethernet layer is often used in real-time networks.

It is possible to get real-time performance on an Ethernet network,if the access to the medium is scheduled in some way, i.e.,collisions are avoided

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CSMA - CR

Carrier-Sense Multiple Access with Bit-Wise Arbitration,also called CSMA/CR (Collision Resolution)

Bit-wise arbitration with non-destructive collisions

Upon collision, highest priority node is unaffected. Nodeswith lower priorities retry right after

Deterministic in priority arbitration

Example: CAN

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Communication alternatives

Event-Triggered (ET) communication Communication activities are driven by occurrences of events

More flexible

Time-Triggered (TT) communication Communication activities are driven by the progress of time Dependability is much easier to ensure using a time-triggered bus

Hybrid ET+TT communication

Examples in the automobile industry Event-triggered: LIN, CAN* Time-triggered: TTP/A, TTP/C Hybrid: TT-CAN, FlexRay

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Summary

MAC is crucial for real-time performance Master-slave

Token passing

TDMA

CSMA CSMA-CD

CSMA-CR

ET vs. TT architectures Examples in the automobile industry Event-triggered: LIN, CAN*

Time-triggered: TTP/A, TTP/C

Hybrid: TT-CAN, FlexRayMarch 1, 2014 23

Real-time Communication:

The CAN bus

Zhonghai Lu, Yang Chen

KTH Royal Institute of Technology


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Automobile Communication

Network

March 1, 2014 25Renesas Electronics Corp. Introduction to CAN.

AutomobileCommunication Network

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Control ler Area Network

Birth of CAN Developed originally by BOSCH in 1983,

standardrised in 1993 (ISO 11898)

Purpose Find a bus protocol with good real-time

performance and which can be analysed to showworst case timing behaviour

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History

Developed by BOSCH Starting in 1983 presented at SAE in 1986

Standardized by ISO in 1993 (11898)

First CAN controller chips Intel (82526), Philips (82C200) in 1987

First production car using CAN 1991 Mercedes-Benz S-class (W140)

Milestone of success 1995 Volvo S80 (P23) (with hard real-time guarantee)

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Nowadays

Today almost everynew car sold in Europeuses CAN bus

Sales ofmicroprocessors withon chip CAN capability

increased from under50 million in 1999 toover 750 million in 2010

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Advantages

Token Ring the previous solution Pass the token among processors. The processor

holding the token has the right to transmit

Urgent messages have to wait for the token, whichcauses low real-time performance

Short intervals cause large protocol overhead

Advantages of CAN Associate a priority with each message and use

priority based arbitration mechanism to ensure thatthe higher priority message has shorter delay

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CAN Bus

CAN is a multi-master CSMA/CR serial bus 1 Mbit/sec with the length no longer than 50m 500Kbit/sec with the length no longer than 100m Arbitrary number of CAN stations can be connected

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Host

Processor

CAN

Controller

CAN BUS

ECU

Host

Processor

CAN

Controller

ECU

ECU:Execution Control Unit

CAN bus features

Priority-based Messages have a unique (pre-defined) global identifiers.

Report error if two messages are found with the sameidentifier.

Each node can send many messages Non-preemptive

Message being transmitted can not be preempted byhigher priority messages

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Distributed priority arbitration

CAN physical layer sends a bit stream, with each bitsupports two states: 0dominant, 1recessive

Transmit ID from the most significant ones. Collisions among identifier bits are resolved by the

logical AND semantics, and each node reads thebus state while it transmits

Node reads its identifier bits on the medium without

any change, it realizes it is the winner

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Transmission

Transmission steps CAN nodes wait for bus idle before starting

transmission

Synchronise on the SOF bit (0) Each node starts to transmit the ID for arbitration Node transmitting highest priority message wins

arbitration, which then completes the transmission

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Example of Bus Arbitration

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Standard11-bit identifier

Smaller Id value, higher priority.

CAN Frames

Messages are transmitted in frames. Frame Types:

Data: data information

Remote: request transmission of a Data frame

Error: transmitted whenever a node detects an error

Overload: for flow control to request an additionaltime delay

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CAN Frame Format

Arbitration field: ID with 11 bit (standard) or 29 bit (extended)

ID associates priority A message with smaller ID has higher priority

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Bit Stuffing

If 5 consecutive bits with the same levelappear, one bit of inverted data is added 000000 and 111111 are used to signal errors

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How many stuffing bits are needed?

'5

ll

l ' l1

4

21 bits

26 bits


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Message

maximum transmission time

Worst case bit stuffing

l is the size of bits that are related to bit stuffing

Maximum transmission time si is message size in byte g is the sum size of

arbitration, control andCRC field

tbit is the time to transmitone bit

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l ' l1

4

Ci g 8si 1

4

g 8si 13

tbit

Standard 11-bit identifier

What is the maximum size of a frame?

