Timer Drivers
=============

Files supporting the timer driver can be found in the following
locations:

-  **Interface Definition**. The header file for the NuttX timer
   driver reside at ``include/nuttx/timers/timer.h``. This header
   file includes both the application level interface to the timer
   driver as well as the interface between the "upper half" and
   "lower half" drivers. The timer driver uses a standard
   character driver framework.
-  **"Upper Half" Driver**. The generic, "upper half" timer driver
   resides at ``drivers/timers/timer.c``.
-  **"Lower Half" Drivers**. Platform-specific timer drivers
   reside in ``arch/<architecture>/src/<hardware>``
   directory for the specific processor ``<architecture>`` and for
   the specific ``<chip>`` timer peripheral devices.

There are two ways to enable Timer Support along with the Timer Example. The
first is faster and simpler. Just run the following command to use a ready
config file with timer support and example included. You need to check if there's
a timer config file for your specific chip. You may check it at the specific
board's path: ``/boards/<arch>/<chip>/<variant>/config``.

.. code-block:: console

   $ ./tools/configure.sh <variant>:timer

And the second way is creating your own config file. To do so, follow the next
instructions.

Enabling the Timer Support and Example in ``menuconfing``
----------------------------------------------------------

  1. Select Timer Instances

 To select these timers browse in the ``menuconfig`` using the following path:

  Go into menu :menuselection:`System Type --> <Chip> Peripheral Selection` and press :kbd:`Enter`.

  Then select one or more timers according to availability.

  2. Enable the Timer Support

  Go into menu :menuselection:`Device Drivers --> Timer Driver Support` and press :kbd:`Enter`. Then enable:

  - [x] Timer Support
  - [x] Timer Arch Implementation

  3. Include the Timer Example

  Go into menu :menuselection:`Application Configuration --> Examples` and press :kbd:`Enter`. Then select the Timer Example.

  - [x] Timer example

  Below the option, it is possible to manually configure some parameters as the
  standard timer device path, the timeout, the sample rate in which the counter
  will be read, the number of samples to be executed, and other parameters.

Timer Example
-------------

The previously selected example will basically consult the timer status, set a
timer alarm interval, set a timer signal handler function to be notified at the
alarm, which only increments a variable, and then it will start the timer. The
application will periodically consult the timer status at the sample rate
previously configured through the ``menuconfig`` to follow the time left until
the timer expires. After the samples have been read, the application stops the
timer.

The `example code <https://github.com/apache/nuttx-apps/blob/master/examples/timer/timer_main.c>`_
may be explored, its path is at ``/examples/timer/timer_main.c`` in the apps' repository.

In NuttX, the timer driver is a character driver and when a chip supports multiple
timers, each one is accessible through its respective file in ``/dev`` directory.
Each timer is registered using a unique numeric identifier (i.e. ``/dev/timer0``,
``/dev/timer1``, ...).

Use the following command to run the example:

.. code-block:: console

  `nsh> timer`

This command will use the timer 0. To use the others, specify it through a
parameter (where x is the timer number):

.. code-block:: console

  `nsh> timer -d /dev/timerx`

Application Level Interface
---------------------------

The first necessary thing to be done in order to use the timer driver in an
application is to include the header file for the NuttX timer driver. It contains
the Application Level Interface to the timer driver. To do so, include:

.. code-block:: c

  #include <nuttx/timers/timer.h>

At an application level, the timer functionalities may be accessed through ``ioctl``
systems calls. The available ``ioctl`` commands are:

 * :c:macro:`TCIOC_START`
 * :c:macro:`TCIOC_STOP`
 * :c:macro:`TCIOC_GETSTATUS`
 * :c:macro:`TCIOC_SETTIMEOUT`
 * :c:macro:`TCIOC_NOTIFICATION`
 * :c:macro:`TCIOC_MAXTIMEOUT`

These ``ioctl`` commands internally call lower-half layer operations and the
parameters are forwarded to these ops through the ``ioctl`` system call. The return
of a system call is the return of an operation.
These ``struct timer_ops_s`` keeps pointers to the implementation of each operation.
Following is the struct.

