DS1339A Datasheet, PDF (9/19 Pages) Maxim Integrated Products – Low-Current, I2C, Serial Real-Time Clock

DS1339A
Low-Current, I²C, Serial Real-Time Clock
Clock Accuracy
The accuracy of the clock is dependent upon the
accuracy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and
the capacitive load for which the crystal was trimmed.
Additional error is added by crystal frequency drift
caused by temperature shifts. External circuit noise
coupled into the oscillator circuit may result in the clock
running fast. Figure 6 shows a typical PC board layout
for isolating the crystal and oscillator from noise. Refer to
Application Note 58: Crystal Considerations with Dallas
Real-Time Clocks for detailed information
RTC Address Map
Table 3 shows the address map for the DS1339A
registers. During a multibyte access, when the address
pointer reaches the end of the register space (10h),
it wraps around to location 00h. On an I²C START or
address pointer incrementing to location 00h, the current
time is transferred to a second set of registers. The time
information is read from these secondary registers, while
the clock may continue to run. This eliminates the need
to re-read the registers in case of an update of the main
registers during a read.
Time and Date Operation
The time and date information is obtained by reading
the appropriate register bytes. Table 3 shows the RTC
registers. 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 BCD format. The
DS1339A 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
20-hour bit (20 to 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 YEAR register overflows from 99 to 00. If the
Century bit is logic 0, the year will be designated as a
Leap Year and February will contain 29 days.
If the Century bit is logic 1, the year will not be designated
as a Leap Year and February will contain 28 days.
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 a START or when the address
pointer rolls over to 00h. The countdown chain is reset
whenever the seconds register is written. Write transfers
occurs 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. If enabled, the 1Hz square-wave
output transitions high 500ms after the seconds data
transfer, provided the oscillator is already running.
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