Event Groups

group EventGroup

An event group is a collection of bits to which an application can assign a meaning.

For example, an application may create an event group to convey the status of various CAN bus related events in which bit 0 might mean “A CAN message has been received and is ready for processing”, bit 1 might mean “The application has queued a message that is ready for sending onto the CAN network”, and bit 2 might mean “It is time to send a SYNC message onto the CAN network” etc. A task can then test the bit values to see which events are active, and optionally enter the Blocked state to wait for a specified bit or a group of specified bits to be active. To continue the CAN bus example, a CAN controlling task can enter the Blocked state (and therefore not consume any processing time) until either bit 0, bit 1 or bit 2 are active, at which time the bit that was actually active would inform the task which action it had to take (process a received message, send a message, or send a SYNC).

The event groups implementation contains intelligence to avoid race conditions that would otherwise occur were an application to use a simple variable for the same purpose. This is particularly important with respect to when a bit within an event group is to be cleared, and when bits have to be set and then tested atomically - as is the case where event groups are used to create a synchronisation point between multiple tasks (a ‘rendezvous’).

EventGroupHandle_t xEventGroupCreate(void)

event_groups.h

Create a new event group.

Internally, within the FreeRTOS implementation, event groups use a [small] block of memory, in which the event group’s structure is stored. If an event groups is created using xEventGropuCreate() then the required memory is automatically dynamically allocated inside the xEventGroupCreate() function. (see http://www.freertos.org/a00111.html). If an event group is created using xEventGropuCreateStatic() then the application writer must instead provide the memory that will get used by the event group. xEventGroupCreateStatic() therefore allows an event group to be created without using any dynamic memory allocation.

Although event groups are not related to ticks, for internal implementation reasons the number of bits available for use in an event group is dependent on the configUSE_16_BIT_TICKS setting in FreeRTOSConfig.h. If configUSE_16_BIT_TICKS is 1 then each event group contains 8 usable bits (bit 0 to bit 7). If configUSE_16_BIT_TICKS is set to 0 then each event group has 24 usable bits (bit 0 to bit 23). The EventBits_t type is used to store event bits within an event group.

Example usage:

// Declare a variable to hold the created event group.
EventGroupHandle_t xCreatedEventGroup;

// Attempt to create the event group.
xCreatedEventGroup = xEventGroupCreate();

// Was the event group created successfully?
if( xCreatedEventGroup == NULL )
{
   // The event group was not created because there was insufficient
   // FreeRTOS heap available.
}
else
{
   // The event group was created.
}

Return

If the event group was created then a handle to the event group is returned. If there was insufficient FreeRTOS heap available to create the event group then NULL is returned. See http://www.freertos.org/a00111.html

EventGroupHandle_t xEventGroupCreateStatic(StaticEventGroup_t *pxEventGroupBuffer)

event_groups.h

Create a new event group.

Internally, within the FreeRTOS implementation, event groups use a [small] block of memory, in which the event group’s structure is stored. If an event groups is created using xEventGropuCreate() then the required memory is automatically dynamically allocated inside the xEventGroupCreate() function. (see http://www.freertos.org/a00111.html). If an event group is created using xEventGropuCreateStatic() then the application writer must instead provide the memory that will get used by the event group. xEventGroupCreateStatic() therefore allows an event group to be created without using any dynamic memory allocation.

Although event groups are not related to ticks, for internal implementation reasons the number of bits available for use in an event group is dependent on the configUSE_16_BIT_TICKS setting in FreeRTOSConfig.h. If configUSE_16_BIT_TICKS is 1 then each event group contains 8 usable bits (bit 0 to bit 7). If configUSE_16_BIT_TICKS is set to 0 then each event group has 24 usable bits (bit 0 to bit 23). The EventBits_t type is used to store event bits within an event group.

Example usage:

// StaticEventGroup_t is a publicly accessible structure that has the same
// size and alignment requirements as the real event group structure.  It is
// provided as a mechanism for applications to know the size of the event
// group (which is dependent on the architecture and configuration file
// settings) without breaking the strict data hiding policy by exposing the
// real event group internals.  This StaticEventGroup_t variable is passed
// into the xSemaphoreCreateEventGroupStatic() function and is used to store
// the event group's data structures
StaticEventGroup_t xEventGroupBuffer;

// Create the event group without dynamically allocating any memory.
xEventGroup = xEventGroupCreateStatic( &xEventGroupBuffer );

Return

If the event group was created then a handle to the event group is returned. If pxEventGroupBuffer was NULL then NULL is returned.