Summary

Messages are transmitted in bit-stream frames,which are prioritized globally. An identifier is transmitted along with data

Bit stuffing to accommodate special-purpose codes Use CSMA-CR as MAC for the serial bus

Natural AND to determine the winner upon collision

Higher priority wins over lower priority frames.

Deterministic maximum messagecommunication time (no bus contention) Non-preemptive transmission

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Real-time Communication

Analysis on CAN

Zhonghai Lu, Yang ChenKTH Royal Institute of Technology

Real-time analysis for CAN

A common misconception in the early 1990s Good at transmitting the highest priority message with low

latency, but not possible to guarantee low priority messages

Real-time analysis for CAN Tindell and Burns (York University, 1994) showed how

research into fixed priority pre-emptive scheduling forsingle processor systems could be adapted and appliedto the scheduling of messages on CAN

In 1995 it was recognized by Volvo Car Corporation andused in the development of the Volvo S80 (P23)

Due to Tindells work, CAN has been widely recognized since1995

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Message modeling

Periodic messages

Each message has Unique priority Pi

smaller i, higher priority

Period Ti Maximum transmission time Ci Queuing jitter Ji Deadline D

i(D

i T

i)

Worst-case Response time(WCRT)Ri

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Queuing Jitter

Queuing jitter The maximum difference between the reference

arrival and the actual arrival of a message

Reason causing queuing jitter Task queuing the message may be finished very

soon (BCRT) or finished with WCRT of the task. E.g., Task j queues message i, Ji (queuing jitter) is

equal to Rj (WCRT BCRT of task j)

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WCRT analysis

Two steps Determine the worst case Calculate WCRT from the worst case

To determine the worst case, need to consider Starting of tasks are asynchronously Different combinations of the starting cause

messages experiencing different response time

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Crit ical Instant

Critical Instant ofMi Mi with maximum queuing jitter is queued with all higher

priority message simultaneously, then each of these higherpriority messages is subsequently queued again after theshortest possible time interval

A lower priority message with maximum execution starts to betransmitted just before the instant, so called priority inversion

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C1 2

C2 1

C3 2


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Priori ty Inversion

Blocking time due to priority inversion A message may suffer from blocking that is the delay

caused by transmission of a lower priority message

Maximum blocking timeBi

Upper-bounded by Example:

Mk is the lower priority message with largest size

Mi is queued just after the SOF bit ofMkhas been sent

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Bi maxklp(i )

(Ck ) 135tbit

135 bitt

WCRT Calculation

Wi: waiting and communication time Hp(i): higher priority messages than i Bi: blocking time caused by lower priority message

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Ri Ji wi

win1 Ci Bi

Jkwin

Tk

khp(i )

Ck


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Example of WCRT Analysis

49

0

2

12 2 2 1

2

2 2 2 1

3

2 2 2 1

0

5

2 7

2 7

w

w C B C

w C B C

w C B C

RiJi wi

win1 Ci Bi

Jk win

Tk

khp(i ) Ck

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C1 2

C2 1

C3 2

1

2

3

1

1

1

J

J

J

T1 5

T2 20

T3 100

2 3 2B C

2 2 2 7 1 8R J w

Compute WCRT of message MA2

Summary

Message modeling Queuing jitter

Priority inversion

WCRT analysis of CAN message Identify the critical instant (higher priority

messages and a lower priority message) WCRT calculation for the critical instant scenario

using the formulas

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Acknowledgements

Karl-Erik rzen , Lecture slides on Realtime systems, LundUniversity.

Ken Tindell and Hans Hansson. Real Time Systems by FixedPriority Scheduling, Uppsala University, October 1997

H. Kopetz. GNTHER BAUER. The Time-Triggered Architecture.PROCEEDINGS OF THE IEEE, VOL. 91, NO. 1, JANUARY 2003.

H. Kopetz. A Comparison of CAN and TTP. Annual Reviews inControl, 24:177188, 2000.

Safety-related architectures and time-triggered networkshttp://www.ttagroup.org/ FlexRay: Automotive Communication Bus Overview. National

Instrument, Aug 21, 2009

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Acknowledgements

Marco Di Natale, Haibo Zeng, Paolo Giusto, Arkadeb Ghosal,Understanding and Using the Controller Area NetworkCommunication Protocol Theory and Practice

Roman ObermaisserTime-Triggered Communication (EmbeddedSystems)Nov. 2011

R.I.Davis, A. Burns, R.J. Bril, and J.J. Lukkien. Controller AreaNetwork (CAN) Schedulability Analysis: Refuted, Revisited andRevised. Real-Time Systems, Volume 35, Number 3, pp. 239-272,April 2007.

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