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A Time-Predictable Open-Source Ttethernet End-System Downloaded from orbit.dtu.dk on: Oct 01, 2021 A time-predictable open-source TTEthernet end-system Kyriakakis, Eleftherios; Lund, Maja; Pezzarossa, Luca; Sparsø, Jens; Schoeberl, Martin Published in: Journal of Systems Architecture Link to article, DOI: 10.1016/j.sysarc.2020.101744 Publication date: 2020 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Kyriakakis, E., Lund, M., Pezzarossa, L., Sparsø, J., & Schoeberl, M. (2020). A time-predictable open-source TTEthernet end-system. Journal of Systems Architecture, 108, [101744]. https://doi.org/10.1016/j.sysarc.2020.101744 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. 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Journal of Systems Architecture 108 (2020) 101744 Contents lists available at ScienceDirect Journal of Systems Architecture journal homepage: www.elsevier.com/locate/sysarc A time-predictable open-source TTEthernet end-system Eleftherios Kyriakakis ∗, Maja Lund, Luca Pezzarossa, Jens Sparsø, Martin Schoeberl Department of Applied Mathematics and Computer Science, Technical University of Denmark, Denmark a b s t r a c t Cyber-physical systems deployed in areas like automotive, avionics, or industrial control are often distributed systems. The operation of such systems requires coordinated execution of the individual tasks with bounded communication network latency to guarantee quality-of-control. Both the time for computing and communication needs to be bounded and statically analyzable. To provide deterministic communication between end-systems, real-time networks can use a variety of industrial Ethernet standards typically based on time-division scheduling and enforced by real-time enabled network switches. For the computation, end-systems need time-predictable processors where the worst-case execution time of the application tasks can be analyzed statically. This paper presents a time-predictable end-system with support for deterministic communication using the open-source processor Patmos. The proposed architecture is deployed in a TTEthernet network, and the protocol software stack is implemented, and the worst-case execution time is statically analyzed. The developed end-system is evaluated in an experimental network setup composed of six TTEthernet nodes that exchange periodic frames over a TTEthernet switch. 1. Introduction In this work, we focus on safety-critical systems and investigate the TTEthernet protocol. TTEthernet uses a fault-tolerant synchronized Advancements in the field of safety-critical systems for industrial communication cycle with strict guarantees for transmission latency control and avionics/automotive automation have brought a recent fo- [17] . In addition, the protocol provides support for rate-constrained traf- cus on distributed real-time system communication [44] . This trend is fic and best-effort traffic classes. TTEthernet has been standardized un- emphasized by the upcoming paradigm of Industry 4.0 and the recent der the aerospace standard SAE AS6802 [43] and has been implemented efforts of the time-sensitive networking (TSN) group [15] to develop a as a communication bus replacement in both automotive and aerospace set of deterministic Ethernet standards that meets the requirement for real-time applications [7,23] . system’s interoperability and real-time communications. This paper presents a time-predictable TTEthernet end-system imple- The correct execution of real-time applications depends both on the mented on the open-source processor Patmos [33] , allowing for static functional correctness of the result as well as the time it takes to pro- WCET analysis of the networking code. This work enables the Patmos duce the result. A typical example that illustrates this criticality of func- processor to communicate through a deterministic network protocol, tional and temporal correctness is a vehicle collision avoidance system. allowing the possibility for end-to-end bounded latency communication In this situation, the processor must correctly detect a possible object, that includes the software stack. We do so by extending the existing Eth- but it is equally important that communication time, from the cam- ernet controller driver and implement a TTEthernet software stack. We era to the processor, and processing time, of the image on the proces- test the performance of the controller driver by evaluating the achieved sor, are deterministically bounded to consider the system as correctly clock synchronization and correct exchange of TT frames and perform functioning. a static WCET analysis of the software stack. To achieve deterministic communication with bounded latency, The main contributions of this work are: real-time systems often employ a time-triggered (TT) communication • A TTEthernet node that combines time-predictable execution of tasks paradigm such as the well-known time-triggered protocol [18] , which with time-triggered communication through TTEthernet is based on a cooperative schedule and a network-wide notion of time • A WCET analyzable Ethernet software stack, which allows to stat- [35] . This approach can be implemented on Ethernet communications, ically guarantee that all deadlines and end-to-end timing require- such as TTEthenet [17] and TSN [10] , to provide the guaranteed net- ments are met working services (latency, bandwidth, and jitter) to distributed real-time • Performing a comparative analysis of the effects of number of inte- applications in avionics, automotive, and industrial control systems. gration cycles against the achieved clock synchronization ∗ Corresponding author. E-mail addresses: [email protected] (E. Kyriakakis), [email protected] (M. Lund), [email protected] (L. Pezzarossa), [email protected] (J. Sparsø), [email protected] (M. Schoeberl). https://doi.org/10.1016/j.sysarc.2020.101744 Received 29 July 2019; Received in revised form 5 January 2020; Accepted 9 February 2020 Available online 26 February 2020 1383-7621/© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license. (http://creativecommons.org/licenses/by/4.0/ ) E. Kyriakakis, M. Lund and L. Pezzarossa et al. Journal of Systems Architecture 108 (2020) 101744 • Implementing a PI controller that improves the achieved clock syn- jitter, and frame size was investigated. A comparison between a com- chronization precision mercial off-the-shelf switch and a TTEthernet switch was presented as a function of link utilization and end-to-end latency. This emphasized To the best of our knowledge, this is the first WCET analyzable the benefits of TTEthernet over standard Ethernet switches. The mea- TTEthernet node that combines time-predictable communication over sured jitter for the system was dependent on frame size, and the authors Ethernet with time-predictable execution of tasks. The presented design observed a jitter of 10 μs for frames smaller than 128 bytes and 30 μs 1 is available in open source. An initial version of this work has been for larger frames. Furthermore, a model of analyzing the worst-case la- presented in [24] . tency of TTEthernet in the existence of rate-constrained traffic load is This paper is organized into six sections: Section 2 presents related presented in [49] . The model shows that it is possible to provide safe work on TTEthernet. Section 3 provides a background on the TTEther- bounds for rate-constrained traffic, and it is evaluated over a simulated net internals. Section 4 describes the design and implementation of a network topology of an Airbus A380 aircraft. time-predictable TTEthernet node. Section 5 evaluates our design with Scheduling of TTEthernet communication has been investigated in measurements and static WCET analysis performed on a system consist- [39] . It proposes a scheduling approach for TT traffic that allows the ing of a switch and six nodes. Section 6 concludes the paper. calculation of the transmission and reception time instants by each connected real-time application. The synthesis for static scheduling for 2. Related work mixed-criticality systems has been investigated in [41]. The concept of schedule porosity was introduced, allowing un-synchronized (best-effort Traditional real-time communication was based on bus protocols or rate-constrained) traffic to be mixed with time-triggered traffic with- such as CAN and PROFIBUS that can send small prioritized frames out suffering from starvation. Moreover, in [40], the authors further with bounded latency. CAN was later extended with TT capabilities. optimize TTEthernet schedules for mixed-criticality applications by pre- This TTCAN bus restricts nodes to only transmit frames in specific time senting a schedule that allocates more bandwidth to best-effort traffic slots, thus increasing the determinism of the communication [21] . The while still preserving determinism of TT traffic. main limitations of both CAN
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