The mbed RTOS API is a simple C++ encapsulation of the CMSIS RTOS C API.

RTOS Library URL: http://mbed.org/projects/libraries-testing/svn/rtos

Thread¶

The Thread class allows 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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Mutex¶

A Mutex is used to synchronize the execution of threads: for example to protect the access to a shared resource.

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Semaphore¶

A Semaphore is particularly useful to manage thread access to a pool of shared resources of a certain type.

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Signals¶

Each Thread can be notified and wait for signals:

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Queue¶

A Queue allows you to queue pointers to data from producers threads to consumers threads:

Queue<message_t, 16> queue;

message_t *message;

queue.put(message);

osEvent evt = queue.get();
if (evt.status == osEventMessage) {
    message_t *message = (message_t*)evt.value.p;

MemoryPool¶

The MemoryPool class is used to define and manage fixed-size memory pools:

MemoryPool<message_t, 16> mpool;

message_t *message = mpool.alloc();

mpool.free(message);

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Mail¶

A Mail works like a queue with the added benefit of providing a memory pool for allocating messages (not only pointers).

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RTOS Timer¶

The RtosTimer class allows 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.

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Interrupt Service Routines¶

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

• Mutex 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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Default Timeouts¶

The mbed rtos API has made the choice of defaulting to 0 timeout (no wait) for the producer methods, and osWaitForever (infinitive wait) for the consumer methods.

A typical scenario for a producer could be a peripheral triggering an interrupt to notify an event: in the corresponding interrupt service routine you cannot wait (this would deadlock the entire system). On the other side, the consumer could be a background thread waiting for events: in this case the desired default behaviour is not using CPU cycles until this event is produced, hence the osWaitForever.

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.

Our initial RTX configuration choices are likely to change in response to the mbed users feedback:

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:

  * status       [osStatus    ]: status code (event or error information)

union: either {
  * value
    * v          [uint32_t    ]: message as 32-bit value 
    * p          [void *      ]: message or mail as void pointer 
} or {
  * def
    * mail_id    [osMailQId   ]: mail id obtained by osMailCreate 
    * message_id [osMessageQId]: message id obtained by osMessageCreate
}