etl/docs/timers/callback-timer.md

---
title: "callback_timer"
---

{{< callout type="info">}}
  Header: `callback_timer.h`  
{{< /callout >}}

**This class has been superseded.**  
Consider using `callback_timer_atomic` or `callback_timer_locked`.  

---

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 2<sup>32</sup>-2 ticks (4,294,967,294).  
At 1ms per tick this would equate to just over 49 days.  

**Defines the following classes**  
```cpp
etl::icallback_timer
etl::callback_timer
etl::callback_timer_data
```

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.

`ETL_CALLBACK_TIMER_USE_ATOMIC_LOCK`

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.  

---

`ETL_CALLBACK_TIMER_USE_INTERRUPT_LOCK`  

The user must supply two macro definitions (`ETL_CALLBACK_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()`.  

---

**Important**  
For correct operation of the timer framework, the routine that calls tick must not be preemptible 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.

## icallback_timer
The base class for all timer controllers.

### Member functions
```cpp
etl::timer::id::type register_timer(void     (*p_callback)(),
                                    uint32_t period,                                        
                                    bool     repeating)
```
**Description**  
Registers a timer calling a free or static function.

**Parameters**  
`p_callback` A pointer to the callback free function that will be called when the timer expires.
`period`     The timer period in ticks.
`repeating`  `false` if single shot, `true` if repeating.

**Return**  
The allocated timer id or `etl::timer::mode::NO_TIMER` if one was not available.

---

```cpp
etl::timer::id::type register_timer(etl::ifunction<void>& callback,
                                    uint32_t              period,                                        
                                    bool                  repeating)
```
**Description**  
Registers a timer calling a free, static or member function through the etl::ifunction interface.

**Parameters**  
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.

**Return**  
The allocated timer id or `etl::timer::id::NO_TIMER` if one was not available.

---

```cpp
etl::timer::id::type register_timer(etl::delegate<void()>& callback,
                                    uint32_t               period,                                        
                                    bool                   repeating)
```
**Description**  
Registers a timer calling a free, static or member function through the etl::delegate interface.
p_callback A reference to the etl::delegate 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.

---

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

---

```cpp
void enable(bool state)
```
**Description**  
Enables or disables the timer manager according to the state.

---

```cpp
bool is_running() const
```
**Description**  
Returns `true` if the timer manager is enabled.

---

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

---

```cpp
bool tick(uint32_t count)
```
**Description**  
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.

---

```cpp
bool start(etl::timer::id::type id, bool immediate = false)
```
**Description**  
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`.

---

```cpp
bool stop(etl::timer::id::type id)
```
**Description**  
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`.

---

```cpp
bool set_period(etl::timer::id::type id, uint32_t period)
```
**Description**  
Stops the timer with the specified id.
Sets a new timer period.
Returns `true` if successful.

---

```cpp
bool set_mode(etl::timer::id::type id, bool repeating)
```
**Description**  
Stops the timer with the specified id.
Sets a new timer mode.
Returns `true` if successful.

---

```cpp
etl::timer::id::type time_to_next()
```
**Description**  
Returns the time to the next timeout.
From: `20.38.0`

### Constants
`MAX_TIMERS`

## callback_timer
**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.

## Example
```cpp
//***************************************************************************
// Class callback via etl::function
//***************************************************************************
class Test
{
public:

  Test()
    : ticks(0)
  {
  }

  void callback()
  {
    ++ticks;
  }

  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()
{
  ++free_ticks;
}

etl::function_fv<free_callback1> free_function_callback;

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

void free_callback2()
{
  ++free_ticks2;
}

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

//***************************************************************************
// The main loop.
//***************************************************************************
int main()
{
  etl::timer::id::type id1 = timer_controller.register_timer(member_callback,
                                                             1000,
                                                             etl::timer::mode::SINGLE_SHOT);

  etl::timer::id::type id2 = timer_controller.register_timer(free_function_callback,
                                                             100,
                                                             etl::timer::mode::REPEATING);

  etl::timer::id::type id3 = timer_controller.register_timer(free_callback2,
                                                             10,
                                                             etl::timer::mode::REPEATING);

  timer_controller.start(id1);
  timer_controller.start(id2);
  timer_controller.start(id3);

  timer_controller.enable(true);

  // 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;
  }
  else
  {
    nticks += TICK;
  }
}
```