Parameters

• pxEventGroupBuffer: pxEventGroupBuffer must point to a variable of type StaticEventGroup_t, which will be then be used to hold the event group’s data structures, removing the need for the memory to be allocated dynamically.

EventBits_t xEventGroupWaitBits(EventGroupHandle_t xEventGroup, const EventBits_t uxBitsToWaitFor, const BaseType_t xClearOnExit, const BaseType_t xWaitForAllBits, TickType_t xTicksToWait)

event_groups.h

[Potentially] block to wait for one or more bits to be set within a previously created event group.

This function cannot be called from an interrupt.

Example usage:

#define BIT_0   ( 1 << 0 )
#define BIT_4   ( 1 << 4 )

void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;
const TickType_t xTicksToWait = 100 / portTICK_PERIOD_MS;

    // Wait a maximum of 100ms for either bit 0 or bit 4 to be set within
    // the event group.  Clear the bits before exiting.
    uxBits = xEventGroupWaitBits(
                xEventGroup,    // The event group being tested.
                BIT_0 | BIT_4,  // The bits within the event group to wait for.
                pdTRUE,         // BIT_0 and BIT_4 should be cleared before returning.
                pdFALSE,        // Don't wait for both bits, either bit will do.
                xTicksToWait ); // Wait a maximum of 100ms for either bit to be set.

    if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
    {
        // xEventGroupWaitBits() returned because both bits were set.
    }
    else if( ( uxBits & BIT_0 ) != 0 )
    {
        // xEventGroupWaitBits() returned because just BIT_0 was set.
    }
    else if( ( uxBits & BIT_4 ) != 0 )
    {
        // xEventGroupWaitBits() returned because just BIT_4 was set.
    }
    else
    {
        // xEventGroupWaitBits() returned because xTicksToWait ticks passed
        // without either BIT_0 or BIT_4 becoming set.
    }
}

Return

The value of the event group at the time either the bits being waited for became set, or the block time expired. Test the return value to know which bits were set. If xEventGroupWaitBits() returned because its timeout expired then not all the bits being waited for will be set. If xEventGroupWaitBits() returned because the bits it was waiting for were set then the returned value is the event group value before any bits were automatically cleared in the case that xClearOnExit parameter was set to pdTRUE.

Parameters

• xEventGroup: The event group in which the bits are being tested. The event group must have previously been created using a call to xEventGroupCreate().
• uxBitsToWaitFor: A bitwise value that indicates the bit or bits to test inside the event group. For example, to wait for bit 0 and/or bit 2 set uxBitsToWaitFor to 0x05. To wait for bits 0 and/or bit 1 and/or bit 2 set uxBitsToWaitFor to 0x07. Etc.
• xClearOnExit: If xClearOnExit is set to pdTRUE then any bits within uxBitsToWaitFor that are set within the event group will be cleared before xEventGroupWaitBits() returns if the wait condition was met (if the function returns for a reason other than a timeout). If xClearOnExit is set to pdFALSE then the bits set in the event group are not altered when the call to xEventGroupWaitBits() returns.
• xWaitForAllBits: If xWaitForAllBits is set to pdTRUE then xEventGroupWaitBits() will return when either all the bits in uxBitsToWaitFor are set or the specified block time expires. If xWaitForAllBits is set to pdFALSE then xEventGroupWaitBits() will return when any one of the bits set in uxBitsToWaitFor is set or the specified block time expires. The block time is specified by the xTicksToWait parameter.
• xTicksToWait: The maximum amount of time (specified in ‘ticks’) to wait for one/all (depending on the xWaitForAllBits value) of the bits specified by uxBitsToWaitFor to become set.

EventBits_t xEventGroupSetBits(EventGroupHandle_t xEventGroup, const EventBits_t uxBitsToSet)

event_groups.h

Set bits within an event group. This function cannot be called from an interrupt. xEventGroupSetBitsFromISR() is a version that can be called from an interrupt.

Setting bits in an event group will automatically unblock tasks that are blocked waiting for the bits.

