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4. Real Time Clock and Time Management


Y2KC - DS1742 real time clock keeps track of time throughout its lifetime (approximately 10 yrs.) Clock registers are identical to SRAMs and are resident in the eight top RAM locations. BCD coded century, year, month, date, day, hour, minute and seconds can be manipulated to validate call starting time and duration of a telephone call. This is done by Z80 by reading appropriate registers of RTC, at the moment of call establishment and hang up.  

Table 1 DS 1742 Register map

OSC = STOP BIT         R = READ BIT                                                FT = FREQUENCY TEST

W     = WRITE BIT        X = not dedicated                      BF = BATTERY FLAG

     4.1.Setting the clock

As shown in Table 2, bit 7 (B7) of the century register is the write bit. Setting the write bit to a one, like the read bit, halts updates the DS1742 registers and the program  can then load them with the correct day, date and time data in 24 hour BCD format. Resetting the write bit to a zero then transfers those values to the actual clock counters and allows normal operation to resume. This time setting is to be done once in itís life time.



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Last modified: July 13, 2002