source: rtems-docs/bsp_howto/real_time_clock.rst @ f916fca

Real-Time Clock Driver
######################

Introduction
============

The Real-Time Clock (*RTC*) driver is responsible for providing an
interface to an *RTC* device.  \[NOTE: In this chapter, the abbreviation*TOD* is used for *Time of Day*.]  The capabilities provided by this
driver are:

- Set the RTC TOD to RTEMS TOD

- Set the RTEMS TOD to the RTC TOD

- Get the RTC TOD

- Set the RTC TOD to the Specified TOD

- Get the Difference Between the RTEMS and RTC TOD

The reference implementation for a real-time clock driver can
be found in ``c/src/lib/libbsp/shared/tod.c``.  This driver
is based on the libchip concept and can be easily configured
to work with any of the RTC chips supported by the RTC
chip drivers in the directory ``c/src/lib/lib/libchip/rtc``.
There is a README file in this directory for each supported
RTC chip.  Each of these README explains how to configure the
shared libchip implementation of the RTC driver for that particular
RTC chip.

The DY-4 DMV177 BSP used the shared libchip implementation of the RTC
driver.  There were no DMV177 specific configuration routines.  A BSP
could use configuration routines to dynamically determine what type
of real-time clock is on a particular board.  This would be useful for
a BSP supporting multiple board models.  The relevant ports of
the DMV177's ``RTC_Table`` configuration table is below:
.. code:: c

    #include <bsp.h>
    #include <libchip/rtc.h>
    #include <libchip/icm7170.h>
    bool dmv177_icm7170_probe(int minor);
    rtc_tbl     RTC_Table[] = {
    { "/dev/rtc0",                /* sDeviceName \*/
    RTC_ICM7170,                /* deviceType \*/
    &icm7170_fns,               /* pDeviceFns \*/
    dmv177_icm7170_probe,       /* deviceProbe \*/
    (void \*) ICM7170_AT_1_MHZ,  /* pDeviceParams \*/
    DMV170_RTC_ADDRESS,         /* ulCtrlPort1 \*/
    0,                          /* ulDataPort \*/
    icm7170_get_register_8,     /* getRegister \*/
    icm7170_set_register_8,     /* setRegister \*/
    }
    };
    unsigned long  RTC_Count = (sizeof(RTC_Table)/sizeof(rtc_tbl));
    rtems_device_minor_number  RTC_Minor;
    bool dmv177_icm7170_probe(int minor)
    {
    volatile unsigned16 \*card_resource_reg;
    card_resource_reg = (volatile unsigned16 \*) DMV170_CARD_RESORCE_REG;
    if ( (\*card_resource_reg & DMV170_RTC_INST_MASK) == DMV170_RTC_INSTALLED )
    return TRUE;
    return FALSE;
    }

Initialization
==============

The ``rtc_initialize`` routine is responsible for initializing the
RTC chip so it can be used.  The shared libchip implementation of this
driver supports multiple RTCs and bases its initialization order on
the order the chips are defined in the ``RTC_Table``.  Each chip
defined in the table may or may not be present on this particular board.
It is the responsibility of the ``deviceProbe`` to indicate the
presence of a particular RTC chip.  The first RTC found to be present
is considered the preferred RTC.

In the shared libchip based implementation
of the driver, the following actions are performed:
.. code:: c

    rtems_device_driver rtc_initialize(
    rtems_device_major_number  major,
    rtems_device_minor_number  minor_arg,
    void                      \*arg
    )
    {
    for each RTC configured in RTC_Table
    if the deviceProbe for this RTC indicates it is present
    set RTC_Minor to this device
    set RTC_Present to TRUE
    break out of this loop
    if RTC_Present is not TRUE
    return RTEMS_INVALID_NUMBER to indicate that no RTC is present
    register this minor number as the "/dev/rtc"
    perform the deviceInitialize routine for the preferred RTC chip
    for RTCs past this one in the RTC_Table
    if the deviceProbe for this RTC indicates it is present
    perform the deviceInitialize routine for this RTC chip
    register the configured name for this RTC
    }

The ``deviceProbe`` routine returns TRUE if the device
configured by this entry in the ``RTC_Table`` is present.
This configuration scheme allows one to support multiple versions
of the same board with a single BSP.  For example, if the first
generation of a board had Vendor A's RTC chip and the second
generation had Vendor B's RTC chip, RTC_Table could contain
information for both.  The ``deviceProbe`` configured
for Vendor A's RTC chip would need to return TRUE if the
board was a first generation one.  The ``deviceProbe``
routines are very board dependent and must be provided by
the BSP.

setRealTimeToRTEMS
==================

The ``setRealTimeToRTEMS`` routine sets the current RTEMS TOD to that
of the preferred RTC.
.. code:: c

    void setRealTimeToRTEMS(void)
    {
    if no RTCs are present
    return
    invoke the deviceGetTime routine for the preferred RTC
    set the RTEMS TOD using rtems_clock_set
    }

setRealTimeFromRTEMS
====================

The ``setRealTimeFromRTEMS`` routine sets the preferred RTC TOD to the
current RTEMS TOD.
.. code:: c

    void setRealTimeFromRTEMS(void)
    {
    if no RTCs are present
    return
    obtain the RTEMS TOD using rtems_clock_get
    invoke the deviceSetTime routine for the preferred RTC
    }

getRealTime
===========

The ``getRealTime`` returns the preferred RTC TOD to the
caller.
.. code:: c

    void getRealTime( rtems_time_of_day \*tod )
    {
    if no RTCs are present
    return
    invoke the deviceGetTime routine for the preferred RTC
    }

setRealTime
===========

The ``setRealTime`` routine sets the preferred RTC TOD to the
TOD specified by the caller.
.. code:: c

    void setRealTime( rtems_time_of_day \*tod )
    {
    if no RTCs are present
    return
    invoke the deviceSetTime routine for the preferred RTC
    }

checkRealTime
=============

The ``checkRealTime`` routine returns the number of seconds
difference between the RTC TOD and the current RTEMS TOD.
.. code:: c

    int checkRealTime( void )
    {
    if no RTCs are present
    return -1
    obtain the RTEMS TOD using rtems_clock_get
    get the TOD from the preferred RTC using the deviceGetTime routine
    convert the RTEMS TOD to seconds
    convert the RTC TOD to seconds
    return the RTEMS TOD in seconds - RTC TOD in seconds
    }

.. COMMENT: COPYRIGHT (c) 1988-2002.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.