Rainbow Electronics DS28DG02 - Manual (Page 16)

DS28DG02: 2kb SPI EEPROM with PIO, RTC, Reset, Battery Monitor, and Watchdog
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In contrast to the V
CC
monitor, the battery monitor is active only for two seconds per hour, and only if it is enabled
through the BME bit in the Multifunction Control/Setup register. In addition to this, the DS28DG02 must have
sufficient V
CC
power and the RTC must be running (OSCE = 1). The battery test takes place a) immediately after
enabling the battery monitor, and, if the battery monitor is enabled, b) every hour on the full hour, and c)
immediately after V
CC
ramps up above V
POR
. Figure 8 shows the details.
The battery test procedure begins with the DS28DG02 internally connecting the test load to the V
BAT
pin. If the
battery is near the end of its lifetime, this extra load causes the battery voltage to fall below V
BTP
, the Battery Trip
Point. After the stabilization window is over, the actual comparison of the battery voltage to the battery trip point
takes place. If at the beginning of or during the battery test window the battery voltage falls below V
BTP
, the battery
alarm flag BATA in the Alarm and Status register is set, which in turn activates the ALMZ output. The BATA flag is
cleared by a) replacing the battery, or b) by writing to the Alarm and Status register. The BATA flag is not cleared if
a subsequent battery test, e.g., one hour later or after power-cycling the DS28DG02, determines that the battery
voltage is above V
BTP
. Note that replacing the battery resets the RTC and clears the Multifunction Control/Setup
register.
Battery monitoring is only useful when performed regularly. Equipment that is powered-down for excessively long
periods can completely drain its battery without receiving any advanced warning. To prevent such an occurrence,
equipment using the battery-monitoring feature should be switched on periodically, e.g., once a month, to perform a
battery test.
Figure 8. Battery Monitor Operation
V
BAT
V
BTP
0V
ALMZ
Test
Load On
off Stabilization
Window
Battery Test
Window
1s 1s
BATA