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Real-Time Clock
Using the battery backed RTC as a calendar/time of day clock

A battery-backed real time clock (RTC) may be optionally included. If so, the RTC’s battery also backs-up the 128K RAM. Note however that if the expanded memory option is chosen, providing 512K or RAM, then none of the RAM can be battery-backed. The accuracy of the clock is better than +/- 2 minutes per month.


Battery discharge rate

A 7 ma-hr Lithium rechargable battery is used. This backup battery charges automatically while +5V is applied to the board. When +5V is not applied the battery keeps alive the RTC and the RAM so that information is not lost. The length of time the battery lasts varies from board to board primarily because RAM chips vary device-to-device in their leakage currents. Their leakage currents also depend strongly on ambient temperature so that while a battery may last a long time at normal or low ambient temperatures, at greater temperature it may discharge quickly.

We have measured battery discharge times for typical devices and have found the following:

  • Actual measurements at normal temperatures (<40°C) show the battery should backup the memory more than two years between recharging. Our actual measurements of the RAM current draw show it to draw much less than its "typical" spec from its datasheet.
  • Assuming the "typical" current specifications of the RAM data sheets the battery should last 139 days; and,
  • At the worst case current specification for the RAM, battery life would be 44 days at normal temperatures, and as little as 6 days if operated continuously at an ambient temperature of 70°C.
  • If the RAM is not battery backed, but the RTC installed, typical RTC retention time at 25°C should be 300 days.

Figure 12-1 illustrates the dependence of battery lifetime between charges while backing up the 128K RAM and powering the RTC, and ambient temperature.

Battery discharge time as a function of ambient temperature

Figure 12-1 Battery discharge time as a function of ambient temperature.

After each cold restart the kernel re-initializes the RTC chip to charge the battery and to use battery backup on removal of power. The maximum battery recharge time is 22 hours.


Setting and reading the real time clock

The built-in library functions SetWatch() and ReadWatch() make it easy to set and read the real time clock. The SetWatch() and ReadWatch() functions use the top 16 bytes of the 68HC11F1’s on-chip RAM as a buffer to hold intermediate results during the data transfers, and the top 8 bytes at addresses 0xB3F8-0xB3FF serve as the watch_results structure that contains the results returned by the most recent call to ReadWatch() as described below.

Because the structure that is written to by ReadWatch() is at a fixed location, this code is not re-entrant. This is not a problem in single-task applications or applications where only one task uses the watch. But it can cause problems if multiple tasks are executing ReadWatch(). For example, assume that task #1 calls ReadWatch(), but before it can access the contents of the structure using the assignment statement, the timeslice interrupt occurs and task#2 proceeds to call ReadWatch(), then the contents of the structure could be changed before task#1 is able to execute its assignment statement. To avoid this situation, the multitasking application can be configured so that only one task calls ReadWatch() and SetWatch() and shares the data with other tasks.

Another option is to define a resource variable to mediate access to the watch. The routines needed to accomplish this are declared in the MTASKER.H file and described in detail in the Control-C Glossary. The following brief example illustrates how to design a re-entrant function that returns the current WATCH_MINUTE:

RESOURCE watch_resource;     // declare resource variable: controls
                             // watch access
  _Q int CurrentMinute(void) // reads watch, returns current minute
   {  int minute;
      GET(&watch_resource);   // get access to watch; Pause() if
                             // another task has it
      ReadWatch();           // updates contents of watch structure
      minute = WATCH_MINUTE;
                             // you can also transfer other contents
                             // from the watch structure to
                             // "task-private" variables here
      RELEASE(&watch_resource); // release access to watch so other
                             // tasks can use it
      return minute;

The CurrentMinute() function can be simultaneously called from multiple tasks without causing any conflicts. The GET() and RELEASE() macros automatically mediate access to the watch, ensuring that only one task has access at a time.

The battery-backed clock is pre-set at the factory to Pacific Time in the United States. To re-set the smart watch to your time zone, your program can call the function:

void SetWatch(hundredth_seconds,seconds,minute,hour,day,date,month,year)

The interactively callable routine named SetTheWatch() is defined in the TIMEKEEP.C file so you can set the watch from your terminal. As explained in the Control-C Glossary, the hour parameter ranges from 0 to 23, the day from 1 to 7 (Monday=1 in this example), and the year parameter ranges from 00 to 99. For example, if it is now 10 seconds past 5:24 PM on Wednesday, May 26, 2004, you could interactively set the watch by typing:

SetTheWatch( 0, 10, 24, 17, 3, 26, 5, 4)↓

The watch is set and read using 24-hour time, where midnight is hour 0, noon is hour 12, and 11 PM is hour 23. Note that you may assign any day of the week as "day number 1".

The function ReadWatch() writes the current time and date information into a structure named watch_results that occupies the top 8 bytes of on-chip RAM at addresses 0xB3F8-0xB3FF. Pre-coded macros name the structure elements so it is easy to access the time and date information. For example, the following simple function in the TIMEKEEP.C file reads the smart watch and prints the time and date:

_Q void SayDate(void)
{   ReadWatch();     // results are placed in watch_results structure
printf("\nIt is now %d:%d:%d on %d/%d/%d.\n",

It simply calls ReadWatch(), and then executes a printf() statement using the pre-coded structure macros to reference the time parameters in the watch_results structure. If you called this function immediately after setting the watch as described in the prior section, the response at your terminal might be:

It is now 17:24:31 on 5/26/04.

After compiling TIMEKEEP.C, you can interactively type at your terminal:

SayDate( )↓

at any time to see a display of the current time and date.

For backwards compatibility with the QED product line, the hundredths of a second field is present but it is not used. The hundredths of a second value is required for SetWatch() but it is ignored and it is always 0 from ReadWatch().

This page is about: Real-Time Clock, Using Battery Backed RTC as Calendar/time of Day Clock – A battery backed real time clock (RTC) may be optionally included. If so, RTC’s battery also backs up 128K RAM. Note however that if expanded memory option is chosen, providing 512K or RAM, then none of RAM can be battery backed. The accuracy of clock is …