m40z300av STMicroelectronics, m40z300av Datasheet - Page 7

• Manufacturer Part Number: m40z300av
• Description: 3v Nvram Supervisor For Up To 8 Lpsrams
• Manufacturer: STMicroelectronics

Data Retention Lifetime Calculation
Most low power SRAMs on the market today can
be used with the M40Z300AV NVRAM SUPERVI-
SOR. There are, however some criteria which
should be used in making the final choice of which
SRAM to use. The SRAM must be designed in a
way where the chip enable input disables all other
inputs to the SRAM. This allows inputs to the
M40Z300AV and SRAMs to be “Don't care” once
V
guarantee data retention down to V
chip enable access time must be sufficient to meet
the system needs with the chip enable propaga-
tion delays included. If the SRAM includes a sec-
ond chip enable pin (E2), this pin should be tied to
V
If data retention lifetime is a critical parameter for
the system, it is important to review the data reten-
tion current specifications for the particular
SRAMs being evaluated. Most SRAMs specify a
data retention current at 3.0V. Manufacturers gen-
erally specify a typical condition for room temper-
ature along with a worst case condition (generally
at elevated temperatures). The system level re-
quirements will determine the choice of which val-
ue to use.
The data retention current value of the SRAMs can
then be added to the I
to determine the total current requirements for
data retention. The available battery capacity for
the SNAPHAT
by this current to determine the amount of data re-
tention available (see
CAUTION: Take care to avoid inadvertent dis-
charge through V
battery has been attached.
For a further more detailed review of lifetime calcu-
lations, please see Application Note AN1012.
Power-on Reset Output
All microprocessors have a reset input which forc-
es them to a known state when starting. The
M40Z300AV has a reset output (RST) pin which is
guaranteed to be low within t
ble 7., page
figuration. An appropriate pull-up resistor should
CC
OUT
falls below V
.
13). This signal is an open drain con-
®
of your choice can then be divided
PFD
OUT
(min). The SRAM should also
BAT
Table 13., page
and E1
value of the M40Z300AV
WPT
CON
of V
CC
- E4
PFD
= 2.0V. The
18).
CON
(see
after
Ta-
be chosen to control the rise time. This signal will
be valid for all voltage conditions, even when V
equals V
Once V
age V
t
Battery Low Pin
The M40Z300AV automatically performs battery
voltage monitoring upon power-up, and at factory-
programmed time intervals of at least 24 hours.
The Battery Low (BL) pin will be asserted if the
battery voltage is found to be less than approxi-
mately 2.5V. The BL pin will remain asserted until
completion of battery replacement and subse-
quent 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
2.5V 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 SNAPHAT
should be replaced with valid V
device.
The M40Z300AV 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. The BL pin is an open drain output and
an appropriate pull-up resistor to V
chosen to control the rise time.
REC
to allow the power supply to stabilize.
PFD
CC
SS
, an internal timer keeps RST low for
CC
.
exceeds the power failure detect volt-
is applied to the device. Thus appli-
CC
M40Z300AV
applied to the
CC
should be
CC
®
7/20
top
CC
is