IDT1337AG
REAL-TIME CLOCK WITH I
2
C SERIAL INTERFACE RTC
IDT®
REAL-TIME CLOCK WITH I
2
C SERIAL INTERFACE 7
IDT1337AG REV C 011514
Clock and Calendar
The time and calendar information is obtained by reading
the appropriate register bytes. The RTC registers are
illustrated in Table 1. The time and calendar are set or
initialized by writing the appropriate register bytes. The
contents of the time and calendar registers are in the
binary-coded decimal (BCD) format.
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 register
pointer rolls over to zero.
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 1 second. The 1Hz
square-wave output, if enable, transitions high 500ms after
the seconds data transfer, provided the oscillator is already
running.
The IDT1337AG can be run in either 12-hour or 24-hour
mode. Bit 6 of the hours register is defined as the 12- 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 reinitialized 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 99–00.
Alarms
The IDT1337AG 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 INTCN bits of the Control
Register) to operate in two different modes—each alarm
can drive its own separate interrupt output or both alarms
can drive a common interrupt output. Bit 7 of each of the
time-of-day/date alarm registers are mask bits (Table 1).
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. Table 2 (Alarm Mask Bits table) 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, the alarm condition
activates one of the interrupt output (INTA
or SQW/INTB)
signals. The match is tested on the once-per-second update
of the time and date registers.
