Rainbow Electronics DS1558Y - Manual (Page 6)

DS1558
6 of 18
BATTERY LONGIVITY
The battery lifetime is dependent on the RAM battery standby current and the DS1558 internal clock
oscillator current. The total battery current is I
OSC
+ I
CCO
. When V
CC
is above V
PF
, I
BAT
current is less than
50nA. The DS1558 has an internal circuit to prevent battery charging. No external protection components
are required, and none should be used. The DS1558 has two battery pins that operate independently; the
DS1558 selects the higher of the two inputs. If only one battery is used, the battery should be attached to
V
BAT1
, and V
BAT2
should be grounded.
INTERNAL BATTERY MONITOR
The DS1558 constantly monitors the battery voltage of the internal battery. The battery-low flag (BLF)
bit of the flags register (B4 of 7FFF0h) is not writable and should always be a 0 when read. If a 1 is ever
present, both battery inputs are below 1.8V and both the contents of the RTC and RAM are questionable.
POWER-ON RESET
A temperature-compensated comparator circuit monitors the level of V
CC
. When V
CC
falls to the power-
fail trip point, the RST signal (open drain) is pulled low. When V
CC
returns to nominal levels, the RST
signal continues to be pulled low for a period of 40ms to 200ms. The power-on reset function is
independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.