m41t256y STMicroelectronics, m41t256y Datasheet - Page 16

• Manufacturer Part Number: m41t256y
• Description: 256 Kbit 32k X8 Serial Rtc
• Manufacturer: STMicroelectronics

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Sleep mode
In order to minimize the battery current draw while in storage, the M41T256Y provides the
user with a battery “sleep mode,” which disconnects the RAM memory array from the
external Lithium battery normally used to provide non-volatile operation in the absence of
V
not needed.
The sleep mode will remove power from the RAM array only and not affect the data retention
of the TIMEKEEPER Registers (7FF0h through 7FFFh - this includes the Calibration
Register).
The sleep mode (SLP) Bit located in register 7FF8h (D6), must be set to a '1' by the user
while the device is powered by V
disconnect the RAM array from power until the next power-down cycle. This protects the
user from immediate data loss in the event he inadvertently sets the SLP Bit. Once V
below V
from the battery, resulting in both a lower battery current, and a loss of RAM data.
Upon initial battery attach or initial power application without the battery, the state of the SLP
Bit will be undetermined. Therefore, the SLP Bit should be initialized to '0' by the user.
Additional current reduction can be achieved by setting the STOP (ST) Bit in register 7FF9h
(D7), turning off the clock oscillator. This combination will result in the longest possible
battery life, but also loss of time and data. When the device is again powered-up, the user
should first read the SLP Bit to determine if the device is currently in sleep mode, then reset
the bit to '0' in order to disable the sleep mode (this will NOT be automatically taken care of
during the power-up).
See AN1570, “M41T256Y Sleep Mode Function” for more information on sleep mode and
battery lifetimes.
CC
. This can significantly extend the lifetime of the battery, when non-volatile operation is
SO
(V
BAT
), the sleep mode circuit will be engaged and the RAM array will be isolated
CC
. This will “arm” the sleep mode latch, but not actually
CC
falls