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CMSIS RTOS

Table of Contents

    RTOS

    1. mbed RTOS
    2. CMSIS RTOS

    The CMSIS-RTOS is a common API for Real-Time operating systems. It provides a standardized programming interface that is portable to many RTOS and enables therefore software templates, middleware, libraries, and other components that can work across supported the RTOS systems.

    Import librarymbed-rtos

    Official mbed Real Time Operating System based on the RTX implementation of the CMSIS-RTOS API open standard.

    Last commit 04 Jul 2017 by mbed official

    Thread

    The Thread Management function group allow defining, creating, and controlling thread functions in the system. The function main is a special thread function that is started at system initialization and has the initial priority osPriorityNormal.

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    main

    The main function is already the first thread scheduled by the rtos.

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    Mutex

    The Mutex Management function group is used to synchronize the execution of threads. This is for example used to protect access to a shared resource, for example a shared memory image.

    ISR

    Mutex Management functions cannot be called from interrupt service routines (ISR).

    /media/uploads/emilmont/mutex.png

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    C standard library mutexes

    The ARM C standard library has already mutexes in place to protect the access to stdio, therefore on the M3 mbed the above example is not necessary. On the contrary, ARM microlib (used on the M0 mbed) does not provide default stdio mutexes making the above example a necessity.

    printf in ISR

    Because of the mutexes in the ARM C standard library you can not use printf in ISR!

    Semaphore

    The Semaphore Management function group is used to manage and protect access to shared resources. For example, with a Semaphore the access to a group of identical peripherals can be managed. The number of available resources is specified as parameter of the osSemaphoreCreate function.

    /media/uploads/emilmont/semaphore.png

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    Signals

    The Signal Management function group allow to control or wait signal flags. Each thread has assigned signal flags.

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    Message Queue

    The Message Queue Management function group allow to control, send, receive, or wait for messages. A message can be a integer or pointer value that is send to a thread or interrupt service routine.

    /media/uploads/emilmont/messagequeue.png

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    Memory Pool

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    Mail Queue

    The Mail Queue Management function group allow to control, send, receive, or wait for mail. A mail is a memory block that is send to a thread or interrupt service routine.

    /media/uploads/emilmont/mailqueue.png

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    Timer

    The Timer Management function group allow creating and and controlling of timer functions in the system. A timer function is called when a time period expires whereby both on-shot and periodic timers are possible. A timer can be started, restarted, or stopped.

    Timers are handled in the thread osTimerThread. Callback functions run under control of this thread and may use CMSIS-RTOS API calls.

    /media/uploads/emilmont/timer.png

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    Interrupt Service Routines

    The same CMSIS-RTOS can be used in ISR. The only two warnings are:

    • Mutexes can not be used.
    • Wait in ISR is not allowed: all the timeouts in method parameters have to be set to 0 (no wait).

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    No wait in ISR

    When calling an rtos object method in an ISR all the timeout parameters have to be set to 0 (no wait): waiting in ISR is not allowed.

    Stack Configuration

    The stack configuration is very dependent from the underling CMSIS-RTOS implementation.

    As first reference implementation we are using RTX. One of the limitation of the RTX implementation is that it requires to statically configure the maximum number of threads and the size of the memory pool for their stacks.

    This is our default configuration:

    mbed NXP LPC11U24mbed NXP LPC1768
    Max number of user threads + (timer)3 + (1)7 + (1)
    Default stack size in bytes0.5 Kb1 Kb

    configuration

    To edit the configuration:

    • Right-click the rtos library and select "Edit library...".
    • Modify RTX_Conf_CM.c as needed.

    Status and Error Codes

    The Status and Error Codes section lists all the return values that the CMSIS-RTOS functions will return:

    • osOK: function completed; no event occurred.
    • osEventSignal: function completed; signal event occurred.
    • osEventMessage: function completed; message event occurred.
    • osEventMail: function completed; mail event occurred.
    • osEventTimeout: function completed; timeout occurred.
    • osErrorParameter: parameter error: a mandatory parameter was missing or specified an incorrect object.
    • osErrorResource: resource not available: a specified resource was not available.
    • osErrorTimeoutResource: resource not available within given time: a specified resource was not available within the timeout period.
    • osErrorISR: not allowed in ISR context: the function cannot be called from interrupt service routines.
    • osErrorISRRecursive: function called multiple times from ISR with same object.
    • osErrorPriority: system cannot determine priority or thread has illegal priority.
    • osErrorNoMemory: system is out of memory: it was impossible to allocate or reserve memory for the operation.
    • osErrorValue: value of a parameter is out of range.
    • osErrorOS: unspecified RTOS error: run-time error but no other error message fits.

    osEvent

    The osEvent data structure is returned by get methods of Queue and Mail objects. This data structure contains both an error code and a pointer to the actual data:

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