The Battery-Backed Real-Time Clock

A battery-backed real time clock (RTC) may be optionally included on the QCard Controller Single Board Computer (SBC). 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. Pre-coded C language functions make it easy to incorporate the real-time data (time of day, day, date, month and year) for instrument control and automation applications.

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",
WATCH_HOUR, WATCH_MINUTE, WATCH_SECONDS,
WATCH_MONTH, WATCH_DATE, WATCH_YEAR);
}

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().

The real time clock and RAM are kept alive when the board is not externally powered by a small, button rechargeable battery. A 7 mA-h lithium-vanadium rechargeable battery¹⁾ provides this data retention capability. 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 main RAM so that information is not lost. (Note that the 1Kbyte of internal RAM on the 68HC11 microcontroller chip is not battery backed.) While the board is unpowered the battery slowly discharges into the RAM and into the reverse leakage current of power steering diodes. The length of time the battery lasts when the board is unpowered, that is, the data retention time, 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 storage temperature it may discharge more 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 300 days between recharging. Our actual measurements of the RAM current draw show it to draw much less than its "typical" spec from its datasheet, so that the RAM does not limit battery life. Instead, it is limited by diode leakage.
• Assuming the "typical" current specifications of the RAM and diode data sheets the battery should last around 139 days; and,
• At the worst case current specification for the RAM and diodes, 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, decreasing to 150 days at 35°C.

The following graph shows the dependence of battery lifetime between charges while backing up the 128KB RAM and powering the RTC, and ambient temperature.

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.

The battery's specified operating temperature is - 20 to + 60 °C. Its charge/discharge characteristics hardly vary over that range, so it may perform adequately over a wider range, but it's ultimate lifetime could be compromised if operated for prolonged times outside of this range. Please see the VL1220 datasheet to examine its charge/discharge characteristics over operating temperature.

For more information, here are the datasheets for the backup battery and RTC chip: