M41T93 Datasheet, PDF (15/49 Pages) STMicroelectronics – Serial SPI bus RTC with battery switchover

M41T93
Operation
2.3
Data retention and battery switch-over (V_SO = V_RST)
Once V_CC falls below the switchover voltage (V_SO = V_RST), the device automatically
switches over to the battery and powers down into an ultra low current mode of operation to
preserve battery life. If V_BAT is less than, or greater than V_RST, the device power is switched
from V_CC to V_BAT when V_CC drops below V_RST (see Figure 17 on page 39). At this time the
clock registers and user RAM will be maintained by the attached battery supply.
When it is powered back up, the device switches back from battery to V_CC at V_SO +
hysteresis. When V_CC rises above V_RST, it will recognize the inputs. For more information
on Battery Storage Life refer to Application Note AN1012.
2.4
Note:
Power-on reset (t_rec)
The M41T93 continuously monitors V_CC. When V_CC falls to the power fail detect trip point,
the RST output pulls low (open drain) and remains low after power-up for t_rec (210ms
typical) after V_CC rises above V_RST (max).
The t_rec period does not affect the RTC operation. Write protect only occurs when V_CC is
below V_RST. When V_CC rises above V_RST, the RTC will be selectable immediately. Only the
RST output is affected by the t_rec period.
The RST pin is an open drain output and an appropriate pull-up resistor to V_CC should be
chosen to control the rise time.
15/49