RTOS

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mbed RTOS
CMSIS RTOS

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

Import library mbed-rtos

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

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.

Import program
rtos_basic - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003  
00004 DigitalOut led1(LED1);
00005 DigitalOut led2(LED2);
00006 Thread thread;
00007  
00008 void led2_thread() {
00009     while (true) {
00010         led2 = !led2;
00011         Thread::wait(1000);
00012     }
00013 }
00014  
00015 int main() {
00016     thread.start(led2_thread);
00017     
00018     while (true) {
00019         led1 = !led1;
00020         Thread::wait(500);
00021     }
00022 }

main

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

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1Thread.html' not found]

A Thread can be in the following states:

RUNNING: The thread that is currently running is in the RUNNING state. Only one thread at a time can be in this state.
READY: Threads which are ready to run are in the READY state. Once the RUNNING thread has terminated or is WAITING the next READY thread with the highest priority becomes the RUNNING thread.
WAITING: Threads that are waiting for an event to occur are in the WAITING state.
INACTIVE: Threads that are not created or terminated are in the INACTIVE state. These threads typically consume no system resources.

/media/uploads/emilmont/threadstatus.png

Mutex¶

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

ISR

The Mutex methods cannot be called from interrupt service routines (ISR).

/media/uploads/emilmont/mutex.png

Import program
rtos_mutex - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003 
00004 Mutex stdio_mutex;
00005 
00006 void notify(const char* name, int state) {
00007     stdio_mutex.lock();
00008     printf("%s: %d\n\r", name, state);
00009     stdio_mutex.unlock();
00010 }
00011 
00012 void test_thread(void const *args) {
00013     while (true) {
00014         notify((const char*)args, 0); Thread::wait(1000);
00015         notify((const char*)args, 1); Thread::wait(1000);
00016     }
00017 }
00018 
00019 int main() {
00020     Thread t2;
00021     Thread t3;
00022 
00023     t2.start(callback(test_thread, (void *)"Th 2"));
00024     t3.start(callback(test_thread, (void *)"Th 3"));
00025 
00026     test_thread((void *)"Th 1");
00027 }

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, malloc & new in ISR

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

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1Mutex.html' not found]

Semaphore¶

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

/media/uploads/emilmont/semaphore.png

Import program
rtos_semaphore - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003 
00004 Semaphore two_slots(2);
00005 
00006 void test_thread(void const *name) {
00007     while (true) {
00008         two_slots.wait();
00009         printf("%s\n\r", (const char*)name);
00010         Thread::wait(1000);
00011         two_slots.release();
00012     }
00013 }
00014 
00015 int main (void) {
00016     Thread t2;
00017     Thread t3;
00018 
00019     t2.start(callback(test_thread, (void *)"Th 2"));
00020     t3.start(callback(test_thread, (void *)"Th 3"));
00021 
00022     test_thread((void *)"Th 1");
00023 }

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1Semaphore.html' not found]

Signals¶

Each Thread can be notified and wait for signals:

Import program
rtos_signals - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003 
00004 DigitalOut led(LED1);
00005 
00006 void led_thread() {
00007     while (true) {
00008         // Signal flags that are reported as event are automatically cleared.
00009         Thread::signal_wait(0x1);
00010         led = !led;
00011     }
00012 }
00013 
00014 int main (void) {
00015     Thread thread;
00016 
00017     thread.start(callback(led_thread));
00018 
00019     while (true) {
00020         Thread::wait(1000);
00021         thread.signal_set(0x1);
00022     }
00023 }

Queue¶

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

/media/uploads/emilmont/messagequeue.png

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;

Import program rtos_queue

Basic example showing the Queue and MemoryPool API

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1Queue.html' not found]

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);