Example usage:

#define BIT_0   ( 1 << 0 )
#define BIT_4   ( 1 << 4 )

void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;

    // Set bit 0 and bit 4 in xEventGroup.
    uxBits = xEventGroupSetBits(
                        xEventGroup,    // The event group being updated.
                        BIT_0 | BIT_4 );// The bits being set.

    if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
    {
        // Both bit 0 and bit 4 remained set when the function returned.
    }
    else if( ( uxBits & BIT_0 ) != 0 )
    {
        // Bit 0 remained set when the function returned, but bit 4 was
        // cleared.  It might be that bit 4 was cleared automatically as a
        // task that was waiting for bit 4 was removed from the Blocked
        // state.
    }
    else if( ( uxBits & BIT_4 ) != 0 )
    {
        // Bit 4 remained set when the function returned, but bit 0 was
        // cleared.  It might be that bit 0 was cleared automatically as a
        // task that was waiting for bit 0 was removed from the Blocked
        // state.
    }
    else
    {
        // Neither bit 0 nor bit 4 remained set.  It might be that a task
        // was waiting for both of the bits to be set, and the bits were
        // cleared as the task left the Blocked state.
    }
}

Return

The value of the event group at the time the call to xEventGroupSetBits() returns. There are two reasons why the returned value might have the bits specified by the uxBitsToSet parameter cleared. First, if setting a bit results in a task that was waiting for the bit leaving the blocked state then it is possible the bit will be cleared automatically (see the xClearBitOnExit parameter of xEventGroupWaitBits()). Second, any unblocked (or otherwise Ready state) task that has a priority above that of the task that called xEventGroupSetBits() will execute and may change the event group value before the call to xEventGroupSetBits() returns.

Parameters

• xEventGroup: The event group in which the bits are to be set.
• uxBitsToSet: A bitwise value that indicates the bit or bits to set. For example, to set bit 3 only, set uxBitsToSet to 0x08. To set bit 3 and bit 0 set uxBitsToSet to 0x09.

xEventGroupSetBitsFromISR(xEventGroup, uxBitsToSet, pxHigherPriorityTaskWoken) xTimerPendFunctionCallFromISR( vEventGroupSetBitsCallback, ( void * ) xEventGroup, ( uint32_t ) uxBitsToSet, pxHigherPriorityTaskWoken )

event_groups.h

A version of xEventGroupSetBits() that can be called from an interrupt.

Setting bits in an event group is not a deterministic operation because there are an unknown number of tasks that may be waiting for the bit or bits being set. FreeRTOS does not allow nondeterministic operations to be performed in interrupts or from critical sections. Therefore xEventGroupSetBitsFromISR() sends a message to the timer task to have the set operation performed in the context of the timer task - where a scheduler lock is used in place of a critical section.

Example usage:

 #define BIT_0  ( 1 << 0 )
 #define BIT_4  ( 1 << 4 )

 // An event group which it is assumed has already been created by a call to
 // xEventGroupCreate().
 EventGroupHandle_t xEventGroup;

 void anInterruptHandler( void )
 {
 BaseType_t xHigherPriorityTaskWoken, xResult;

    // xHigherPriorityTaskWoken must be initialised to pdFALSE.
    xHigherPriorityTaskWoken = pdFALSE;

    // Set bit 0 and bit 4 in xEventGroup.
    xResult = xEventGroupSetBitsFromISR(
                        xEventGroup,    // The event group being updated.
                        BIT_0 | BIT_4   // The bits being set.
                        &xHigherPriorityTaskWoken );

    // Was the message posted successfully?
    if( xResult == pdPASS )
    {
        // If xHigherPriorityTaskWoken is now set to pdTRUE then a context
        // switch should be requested.  The macro used is port specific and
        // will be either portYIELD_FROM_ISR() or portEND_SWITCHING_ISR() -
        // refer to the documentation page for the port being used.
        portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
    }
}

Return

If the request to execute the function was posted successfully then pdPASS is returned, otherwise pdFALSE is returned. pdFALSE will be returned if the timer service queue was full.

Parameters

• xEventGroup: The event group in which the bits are to be set.
• uxBitsToSet: A bitwise value that indicates the bit or bits to set. For example, to set bit 3 only, set uxBitsToSet to 0x08. To set bit 3 and bit 0 set uxBitsToSet to 0x09.
• pxHigherPriorityTaskWoken: As mentioned above, calling this function will result in a message being sent to the timer daemon task. If the priority of the timer daemon task is higher than the priority of the currently running task (the task the interrupt interrupted) then *pxHigherPriorityTaskWoken will be set to pdTRUE by xEventGroupSetBitsFromISR(), indicating that a context switch should be requested before the interrupt exits. For that reason *pxHigherPriorityTaskWoken must be initialised to pdFALSE. See the example code below.