.. c:struct:: timer_ops_s
.. code-block:: c

   struct timer_ops_s
   {
      /* Required methods *******************************************************/

      /* Start the timer, resetting the time to the current timeout */

      CODE int (*start)(FAR struct timer_lowerhalf_s *lower);

      /* Stop the timer */

      CODE int (*stop)(FAR struct timer_lowerhalf_s *lower);

      /* Get the current timer status */

      CODE int (*getstatus)(FAR struct timer_lowerhalf_s *lower,
                              FAR struct timer_status_s *status);

      /* Set a new timeout value (and reset the timer) */

      CODE int (*settimeout)(FAR struct timer_lowerhalf_s *lower,
                              uint32_t timeout);

      /* Call the NuttX INTERNAL timeout callback on timeout.
         * NOTE:  Providing callback==NULL disable.
         * NOT to call back into applications.
         */

      CODE void (*setcallback)(FAR struct timer_lowerhalf_s *lower,
                                 CODE tccb_t callback, FAR void *arg);

      /* Any ioctl commands that are not recognized by the "upper-half" driver
         * are forwarded to the lower half driver through this method.
         */

      CODE int (*ioctl)(FAR struct timer_lowerhalf_s *lower, int cmd,
                        unsigned long arg);

      /* Get the maximum supported timeout value */

      CODE int (*maxtimeout)(FAR struct timer_lowerhalf_s *lower,
                              FAR uint32_t *maxtimeout);
   };

Since  ``ioctl`` system calls expect a file descriptor, before using these commands,
it's necessary to open the timer device special file in order to get a file descriptor.
The following snippet demonstrates how to do so:

.. code-block:: c

  /* Open the timer device */

  printf("Open %s\n", devname);

  fd = open(devname, O_RDONLY);
  if (fd < 0)
    {
      fprintf(stderr, "ERROR: Failed to open %s: %d\n",
              devname, errno);
      return EXIT_FAILURE;
    }

.. c:macro:: TCIOC_START

The ``TCIOC_START`` command calls the ``start`` operation, which is described below.

.. c:function:: int start(void)

  The start operation configures the timer, enables the interrupt if ``TCIOC_NOTIFICATION``
  has already been called and finally starts the timer.

  :return: A Linux System Error Code for failing or 0 for success.

This command may be used like so:

.. code-block:: c

  /* Start the timer */

  printf("Start the timer\n");

  ret = ioctl(fd, TCIOC_START, 0);
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to start the timer: %d\n", errno);
      close(fd);
      return EXIT_FAILURE;
    }

.. c:macro:: TCIOC_STOP

The ``TCIOC_STOP`` command calls the ``stop`` operation, which is described below.

.. c:function:: int stop(void)

  The stop operation stops the timer and disables the interrupt.

  :return: A Linux System Error Code for failing or 0 for success.

This command may be used like so:

.. code-block:: c

  /* Stop the timer */

  printf("\nStop the timer\n");

  ret = ioctl(fd, TCIOC_STOP, 0);
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to stop the timer: %d\n", errno);
      close(fd);
      return EXIT_FAILURE;
    }

.. c:macro:: TCIOC_GETSTATUS

The ``TCIOC_GETSTATUS`` command calls the ``getstatus`` operation, which is described below.

.. c:function:: int getstatus(FAR struct timer_status_s *status)

  The getstatus operation gathers the timer's current information.

  :param status: A writable pointer to a struct ``timer_status_s``. This struct
                 contains 3 fields: ``flags`` (``uint32_t``), ``timeout`` (``uint32_t``)
                 and ``timeleft`` (``uint32_t``). Bit 0 from `flags` indicates the timer's
                 status, 1 indicates that the timer is running, zero it is stopped. Bit 1
                 from `flags` indicates if there's a callback registered. The `timeout`
                 indicates the time interval that was configured to trigger an alarm, it
                 is in microseconds. The `timeleft` interval indicates how many microseconds
                 it's missing to trigger the alarm. The following snippet demonstrates how
                 to use it and how to access these fields.

