Introduction to Real-Time Operating Systems with RTEMS

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1 Introduction to RTOS with RTEMS Introduction to Real-Time Operating Systems with RTEMS Aleix Conchillo Flaqué

2 Introduction to RTOS with RTEMS Introduction RTEMS overview RTEMS on the LISA Pathfinder DMU What's next?

3 Introduction to RTOS with RTEMS Introduction What is an embedded system? Embedded system examples What is a real-time system? What is a Real-Time Operating System? RTOS examples RTOS in space

4 What is an embedded system? A computer system designed to perform one or few dedicated functions often with real-time computing constraints. Wikipedia A combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a specific function. Programming Embedded Systems (Michael Barr)

5 Embedded system examples Consumer electronics (DVD and music players, mobile phones, GPS...). Medical equipment (heart pacemaker, apnea monitor, blood pressure monitor...). Automobile (Electronic stability control, fuel control...).

6 What is an embedded system? A computer system designed to perform one or few dedicated functions often with real-time computing constraints. Wikipedia A combination of computer hardware and software, and perhaps additional mechanical or other parts, designed to perform a specific function. Programming Embedded Systems (Michael Barr)

7 What is a real-time system? A computer system (hardware or software) with timing constraints. Hard deadlines: Avionics. Anti-lock brake system. Medical devices. Soft deadlines: Consumer electronics. Satellite communication.

8 What is a Real-Time Operating System? An Operating System designed for real-time systems. Generic Operating System features: Input/Ouput (drivers). Memory allocation. Interrupts. Tasks. Scheduling algorithm (for taks). Inter-tasks communication and synchronization. Message queues, events, semaphores... RTOS features: Small footprint. Offers more control over process priorities. Designed for real-time (e.g. minimal interrupt latency). Normally designed for low set of tasks.

9 RTOS examples Avionics, space, military (hard real-time): VxWorks (1987, Wind River Intel). LynxOS (1986, LynuxWorks) RTEMS (1988, OAR Corporation) RTLinux (1999, FSM Labs Wind River Intel) Open RTLinux (academic, research...). Wind River Real-Time Core (commercial). Mobile devices / Smartphones: Symbian OS (2001, Symbian Software Ltd Nokia) Windows CE (1996/97, Microsoft)

RTOS in space Spirit and Opportunity Mars Exploration Rovers: Landed, January 2004.

10 RTOS in space Spirit and Opportunity Mars Exploration Rovers: Landed, January Spirit stuck on May 2009, re-tasked as a stationary platform. Opportunity still on the way. VxWorks on a radiationhardened 20 MHz RAD6000 CPU.

11 RTOS in space Herschel and Planck: Planck Launched in 2009 by ESA. Space observatories (infrared telescope and cosmic microwave background). Flight computer with RTEMS. RTEMS

12 RTOS in space LISA Pathfinder: Pathfinder Test technology for LISA. On-Board Computer and DMU software developed with RTEMS for ERC32 architecture. Propaganda: DMU is the IEEC/ICE contribution! DMU is mission-critical!

13 Introduction to RTOS with RTEMS RTEMS overview Why RTEMS? History Features Architecture Managers Configuration Sample application

14 Why RTEMS? GNU GPL license v2. Very nice user community. Supports most used architectures. Approved by ESA. Has already flown in many projects.

15 RTEMS History 1988: U.S. Army Missile Command requests royalty free, standards-based real-time solution with full source. Dec 31, 1988: RTEMS is born (Real-Time Executive for Missile Systems) (Missile Military) : Many architectures: i386, i960, m68k... Uses and ships with GNU tools. 1992: First non-army user. 1994: Public available via FTP. 1994: Added ESTEC sponsor SPARC/ERC32 port. 1996: Last Army release. Until now: More than 10 architectures, networking, file systems, standards compliant (POSIX, RTEID, ITRON...).

16 RTEMS Features Standards compliant: POSIX b API including threads. RTEID/ORKID based Classic API TCP/IP including BSD sockets µitron. Kernel: Multitasking. Multiprocessor. Rate-monotonic scheduler. Inter-task communication and synchronization: events, message queues... Dynamic memory allocation. Networking: FreeBSD TCP/IP stack Clients (DNS, TFTP, NFS) Servers (FTP, Web server, Telnet, RPC...) Filesystems (In-memory FS, FAT16, FAT32, NFS...)

17 RTEMS Architecture (1)

18 RTEMS Architecture (2) Just hardware! We need to take control!

19 RTEMS Architecture (3) Startup, clocks, interrupts, context switching...

20 RTEMS Architecutre (4) Events, message queues, semaphores, tasks...

21 RTEMS Architecutre (5) API interfaces. More than one API might be used at the same time.

22 RTEMS Architecutre (6) Classic API is the RTEMS API (non-standard)

23 RTEMS Managers (1)

24 RTEMS Managers (2)

25 RTEMS Managers (3) - Initialization Initialization: Initialize and shutdown RTEMS. Create initialization tasks. Initialization tasks: System Initialization task (device drivers). Idle task (lower priority task in the system). User Initialization task (create application tasks). Interesting directives: rtems_initialize_executive Already calls the User initialization task rtems_shutdown_executive

26 RTEMS Managers (4) - Initialization

27 RTEMS Managers (5) - Tasks What is a task? task Smallest thread of execution which can compete on its own for system resources. Task or process? process Process have separate memory space, tasks do not. What is a preemptable task? Tasks might be interrupted by a higher priority task. Task modes and attributes: Modes: preemption, timeslicing... Attributes: floating point, global or local

28 RTEMS Managers (6) - Tasks Some task directives: rtems_task_create (specify attributes and mode). rtems_task_set_priority (from 1 to 255, where 1 has higher priority than 255). rtems_task_start (task is not started until this directive is called) rtems_task_suspend and rtos_task_resume.