EventBits_t xEventGroupClearBits(EventGroupHandle_t xEventGroup, const EventBits_t uxBitsToClear)

event_groups.h

Clear bits within an event group. This function cannot be called from an interrupt.

Example usage:

#define BIT_0   ( 1 << 0 )
#define BIT_4   ( 1 << 4 )

void aFunction( EventGroupHandle_t xEventGroup )
{
EventBits_t uxBits;

    // Clear bit 0 and bit 4 in xEventGroup.
    uxBits = xEventGroupClearBits(
                            xEventGroup,    // The event group being updated.
                            BIT_0 | BIT_4 );// The bits being cleared.

    if( ( uxBits & ( BIT_0 | BIT_4 ) ) == ( BIT_0 | BIT_4 ) )
    {
        // Both bit 0 and bit 4 were set before xEventGroupClearBits() was
        // called.  Both will now be clear (not set).
    }
    else if( ( uxBits & BIT_0 ) != 0 )
    {
        // Bit 0 was set before xEventGroupClearBits() was called.  It will
        // now be clear.
    }
    else if( ( uxBits & BIT_4 ) != 0 )
    {
        // Bit 4 was set before xEventGroupClearBits() was called.  It will
        // now be clear.
    }
    else
    {
        // Neither bit 0 nor bit 4 were set in the first place.
    }
}

Return

The value of the event group before the specified bits were cleared.

Parameters

• xEventGroup: The event group in which the bits are to be cleared.
• uxBitsToClear: A bitwise value that indicates the bit or bits to clear in the event group. For example, to clear bit 3 only, set uxBitsToClear to 0x08. To clear bit 3 and bit 0 set uxBitsToClear to 0x09.

xEventGroupClearBitsFromISR(xEventGroup, uxBitsToClear) xTimerPendFunctionCallFromISR( vEventGroupClearBitsCallback, ( void * ) xEventGroup, ( uint32_t ) uxBitsToClear, NULL )

event_groups.h

A version of xEventGroupClearBits() that can be called from an interrupt.

Setting bits in an event group is not a deterministic operation because there are an unknown number of tasks that may be waiting for the bit or bits being set. FreeRTOS does not allow nondeterministic operations to be performed while interrupts are disabled, so protects event groups that are accessed from tasks by suspending the scheduler rather than disabling interrupts. As a result event groups cannot be accessed directly from an interrupt service routine. Therefore xEventGroupClearBitsFromISR() sends a message to the timer task to have the clear operation performed in the context of the timer task.

Example usage:

 #define BIT_0  ( 1 << 0 )
 #define BIT_4  ( 1 << 4 )

 // An event group which it is assumed has already been created by a call to
 // xEventGroupCreate().
 EventGroupHandle_t xEventGroup;

 void anInterruptHandler( void )
 {
    // Clear bit 0 and bit 4 in xEventGroup.
    xResult = xEventGroupClearBitsFromISR(
                        xEventGroup,     // The event group being updated.
                        BIT_0 | BIT_4 ); // The bits being set.

    if( xResult == pdPASS )
    {
        // The message was posted successfully.
    }
}

Return

If the request to execute the function was posted successfully then pdPASS is returned, otherwise pdFALSE is returned. pdFALSE will be returned if the timer service queue was full.

Parameters

• xEventGroup: The event group in which the bits are to be cleared.
• uxBitsToClear: A bitwise value that indicates the bit or bits to clear. For example, to clear bit 3 only, set uxBitsToClear to 0x08. To clear bit 3 and bit 0 set uxBitsToClear to 0x09.

xEventGroupGetBits(xEventGroup) xEventGroupClearBits( xEventGroup, 0 )

event_groups.h

Returns the current value of the bits in an event group. This function cannot be used from an interrupt.

Return

The event group bits at the time xEventGroupGetBits() was called.

Parameters

• xEventGroup: The event group being queried.

EventBits_t xEventGroupGetBitsFromISR(EventGroupHandle_t xEventGroup)

event_groups.h

A version of xEventGroupGetBits() that can be called from an ISR.

Return

The event group bits at the time xEventGroupGetBitsFromISR() was called.

Parameters

• xEventGroup: The event group being queried.

EventBits_t xEventGroupSync(EventGroupHandle_t xEventGroup, const EventBits_t uxBitsToSet, const EventBits_t uxBitsToWaitFor, TickType_t xTicksToWait)

event_groups.h

Atomically set bits within an event group, then wait for a combination of bits to be set within the same event group. This functionality is typically used to synchronise multiple tasks, where each task has to wait for the other tasks to reach a synchronisation point before proceeding.

