11. Real-Time Clock Driver

11.1. Introduction

The Real-Time Clock (RTC) driver is responsible for providing an interface to an RTC device. 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

Note

In this chapter, the abbreviation TOD is used for Time of Day.

The reference implementation for a real-time clock driver can be found in bsps/shared/dev/rtc/rtc-support.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 bsps/shared/dev/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:

#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 uint16_t *card_resource_reg;
  card_resource_reg = (volatile uint16_t *) DMV170_CARD_RESORCE_REG;
  if ( (*card_resource_reg & DMV170_RTC_INST_MASK) == DMV170_RTC_INSTALLED )
    return TRUE;
  return FALSE;
}

11.2. 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:

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.

11.3. setRealTimeToRTEMS

The setRealTimeToRTEMS routine sets the current RTEMS TOD to that of the preferred RTC.

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
}

11.4. setRealTimeFromRTEMS

The setRealTimeFromRTEMS routine sets the preferred RTC TOD to the current RTEMS TOD.

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
}

11.5. getRealTime

The getRealTime returns the preferred RTC TOD to the caller.

void getRealTime( rtems_time_of_day *tod )
{
  if no RTCs are present
  return

  invoke the deviceGetTime routine for the preferred RTC
}

11.6. setRealTime

The setRealTime routine sets the preferred RTC TOD to the TOD specified by the caller.

void setRealTime( rtems_time_of_day *tod )
{
  if no RTCs are present
    return

  invoke the deviceSetTime routine for the preferred RTC
}

11.7. checkRealTime

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

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
}