Extremely Accurate I
2
C-Integrated
RTC/TCXO/Crystal
10 Maxim Integrated
DS3231
32kHz TCXO
The temperature sensor, oscillator, and control logic
form the TCXO. The controller reads the output of the
on-chip temperature sensor and uses a lookup table to
determine the capacitance required, adds the aging
correction in AGE register, and then sets the capaci-
tance selection registers. New values, including
changes to the AGE register, are loaded only when a
change in the temperature value occurs, or when a
user-initiated temperature conversion is completed.
Temperature conversion occurs on initial application of
V
CC
and once every 64 seconds afterwards.
Power Control
This function is provided by a temperature-compensat-
ed voltage reference and a comparator circuit that
monitors the V
CC
level. When V
CC
is greater than V
PF
,
the part is powered by V
CC
. When V
CC
is less than V
PF
but greater than V
BAT
, the DS3231 is powered by V
CC
.
If V
CC
is less than V
PF
and is less than V
BAT
, the
device is powered by V
BAT
. See Table 1.
To preserve the battery, the first time V
BAT
is applied to
the device, the oscillator will not start up until V
CC
exceeds V
PF
, or until a valid I
2
C address is written to
the part. Typical oscillator startup time is less than one
second. Approximately 2 seconds after V
CC
is applied,
or a valid I
2
C address is written, the device makes a
temperature measurement and applies the calculated
correction to the oscillator. Once the oscillator is run-
ning, it continues to run as long as a valid power
source is available (V
CC
or V
BAT
), and the device con-
tinues to measure the temperature and correct the
oscillator frequency every 64 seconds.
On the first application of power (V
CC
) or when a valid
I
2
C address is written to the part (V
BAT
), the time and
date registers are reset to 01/01/00 01 00:00:00
(DD/MM/YY DOW HH:MM:SS).
V
BAT
Operation
There are several modes of operation that affect the
amount of V
BAT
current that is drawn. While the device
is powered by V
BAT
and the serial interface is active,
active battery current, I
BATA
, is drawn. When the serial
interface is inactive, timekeeping current (I
BATT
), which
includes the averaged temperature conversion current,
I
BATTC
, is used (refer to Application Note 3644:
Power
Considerations for Accurate Real-Time Clocks
for
details). Temperature conversion current, I
BATTC
, is
specified since the system must be able to support the
periodic higher current pulse and still maintain a valid
voltage level. Data retention current, I
BATTDR
, is the
current drawn by the part when the oscillator is
stopped (EOSC = 1). This mode can be used to mini-
mize battery requirements for times when maintaining
time and date information is not necessary, e.g., while
the end system is waiting to be shipped to a customer.
Pushbutton Reset Function
The DS3231 provides for a pushbutton switch to be
connected to the RST output pin. When the DS3231 is
not in a reset cycle, it continuously monitors the RST
signal for a low going edge. If an edge transition is
detected, the DS3231 debounces the switch by pulling
the RST low. After the internal timer has expired
(PB
DB
), the DS3231 continues to monitor the RST line.
If the line is still low, the DS3231 continuously monitors
the line looking for a rising edge. Upon detecting
release, the DS3231 forces the RST pin low and holds it
low for t
RST
.
RST is also used to indicate a power-fail condition.
When V
CC
is lower than V
PF
, an internal power-fail sig-
nal is generated, which forces the RST pin low. When
V
CC
returns to a level above V
PF
, the RST pin is held
low for approximately 250ms (t
REC
) to allow the power
supply to stabilize. If the oscillator is not running (see
the
Power Control
section) when V
CC
is applied, t
REC
is
bypassed and RST immediately goes high. Assertion of
the RST output, whether by pushbutton or power-fail
detection, does not affect the internal operation of the
DS3231.
Real-Time Clock
With the clock source from the TCXO, the RTC provides
seconds, minutes, hours, day, date, month, and year
information. The date at the end of the month is auto-
matically adjusted for months with fewer than 31 days,
including corrections for leap year. The clock operates
in either the 24-hour or 12-hour format with an AM/PM
indicator.
The clock provides two programmable time-of-day
alarms and a programmable square-wave output. The
INT/SQW pin either generates an interrupt due to alarm
condition or outputs a square-wave signal and the
selection is controlled by the bit INTCN.