29 RTEMS Managers (7) - Tasks

30 RTEMS Managers (8) - Interrupts Interrupt manager: Process externally generated interrupts. Allows nested interrupts. User ISR (Interrupt Service Routine) responsible for processing interrupt. Recommendations: ISRs should be as fast as possible. Do not disable interrupts for long periods. Directives: rtems_interrupt_catch (Install user ISR). rtems_interrupt_enable and rtems_interrupt_disable.

31 RTEMS Managers (9) - Interrupts

32 RTEMS Managers (10)

33 RTEMS Managers (11) - Events Event Manager: Simple synchronization. No data is transmitted. Events are not queued. Tasks can wait to more than one event. Can be sent from interrupts. Directives: rtems_event_send (send a set of events). rtems_event_receive (wait for event condition).

34 RTEMS Managers (12) - Events

35 RTEMS Managers (13) - Events

36 RTEMS Managers (14) Sempahores concepts What is a critical section? section An area of code that needs exclusive access. What is a semaphore? semaphore A way to restrict access to a critical section. Types of semaphores: Mutex or binary semaphores: semaphores the critical section (or mutually exclusive area) can only be accessed by only one task at a time. Counting semaphores: semaphores the critical section can be accessed by more than one task at the same time.

37 RTEMS Managers (15) Sempahores examples

38 RTEMS Managers (16) Sempahores concepts Advanced concepts: Priority inversion problem: What happens if a high priority task requests access to a shared resource previously acquired by a lower priority task? task Solution to priority inversion: inheritance task of lower priority is Priority inheritance: upgraded to the priority of the highest priority task waiting. ceiling task of lower priority is upgraded Priority ceiling: to the priority of the highest priority task which will EVER wait (creation time).

39 RTEMS Managers (17) - Events

40 RTEMS Managers (18) - Events

41 RTEMS Managers (19) - Events

42 RTEMS Managers (20)

43 RTEMS Managers (21) Memory management Two type of managers: Partition manager: Allocate memory in fixed-size units. Region manager: Allocate memory in variable-size units (user defined limits). Dynamic memory allocation is dangerous. Normally forbidden in flight software.

44 RTEMS Managers (22) Configuration System clock: #define CONFIGURE_MICROSECONDS_PER_TICK 1000 Tasks: #define CONFIGURE_MAXIMUM_TASKS 10 Semaphores: #define CONFIGURE_MAXIMUM_SEMAPHORES 5 Message queues: #define CONFIGURE_MAXIMUM_MESSAGE_QUEUES 3 Partitions and regions: #define CONFGIURE_MAXIMUM_PARTITIONS 2 #define CONFIGURE_MAXIMUM_REGIONS 2

45 Introduction to RTOS with RTEMS RTEMS on the LISA Pathfinder DMU What is the DMU? Boot Software Application Software DMU software architecture

46 What is the DMU? (1) DMU is the Data Management Unit: Is part of the LISA Pathfinder payload (known as LISA Test Package). Consists on (redundant): Power Distribution Unit (PDU) Data Processing Unit (DPU). Data Acquisition Unit (DAU). Acts as a bridge between the LTP and the On-Board Computer. Goals: Deliver scientific data to ground. Remote LTP control from the OBC.

47 What is the DMU? (2) Data Processing Unit DPU (Data Processing Unit) hardware: 12 MHz rad-hard ERC32 processor (SPARC). 2 Mbytes RAM (software in execution + data). 1 Mbyte EEPROM (stored software). 64 Kbytes PROM (boot loader). 2 bus controllers (MIL-STD-1553): OBC to DMU DMU to LTP

Custom non-rtos implementation: Only 64 Kbytes available!

48 Boot Software Goals: Initialize and execute Application Software. Allow ASW updates from earth. Hardware failure diagnosis. Custom non-rtos implementation: Only 64 Kbytes available! Simple scheduler for periodic and aperiodic tasks (non-preemptive). Interrupt manager. Memory detection and correction (EDAC). Post mortem information for failure diagnosis.

49 Application Software ASW is stored in the EEPROM. RTEMS has been used. Data Acquisition Unit Goals: Same services as the Boot Software. Receive and process scientific data: DAU and Radiation Monitor (also developed by IEEC). Phasemeter data acquisition. Laser loop controls. Acts as a bus controller for all LTP subsystems: TC routing between OBC and LTP.

50 DMU software architecture The RTOS layer allows sharing applications between BSW and ASW or even change RTEMS for another RTOS.

51 What's next? Or... Pie in the sky. Create a generic layer on top different RTOS, allowing services to be re-used. Start layering RTEMS. Make RTEMS an ICE/IEEC standard for embedded systems (at least for generic ones). Flexible software debugging via embedded interpreters (advanced programming languages). Give more talks about software! (testing, re-usability, maintainability, flexibility = more productivity!!!)

52 The end

RTOS Real T i Time me Operating System System Concepts Part 2

RTOS Real T i Time me Operating System System Concepts Part 2 RTOS Real Time Operating System Concepts Part 2 Real time System Pitfalls - 4: The Ariane 5 satelite launch rocket Rocket self destructed in 4 June -1996. Exactly after 40 second of lift off at an attitude