M41T81S datasheet(9/31 Pages) STMICROELECTRONICS

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M41T81S

Operation

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Operation

The M41T81S 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:

1. Tenths/Hundredths of a Second Register

2. Seconds Register

3. Minutes Register

4. Century/Hours Register

5. Day Register

6. Date Register

7. Month Register

8. Year Register

9. Calibration Register

10. Watchdog Register

11 - 15. Alarm Registers

16. Flags Register

17 - 19. Reserved

20. Square Wave Register

The M41T81S clock continually monitors VCC for an out-of-tolerance condition. Should VCC

fall below VPFD, the device terminates an access in progress and resets the device address

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. Once VCC falls below the

switchover voltage (VSO), the device automatically switches over to the battery and powers

down into an ultra-low current mode of operation to preserve battery life. If VBAT is less than

VPFD, the device power is switched from VCC to VBAT when VCC drops below VBAT. If VBAT is

greater than VPFD, the device power is switched from VCC to VBAT when VCC drops below

VPFD. Upon power-up, the device switches from battery to VCC at VSO. When VCC rises

above VPFD, it will recognize the inputs.

For more information on Battery Storage Life refer to Application Note AN1012.

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:

●

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.

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