Import program
rtos_queue - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003 
00004 typedef struct {
00005     float    voltage;   /* AD result of measured voltage */
00006     float    current;   /* AD result of measured current */
00007     uint32_t counter;   /* A counter value               */
00008 } message_t;
00009 
00010 MemoryPool<message_t, 16> mpool;
00011 Queue<message_t, 16> queue;
00012 
00013 /* Send Thread */
00014 void send_thread (void) {
00015     uint32_t i = 0;
00016     while (true) {
00017         i++; // fake data update
00018         message_t *message = mpool.alloc();
00019         message->voltage = (i * 0.1) * 33; 
00020         message->current = (i * 0.1) * 11;
00021         message->counter = i;
00022         queue.put(message);
00023         Thread::wait(1000);
00024     }
00025 }
00026 
00027 int main (void) {
00028     Thread thread;
00029     thread.start(callback(send_thread));
00030     
00031     while (true) {
00032         osEvent evt = queue.get();
00033         if (evt.status == osEventMessage) {
00034             message_t *message = (message_t*)evt.value.p;
00035             printf("\nVoltage: %.2f V\n\r"   , message->voltage);
00036             printf("Current: %.2f A\n\r"     , message->current);
00037             printf("Number of cycles: %u\n\r", message->counter);
00038             
00039             mpool.free(message);
00040         }
00041     }
00042 }

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1MemoryPool.html' not found]

Mail¶

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

/media/uploads/emilmont/mailqueue.png

Import program
rtos_mail - main.cpp

00001 #include "mbed.h"
00002 #include "rtos.h"
00003 
00004 /* Mail */
00005 typedef struct {
00006   float    voltage; /* AD result of measured voltage */
00007   float    current; /* AD result of measured current */
00008   uint32_t counter; /* A counter value               */
00009 } mail_t;
00010 
00011 Mail<mail_t, 16> mail_box;
00012 
00013 void send_thread (void) {
00014     uint32_t i = 0;
00015     while (true) {
00016         i++; // fake data update
00017         mail_t *mail = mail_box.alloc();
00018         mail->voltage = (i * 0.1) * 33; 
00019         mail->current = (i * 0.1) * 11;
00020         mail->counter = i;
00021         mail_box.put(mail);
00022         Thread::wait(1000);
00023     }
00024 }
00025 
00026 int main (void) {
00027     Thread thread;
00028     thread.start(callback(send_thread));
00029     
00030     while (true) {
00031         osEvent evt = mail_box.get();
00032         if (evt.status == osEventMail) {
00033             mail_t *mail = (mail_t*)evt.value.p;
00034             printf("\nVoltage: %.2f V\n\r"   , mail->voltage);
00035             printf("Current: %.2f A\n\r"     , mail->current);
00036             printf("Number of cycles: %u\n\r", mail->counter);
00037             
00038             mail_box.free(mail);
00039         }
00040     }
00041 }

[Repository '/users/mbed_official/code/rtos/docs/tip/classrtos_1_1Mail.html' not found]

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.

/media/uploads/emilmont/rtostimer.png

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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.

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.

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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All wikipages

RTOS

Table of Contents

RTOS

Import library mbed-rtos

Thread¶

Import program
rtos_basic - main.cpp

main

Mutex¶

ISR

Import program
rtos_mutex - main.cpp

C standard library mutexes

printf, malloc & new in ISR

Semaphore¶

Import program
rtos_semaphore - main.cpp

Signals¶

Import program
rtos_signals - main.cpp

Queue¶

Import program rtos_queue

MemoryPool¶

Import program
rtos_queue - main.cpp

Mail¶

Import program
rtos_mail - main.cpp

RTOS Timer¶

Interrupt Service Routines¶

Default Timeouts¶

No wait in ISR

Status and Error Codes¶

osEvent¶

Recent changes

Important Information for this Arm website

Table of Contents

RTOS

Thread¶

main

Mutex¶

ISR

C standard library mutexes

printf, malloc & new in ISR

Semaphore¶

Signals¶

Queue¶

MemoryPool¶

Mail¶

RTOS Timer¶

Interrupt Service Routines¶

Default Timeouts¶

No wait in ISR

Status and Error Codes¶

osEvent¶

Important Information for this Arm website

Access Warning