A C++ template library for embedded applications
Designed and maintained by
Aster Consulting Ltd

Callback Timer

A software timer class that can manage up to 254 timers. Each one may be repeating or single shot.
When a timer triggers it will call the selected function. The function may be a class member or free function.
The timers are driven from a call to tick(uint32_t count). This call would normally be made from a high priority
interrupt routine. The destination function will receive the callback in the same context as the tick call.
The call to tick has a low overhead when a timer is not 'due'. Internally the timers are stored in 'first timeout' order so only
the head of the list needs to be checked.

Each timer may have a period of up to 232-2 ticks (4,294,967,294).
At 1ms per tick this would equate to just over 49 days.

Defines the following classes:-

Uses definitions from timer.h

An example Keil project for the Nucleo 401RE may be found in examples/ArmTimerCallbacks - C++

Usage notes

The message timer is designed to be used in a timer interrupt/thread - single foreground task environment. The timer tick
call may pre-empt the foreground task, except when a timer function is within a ETL_DISABLE_TIMER_UPDATES /
ETL_ENABLE_TIMER_UPDATES section. These macros will either use an atomic semaphore or contain code to disable or
enable the relevant timer interrupts.

There are two macros that control which mechanism is used.

The framework relies on an atomic counter type. By default this is defined as etl::timer_semaphore_t.
This in turn is either defined as std::atomic<uint32_t>, if the compiler supports std::atomic, or
etl::atomic<uint32_t> if there is an atomic_xxx.h defined for the platform. A user defined type may be used for the
semaphore by defining ETL_TIMER_SEMAPHORE_TYPE. Only the timer interrupt/thread and one foreground  task may call
register_timer, unregister_timer, clear, start or stop.
With this mechanism, calls to tick are never disabled. If the foreground thread is within a disable/enable section when the
timer interrupt/thread is activated then the tick update will be deferred until the next tick period. The timer
interrupt/thread may interrogate the return value of tick() to check whether the update was deferred.

The user must supply two macro definitions (ETL_MESSAGE_TIMER_DISABLE_INTERRUPTS and
ETL_MESSAGE_TIMER_ENABLE_INTERRUPTS) to control interrupt enables. These macros must enable/disable all interrupts
that may call tick, register_timer, unregister_timer, clear, start or stop.
The user should ensure that mechanisms, such as memory barriers are used to disable re-ordering of the instructions.
If the foreground task is within a disable/enable section when the timer interrupt is triggered then the tick update will be
deferred until the interrupts are re-enabled. Depending on the resolution of the timers, the interrupt routine may be
able to compensate for the delay by passing a modified tick count to tick().

For correct operation of the timer framework, the routine that calls tick must not be pre-emptible by another routine
that calls a timer function. Also, calls to the timer framework may only be made from the caller of tick and one other, lower
priority, thread of execution


The base class for all timer controllers.

Member functions

etl::timer::id::type register_timer(void     (*p_callback)(),
                                    uint32_t period,                                       
                                    bool     repeating)
Registers a timer calling a free or static function.
p_callback A pointer to the callback free funtion that will be callled when the timer expires.
period     The timer period in ticks.
repeating  false if single shot, true if repeating.

Returns the allocated timer id or etl::timer::mode::NO_TIMER if one was not available.

etl::timer::id::type register_timer(etl::ifunction<void>& callback,
                                    uint32_t              period,                                       
                                    bool                  repeating)
Registers a timer calling a free, static or member function through the etl::ifunction interface.
p_callback A reference to the etl::function callback function that will be called when the timer expires.
period     The timer period in ticks.
repeating  false if single shot, true if repeating.

Returns the allocated timer id or etl::timer::id::NO_TIMER if one was not available.

bool unregister_timer(etl::timer::id::type id)
Unregisters a timer.
If the timer is active then it will be stopped.
Returns true if a timer with the id was successfully unregistered.

void enable(bool state)
Enables or disables the timer manager according to the state.

bool is_running() const
Returns true if the timer manager is enabled.

void clear()
Clears the callback timer back to the initial state. All timers will be stopped and unregistered.

bool tick(uint32_t count)
This function updates the internal tick counter (if enabled) and must pass the number of ticks that have occurred since
the last call. If the count encompasses more than one period of a repeating timer then the timer will be triggered
multiple times in one call to tick.
Returns true if the tick counter was updated, otherwise false. This may be used by the calling routine to accumulate
unprocessed tick counts.

bool start(etl::timer::id::type id, bool immediate = false)
Starts the timer with the specified id.
If the timer is already running then the timer Is restarted from the current tick count.
If immediate is true then the timer is triggered on the next call to tick(). Note: Single shot timers will only trigger once.
If the id does not correspond to a registered timer then returns false.

bool stop(etl::timer::id::type id_)
Stops the timer with the specified id.
Does nothing if the timer is already stopped.
if the id does not correspond to a registered timer then returns false.

bool set_period(etl::timer::id::type id_, uint32_t period)
Sets a new timer period.
Restarts the timer.
Returns true if successful.

bool set_mode(etl::timer::id::type id_, bool repeating)
Sets a new timer mode.
Restarts the timer.
Returns true if successful.




Template parameters

MAX_TIMERS The number of timers to be supported. The maximum number is 254.
A value of 255 will result in a compile error.


// Class callback via etl::function
class Test

    : ticks(0)

  void callback()

  int ticks;
Test test;
etl::function_mv<Test, &Test::callback> member_callback(test);

// Free function callback via etl::function
int free_ticks1 = 0;

void free_callback1()

etl::function_fv<free_callback1> free_function_callback;

// Free function callback via function pointer
int free_ticks2 = 0;

void free_callback2()

// Timer controller.
etl::callback_timer<3> timer_controller;

// The main loop.
int main()
  etl::timer::id::type id1 = timer_controller.register_timer(member_callback,

  etl::timer::id::type id2 = timer_controller.register_timer(free_function_callback,

  etl::timer::id::type id3 = timer_controller.register_timer(free_callback2,



  // Start timer interrupts here.

  while (true)
    // Loop forever.

  return 0;

// The interrupt timer callback.
void timer_interrupt()
  const uint32_t TICK = 1;
  static uint32_t nticks = TICK;

  if (timer_controller.tick(nticks))
    nticks = TICK;
    nticks += TICK;