Extremely Accurate I
2
C-Integrated
RTC/TCXO/Crystal
Maxim Integrated 15
DS3231
Temperature Registers (11h–12h)
Temperature is represented as a 10-bit code with a res-
olution of 0.25°C and is accessible at location 11h and
12h. The temperature is encoded in two’s complement
format. The upper 8 bits, the integer portion, are at
location 11h and the lower 2 bits, the fractional portion,
are in the upper nibble at location 12h. For example,
00011001 01b = +25.25°C. Upon power reset, the reg-
isters are set to a default temperature of 0°C and the
controller starts a temperature conversion. The temper-
ature is read on initial application of V
CC
or I
2
C access
on V
BAT
and once every 64 seconds afterwards. The
temperature registers are updated after each user-initi-
ated conversion and on every 64-second conversion.
The temperature registers are read-only.
I
2
C Serial Data Bus
The DS3231 supports a bidirectional I
2
C bus and data
transmission protocol. A device that sends data onto
the bus is defined as a transmitter and a device receiv-
ing data is defined as a receiver. The device that con-
trols the message is called a master. The devices that
are controlled by the master are slaves. The bus must
be controlled by a master device that generates the
serial clock (SCL), controls the bus access, and gener-
ates the START and STOP conditions. The DS3231
operates as a slave on the I
2
C bus. Connections to the
bus are made through the SCL input and open-drain
SDA I/O lines. Within the bus specifications, a standard
mode (100kHz maximum clock rate) and a fast mode
(400kHz maximum clock rate) are defined. The DS3231
works in both modes.
The following bus protocol has been defined (Figure 2):
• Data transfer may be initiated only when the bus is
not busy.
• During data transfer, the data line must remain stable
whenever the clock line is high. Changes in the data
line while the clock line is high are interpreted as
control signals.
Accordingly, the following bus conditions have been
defined:
Bus not busy: Both data and clock lines remain
high.
START data transfer: A change in the state of the
data line from high to low, while the clock line is high,
defines a START condition.
STOP data transfer: A change in the state of the
data line from low to high, while the clock line is high,
defines a STOP condition.
Data valid: The state of the data line represents
valid data when, after a START condition, the data
line is stable for the duration of the high period of the
clock signal. The data on the line must be changed
during the low period of the clock signal. There is
one clock pulse per bit of data.
Each data transfer is initiated with a START condition
and terminated with a STOP condition. The number
of data bytes transferred between the START and
the STOP conditions is not limited, and is determined
by the master device. The information is transferred
byte-wise and each receiver acknowledges with a
ninth bit.
Acknowledge: Each receiving device, when
addressed, is obliged to generate an acknowledge
after the reception of each byte. The master device
must generate an extra clock pulse, which is associ-
ated with this acknowledge bit.
A device that acknowledges must pull down the SDA
line during the acknowledge clock pulse in such a
way that the SDA line is stable low during the high
period of the acknowledge-related clock pulse. Of
course, setup and hold times must be taken into
account. A master must signal an end of data to the
Temperature Register (Upper Byte) (11h)
Temperature Register (Lower Byte) (12h)
