Operation
M41T81
2
Operation
The M41T81 clock operates as a slave device on the serial bus. Access is obtained by
implementing a start condition followed by the correct slave address (D0h). The 20 bytes
contained in the device can then be accessed sequentially in the following order:
st
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1 Byte: tenths/hundredths of a second register
nd
2
3
Byte: seconds register
Byte: minutes register
rd
th
4 Byte: century/hours register
th
5 Byte: day register
th
6 Byte: date register
th
7 Byte: month register
th
8 Byte: year register
th
9 Byte: control register
th
10 Byte: watchdog register
th
th
11 - 16 Bytes: alarm registers
th
th
17 - 19 Bytes: reserved
th
20 Byte: square wave register
The M41T81 clock continually monitors V for an out-of-tolerance condition. Should V
CC
CC
fall below V , the device terminates an access in progress and resets the device address
SO
counter. Inputs to the device will not be recognized at this time to prevent erroneous data
from being written to the device from a an out-of-tolerance system. The device also
automatically switches over to the battery and powers down into an ultra low current mode
of operation to conserve battery life. As system power returns and V rises above V , the
CC
SO
battery is disconnected, and the power supply is switched to external V
.
CC
For more information on Battery Storage Life refer to Application Note AN1012.
2.1
2-wire bus characteristics
The bus is intended for communication between different ICs. It consists of two lines: a bi-
directional data signal (SDA) and a clock signal (SCL). Both the SDA and SCL lines must be
connected to a positive supply voltage via a pull-up resistor.
The following protocol has been defined:
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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, will be interpreted as control
signals.
Accordingly, the following bus conditions have been defined:
2.1.1
Bus not busy
Both data and clock lines remain High.
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STMICROELECTRONICS [ ST ]