  :return: A Linux System Error Code for failing or 0 for success.

This command may be used like so:

.. code-block:: c

  /* Get timer status */

  ret = ioctl(fd, TCIOC_GETSTATUS, (unsigned long)((uintptr_t)&status));
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to get timer status: %d\n", errno);
      close(fd);
      exit(EXIT_FAILURE);
    }

  /* Print the timer status */

  printf("  flags: %08lx timeout: %lu timeleft: %lu\n",
         (unsigned long)status.flags, (unsigned long)status.timeout,
         (unsigned long)status.timeleft);

.. c:macro:: TCIOC_SETTIMEOUT

The ``TCIOC_SETTIMEOUT`` command calls the ``settimeout`` operation, which is described below.

.. c:function:: int settimeout(uint32_t timeout)

  The getstatus operation sets a timeout interval to trigger the alarm and then
  trigger an interrupt. It defines the timer interval in which the handler will
  be called.

  :param timeout: An argument of type ``uint32_t`` with the timeout value in microseconds.

  :return: A Linux System Error Code for failing or 0 for success.

This command may be used like so:

.. code-block:: c

  /* Set the timer interval */

  printf("Set timer interval to %lu\n",
         (unsigned long)CONFIG_EXAMPLES_TIMER_INTERVAL);

  ret = ioctl(fd, TCIOC_SETTIMEOUT, CONFIG_EXAMPLES_TIMER_INTERVAL);
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to set the timer interval: %d\n", errno);
      close(fd);
      return EXIT_FAILURE;
    }

.. c:macro:: TCIOC_NOTIFICATION

The ``TCIOC_NOTIFICATION`` is used to configure the timer callback to notify the
application via a signal when the timer expires. This command calls the ``setcallback``
operation. Which will not be described here, since the application does not set a
callback directly. Instead, the user should configure a signal handler to catch
notifications, and then, configure a timer notifier to notify and to signal the
previously configured signal handler. For a better performance, a separate pthread
may be configured to wait on sigwaitinfo() for timer events.

In any case, this command expects a read-only pointer to a struct `timer_notify_s`.
This struct contains 2 fields: ``pid`` (``pid_t``), that indicates the ID of the
task/thread to receive the signal and ``event`` (``struct sigevent``), which
describes the way a task will be notified.

This command may be used like so:

.. code-block:: c

  printf("Configure the notification\n");

  notify.pid   = getpid();
  notify.event.sigev_notify = SIGEV_SIGNAL;
  notify.event.sigev_signo  = CONFIG_EXAMPLES_TIMER_SIGNO;
  notify.event.sigev_value.sival_ptr = NULL;

  ret = ioctl(fd, TCIOC_NOTIFICATION, (unsigned long)((uintptr_t)&notify));
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to set the timer handler: %d\n", errno);
      close(fd);
      return EXIT_FAILURE;
    }

.. c:macro:: TCIOC_MAXTIMEOUT

The ``TCIOC_MAXTIMEOUT`` command calls the ``maxtimeout`` operation, which is described below.

.. c:function:: int maxtimeout(uint32_t *status)

  The maxtimeout operation  gets the maximum timeout value that can be configured.

  :param maxtimeout: A writable pointer to a variable of ``uint32_t`` type in
                     which the value will be stored.
  :return: A Linux System Error Code for failing or 0 for success.

This command may be used like so:

.. code-block:: c

  /* Get the maximum timer timeout  */

  printf("Get the maximum timer timeout\n");

  ret = ioctl(fd, TCIOC_MAXTIMEOUT, (uint32_t*)(&max_timeout));
  if (ret < 0)
    {
      fprintf(stderr, "ERROR: Failed to reat the timer's maximum timeout: %d\n", errno);
      close(fd);
      return EXIT_FAILURE;
    }

  /* Print the maximum supported timeout */

  printf("Maximum supported timeout: %" PRIu32 "\n", max_timeout);

Those snippets were taken from the Example which provides a great resource to
demonstrate how to use those ``ioctl`` commands.