m41st84w STMicroelectronics, m41st84w Datasheet - Page 21

• Manufacturer Part Number: m41st84w
• Description: 3.0/3.3v I 2c Serial Rtc With Supervisory Functions
• Manufacturer: STMicroelectronics

Power-fail INPUT/OUTPUT
The Power-Fail Input (PFI) is compared to an in-
ternal reference voltage (1.25V). If PFI is less than
the power-fail threshold (V
Output (PFO) will go low. This function is intended
for use as an under-voltage detector to signal a
failing power supply. Typically PFI is connected
through an external voltage divider (see
5., page
is available) or the regulated output of the V
ulator. The voltage divider can be set up such that
the voltage at PFI falls below V
onds before the regulated V
M41ST84W or the microprocessor drops below
the minimum operating voltage.
During battery back-up, the power-fail comparator
turns off and PFO goes (or remains) low. This oc-
curs after V
er returns, PFO is forced high, irrespective of V
for the write protect time (t
from V
the end of this time, the power-fail comparator is
enabled and PFO follows PFI. If the comparator is
unused, PFI should be connected to V
left unconnected.
Century Bit
Bits D7 and D6 of Clock Register 03h contain the
CENTURY ENABLE Bit (CEB) and the CENTURY
Bit (CB). Setting CEB to a “1” will cause CB to tog-
gle, either from a “0” to “1” or from “1” to “0” at the
turn of the century (depending upon its initial
state). If CEB is set to a “0”, CB will not toggle.
Output Driver Pin
When the FT Bit, AFE Bit and watchdog register
are not set, the IRQ/FT/OUT pin becomes an out-
put driver that reflects the contents of D7 of the
Control Register. In other words, when D7 (OUT
Bit) and D6 (FT Bit) of address location 08h are a
'0,' then the IRQ/FT/OUT pin will be driven low.
Note: The IRQ/FT/OUT pin is an open drain which
requires an external pull-up resistor.
Battery Low Warning
The M41ST84W automatically performs battery
voltage monitoring upon power-up and at factory-
programmed time intervals of approximately 24
PFD
6) to either the unregulated DC input (if it
(max) until the inputs are recognized. At
CC
drops below V
rec
PFD
PFI
), which is the time
PFI
), the Power-Fail
CC
(min). When pow-
several millisec-
input to the
SS
and PFO
CC
Figure
reg-
PFI
hours. The Battery Low (BL) Bit, Bit D4 of Flags
Register 0Fh, will be asserted if the battery voltage
is found to be less than approximately 2.5V. The
BL Bit will remain asserted until completion of bat-
tery replacement and subsequent battery low
monitoring tests, either during the next power-up
sequence or the next scheduled 24-hour interval.
If a battery low is generated during a power-up se-
quence, this indicates that the battery is below ap-
proximately 2.5 volts and may not be able to
maintain data integrity in the SRAM. Data should
be considered suspect and verified as correct. A
fresh battery should be installed.
If a battery low indication is generated during the
24-hour interval check, this indicates that the bat-
tery is near end of life. However, data is not com-
promised due to the fact that a nominal V
supplied. In order to insure data integrity during
subsequent periods of battery back-up mode, the
battery should be replaced. The battery may be re-
placed while V
The M41ST84W only monitors the battery when a
nominal V
cations which require extensive durations in the
battery back-up mode should be powered-up peri-
odically (at least once every few months) in order
for this technique to be beneficial. Additionally, if a
battery low is indicated, data integrity should be
verified upon power-up via a checksum or other
technique.
t
Bit D7 of Clock Register 04h contains the t
(TR). t
the deselect time after V
lows for a voltage setting time before WRITEs may
again be performed to the device after a power-
down condition. The t
set the length of this deselect time as defined by
Table 7., page
Initial Power-on Defaults
Upon initial application of power to the device, the
following register bits are set to a '0' state: Watch-
dog Register, TR, FT, AFE, ABE, and SQWE. The
following bits are set to a '1' state: ST, OUT, and
HT (see
rec
Bit
rec
Table 8., page
CC
refers to the automatic continuation of
is applied to the device. Thus appli-
CC
22.
is applied to the device.
rec
22).
CC
Bit will allow the user to
reaches V
M41ST84W
PFD
. This al-
rec
CC
21/29
Bit
is