The Battery-Backed Real-Time Clock

A battery-backed real time clock (RTC) may be optionally included on the PDQ Single Board Computer (SBC). 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.

An onboard 7 ma-hr Lithium rechargeable 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 so that information is not lost. The battery does not back up the RAM on the PDQ Board or its Freescale 9S12 (HCS12) microcontroller. The length of time the battery lasts varies from board to board primarily because 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 typical RTC retention time at 25°C should be 300 days.

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 built-in library functions SetWatch and ReadWatch declared in C:\MosaicPlus\c\libraries\include\mosaic\WATCH.h make it easy to set and read the real time clock. The SetWatch and ReadWatch functions use an 8-byte watch_results structure in reserved system RAM 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 C Function Glossary (A-H). 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 C Function Glossary (A-H), 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, 2009, you could interactively set the watch by typing:

SetTheWatch( 0, 10, 24, 17, 3, 26, 5, 9)↓

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 the watch_results sructure. 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);
}

Open a new Timekeep Demo project for a downloadable version of this code.

Look for this icon under Project→ New Project:

Timekeep Demo

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 5/26/09.

After compiling a Timekeep Demo project, 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 input parameter is required for SetWatch but it is ignored, and it is always returned as a 0 value by ReadWatch.

See also → A Turnkeyed C Application Program