Rainbow Electronics DS1339 - Manual (Page 6)

DS1339
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TIME AND DATE OPERATION
The time and date information is obtained by reading the appropriate register bytes. The real-time clock
registers are illustrated in Figure 2. The time and date are set or initialized by writing the appropriate
register bytes. The contents of the time and date registers are in the binary coded decimal (BCD) format.
The DS1339 can be run in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the
12-hour or 24-hour mode-select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5
is the AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the second 10-hour bit (20–23
hours). All hours values, including the alarms, must be re-entered whenever the 12/24-hour mode bit is
changed. The century bit (bit 7 of the month register) is toggled when the years register overflows from
99 to 00. The day-of-week register increments at midnight. Values that correspond to the day of week are
user-defined, but must be sequential (i.e., if 1 equals Sunday, then 2 equals Monday, and so on). Illogical
time and date entries result in undefined operation.
When reading or writing the time and date registers, secondary (user) buffers are used to prevent errors
when the internal registers update. When reading the time and date registers, the user buffers are
synchronized to the internal registers on any START or STOP, and when the address pointer rolls over to
0. The countdown chain is reset whenever the seconds register is written. Write transfers occur on the
acknowledge pulse from the device. To avoid rollover issues, once the countdown chain is reset, the
remaining time and date registers must be written within one second. The 1Hz square-wave output, if
enabled, transitions high 500ms after the seconds data transfer, provided the oscillator is already running.
ALARMS
The DS1339 contains two time-of-day/date alarms. Alarm 1 can be set by writing to registers 07h to 0Ah.
Alarm 2 can be set by writing to registers 0Bh to 0Dh. The alarms can be programmed (by the alarm
enable and INTCN bits of the control register) to activate the SQW/
INT
output on an alarm match
condition. Bit 7 of each of the time-of-day/date alarm registers are mask bits (Figure 3). When all of the
mask bits for each alarm are logic 0, an alarm only occurs when the values in the timekeeping registers
00h–06h match the values stored in the time-of-day/date alarm registers. The alarms can also be
programmed to repeat every second, minute, hour, day, or date. Figure 3 shows the possible settings.
Configurations not listed in the table result in illogical operation.
The DY/DT bits (bit 6 of the alarm day/date registers) control whether the alarm value stored in bits 0 to
5 of that register reflects the day of the week or the date of the month. If DY/DT is written to a logic 0,
the alarm is the result of a match with date of the month. If DY/DT is written to a logic 1, the alarm is the
result of a match with day of the week.
When the RTC register values match alarm register settings, the corresponding alarm flag (A1F or A2F)
bit is set to logic 1. If the corresponding alarm interrupt enable (A1IE or A2IE) is also set to logic 1 and
the INTCN bit is set to logic 1, the alarm condition activates the SQW/
INT
signal.