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M48Z512A, M48Z512AY
Figure 11. Supply Voltage Protection
AI02169
VCC
0.1
μ
F
DEVICE
VCC
VSS
DATA RETENTION MODE
With valid V
CC
applied, the M48Z512A/512AY op-
erates as a conventional BYTEWIDE
static
RAM. Should the supply voltage decay, the RAM
will automatically power-fail deselect, write pro-
tecting itself t
WP
after V
CC
falls below V
PFD
. All
outputs become high impedance, and all inputs
are treated as ”don’t care.”
If power fail detection occurs during a valid ac-
cess, thememory cycle continues to completion. If
the memory cycle fails to terminate within the time
t
WP
, writeprotection takes place. When V
CC
drops
below V
SO
, the control circuit switches power to
the internal energy source which preserves data.
The internal coin cell will maintain data in the
M48Z512A/512AY after the initial application of
V
CC
for an accumulated period of at least 10 years
when V
CC
is less than V
SO
. As system power re-
turns and V
CC
rises above V
SO
, the battery is dis-
connected, and the power supply is switched to
external V
CC
. Writeprotection continues for t
ER
af-
ter V
CC
reaches V
PFD
to allow for processor stabi-
lization. After t
ER
, normal RAM operation can
resume.
For more information on Battery Storage Life refer
to the Application Note AN1012.
POWER SUPPLY DECOUPLING
and UNDERSHOOT PROTECTION
I
CC
transients, including those produced by output
switching, can produce voltage fluctuations, re-
sulting in spikes on the V
CC
bus. These transients
can be reduced if capacitors are used to store en-
ergy, which stabilizes the V
CC
bus. The energy
stored in the bypass capacitors will be released as
low going spikes are generated or energy will be
absorbed when overshoots occur. A ceramic by-
pass capacitor value of 0.1
μ
F (as shown in Figure
11) is recommended in order to provide the need-
ed filtering.
In addition to transients that are caused by normal
SRAM operation, power cycling cangenerate neg-
ative voltage spikes on V
CC
that drive it to values
below V
SS
by as much as one Volt. These nega-
tive spikes can cause data corruptionin the SRAM
while in battery backup mode. To protect from
these voltage spikes, itis recommeded to connect
a schottky diode from V
CC
to V
SS
(cathode con-
nected to V
CC
, anode to V
SS
). Schottky diode
1N5817 is recommended for through hole and
MBRS120T3 is recommended for surface mount.