This function cannot be used from an interrupt.

The function will return before its block time expires if the bits specified by the uxBitsToWait parameter are set, or become set within that time. In this case all the bits specified by uxBitsToWait will be automatically cleared before the function returns.

Example usage:

// Bits used by the three tasks.
#define TASK_0_BIT      ( 1 << 0 )
#define TASK_1_BIT      ( 1 << 1 )
#define TASK_2_BIT      ( 1 << 2 )

#define ALL_SYNC_BITS ( TASK_0_BIT | TASK_1_BIT | TASK_2_BIT )

// Use an event group to synchronise three tasks.  It is assumed this event
// group has already been created elsewhere.
EventGroupHandle_t xEventBits;

void vTask0( void *pvParameters )
{
EventBits_t uxReturn;
TickType_t xTicksToWait = 100 / portTICK_PERIOD_MS;

    for( ;; )
    {
    // Perform task functionality here.

    // Set bit 0 in the event flag to note this task has reached the
    // sync point.  The other two tasks will set the other two bits defined
    // by ALL_SYNC_BITS.  All three tasks have reached the synchronisation
    // point when all the ALL_SYNC_BITS are set.  Wait a maximum of 100ms
    // for this to happen.
    uxReturn = xEventGroupSync( xEventBits, TASK_0_BIT, ALL_SYNC_BITS, xTicksToWait );

    if( ( uxReturn & ALL_SYNC_BITS ) == ALL_SYNC_BITS )
    {
        // All three tasks reached the synchronisation point before the call
        // to xEventGroupSync() timed out.
    }
   }
}

void vTask1( void *pvParameters )
{
    for( ;; )
    {
    // Perform task functionality here.

    // Set bit 1 in the event flag to note this task has reached the
    // synchronisation point.  The other two tasks will set the other two
    // bits defined by ALL_SYNC_BITS.  All three tasks have reached the
    // synchronisation point when all the ALL_SYNC_BITS are set.  Wait
    // indefinitely for this to happen.
    xEventGroupSync( xEventBits, TASK_1_BIT, ALL_SYNC_BITS, portMAX_DELAY );

    // xEventGroupSync() was called with an indefinite block time, so
    // this task will only reach here if the syncrhonisation was made by all
    // three tasks, so there is no need to test the return value.
    }
}

void vTask2( void *pvParameters )
{
    for( ;; )
    {
    // Perform task functionality here.

    // Set bit 2 in the event flag to note this task has reached the
    // synchronisation point.  The other two tasks will set the other two
    // bits defined by ALL_SYNC_BITS.  All three tasks have reached the
    // synchronisation point when all the ALL_SYNC_BITS are set.  Wait
    // indefinitely for this to happen.
    xEventGroupSync( xEventBits, TASK_2_BIT, ALL_SYNC_BITS, portMAX_DELAY );

    // xEventGroupSync() was called with an indefinite block time, so
    // this task will only reach here if the syncrhonisation was made by all
    // three tasks, so there is no need to test the return value.
   }
}

Return

The value of the event group at the time either the bits being waited for became set, or the block time expired. Test the return value to know which bits were set. If xEventGroupSync() returned because its timeout expired then not all the bits being waited for will be set. If xEventGroupSync() returned because all the bits it was waiting for were set then the returned value is the event group value before any bits were automatically cleared.

Parameters

• xEventGroup: The event group in which the bits are being tested. The event group must have previously been created using a call to xEventGroupCreate().
• uxBitsToSet: The bits to set in the event group before determining if, and possibly waiting for, all the bits specified by the uxBitsToWait parameter are set.
• uxBitsToWaitFor: A bitwise value that indicates the bit or bits to test inside the event group. For example, to wait for bit 0 and bit 2 set uxBitsToWaitFor to 0x05. To wait for bits 0 and bit 1 and bit 2 set uxBitsToWaitFor to 0x07. Etc.
• xTicksToWait: The maximum amount of time (specified in ‘ticks’) to wait for all of the bits specified by uxBitsToWaitFor to become set.

void vEventGroupDelete(EventGroupHandle_t xEventGroup)

event_groups.h

Delete an event group that was previously created by a call to xEventGroupCreate(). Tasks that are blocked on the event group will be unblocked and obtain 0 as the event group’s value.

Parameters

• xEventGroup: The event group being deleted.