M48T37 ST Microelectronics, M48T37 Datasheet - Page 15

• Manufacturer Part Number: M48T37
• Description: 3.3V-5V 256 Kbit 32Kb x8 TIMEKEEPER SRAM
• Manufacturer: ST Microelectronics

Table 13. Default Values
Note: 1. WDS, BMB0-BMB4, RBO, RB1.
Figure 13. Supply Voltage Protection
BATTERY LOW FLAG
The M48T37Y/37V automatically performs period-
ic battery voltage monitoring upon power-up. The
Battery Low Flag (BL), Bit D4 of Flags Register
7FF0h, will be asserted high if the SNAPHAT bat-
tery is found to be less than approximately 2.5V.
The BL flag will remain active until completion of
battery replacement and subsequent battery low
monitoring tests, during the next power-up se-
quence.
If a battery low is generated during a power-up se-
quence, this indicates that the battery voltage is
below 2.5V (approximately), which may be insuffi-
cient to maintain data integrity. Data should be
considered suspect and verified as correct. A fresh
battery should be installed.
Note: Battery monitoring is a useful technique only
when performed periodically. The M48T37Y/37V
only monitors the battery when a nominal V
Initial Power-up
(Battery Attach for SNAPHAT)
Subsequent Power-up / RESET
Power-down
2. State of other control bits undefined.
3. State of other control bits remains unchanged.
4. Assuming these bits set to ’1’ prior to power-down.
V CC
0.1 F
(4)
Conditio n
(2)
(3)
V CC
V SS
DEVICE
AI02169
W
0
0
0
CC
is
R
0
0
0
applied to the device. Thus applications which re-
quire extensive durations in the battery back-up
mode should be powered-up periodically (at least
once every few months) in order for this technique
to be beneficial. Additionally, if a battery low is in-
dicated, data integrity should be verified upon
power-up via a checksum or other technique.
POWER-ON DEFAULTS
Upon application of power to the device, the fol-
lowing register bits are set to a ’0’ state: WDS;
BMB0-BMB4; RB0-RB1; AFE; ABE; W; R; FT.
(See Table 13).
POWER SUPPLY DECOUPLING and
UNDERSHOOT PROTECTION
I
switching, can produce voltage fluctuations, re-
sulting in spikes on the V
can be reduced if capacitors are used to store en-
ergy, which stabilizes the V
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 F (as shown in Figure
13) is recommended in order to provide the need-
ed filtering.
In addition to transients that are caused by normal
SRAM operation, power cycling can generate neg-
ative voltage spikes on V
below V
tive spikes can cause data corruption in the SRAM
while in battery backup mode. To protect from
these voltage spikes, it is recommended to con-
nect a schottky diode from V
connected to V
1N5817 is recommended for through hole and
MBRS120T3 is recommended for surface mount.
CC
transients, including those produced by output
FT
0
0
0
SS
by as much as one Volt. These nega-
CC
AFE
0
0
1
, anode to V
CC
CC
ABE
M48T37Y, M48T37V
0
0
1
bus. These transients
that drive it to values
CC
CC
SS
bus. The energy
). Schottky diode
to V
WATCHDOG
Register
SS
0
0
0
(cathode
(1)
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