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
it’s life time.