AN1012
Application note
Predicting the battery life and data
retention period of NVRAMs and serial RTCs
Introduction
Standard SRAM devices have the advantage, over EEPROM and Flash memory, of high
write-speed when used as main memory for a processor or microcontroller. Their
disadvantage is that they are volatile, and lose their contents as soon as the power supply is
removed (whether this is for a prolonged period due to being turned off, or due to an
unexpected glitch or loss of the power supply).
STMicroelectronics manufactures a line of non-volatile SRAMs (NVRAMs), known as
ZEROPOWER
®
or TIMEKEEPER
®
NVRAMs, supervisors, and serial RTCs which offer the
best of both worlds: memory devices that are non-volatile like EEPROM, yet have the fast
access of SRAM. These devices consist of an array of low-power CMOS SRAM, plus a
small long-life lithium power cell (along with a high-accuracy quartz crystal, in the case of
the TIMEKEEPER). While the external power supply is within its specified limits, the
memory behaves as standard SRAM; but as soon as the external power supply strays out of
tolerance, the SRAM becomes write-protected, and its contents are preserved by a small
trickle current supplied by the internal power cell.
Unlike EEPROM, where the data contents are guaranteed to be preserved for 10 years (and
typically last for much longer), the contents of NVRAM will only be retained while the internal
cell is able to supply sufficient current to maintain the array. This document summarizes the
factors involved in predicting the battery life, and consequently data retention under various
operating conditions.
Many of the ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC devices are
packaged in a 600 mil DIP CAPHAT™, a hybrid DIP, or a 330 mil SOIC SNAPHAT
®
. The
SNAPHAT (shown in
Figure 1)
has a removable top that includes both the long-life lithium
cell and, in the case of the TIMEKEEPER, a high-accuracy crystal.
STMicroelectronics has shipped several million SNAPHATs that have been used in a broad
range of applications. From PC-based systems to high-end workstations,
telecommunications, consumer, and automotive applications, these products have provided
highly reliable data storage for the electronics industry.
Figure 1.
Standard ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC
packages
CAPHAT™
SOIC and SNAPHAT
®
Top
Hybrid DIP
September 2011
Doc ID 6395 Rev 4
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Contents
AN1012
Contents
1
2
3
Process technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Battery technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Battery backup current - predicting data retention time . . . . . . . . . . . . 8
3.1
3.2
3.3
3.4
Storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Calculating storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Calculating capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
5
4T cell devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
TIMEKEEPER products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.1
5.2
TIMEKEEPER
®
register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
TIMEKEEPER
®
evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2.1
5.2.2
5.2.3
5.2.4
M48T02 and M48T12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
M48T08 and M48T18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
M48T58 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
M48T35 and M48T37V/Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6
7
8
9
10
Supervisor products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Choosing SRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Industrial temperature devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
U.L. recognition and recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Appendix A Product data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix B ZEROPOWER products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Appendix C TIMEKEEPER
®
products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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AN1012
Contents
Appendix D Serial RTC products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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List of tables
AN1012
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
ZEROPOWER and TIMEKEEPER
®
product categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Typical TIMEKEEPER (M48T37V/Y) register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Typical I
BAT
current for TIMEKEEPER devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
SNAPHAT part numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
M40Z300W (120mAh SNAPHAT) data retention life vs. SRAM type . . . . . . . . . . . . . . . . . 20
M48T201V/Y (120 mAh SNAPHAT) data retention life vs. SRAM type . . . . . . . . . . . . . . . 21
Data for ZEROPOWER
®
and TIMEKEEPER
®
devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Data from hybrid/module devices (V
CC
duty cycle = 0%) . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Data from M48Z02/12 devices (available only in CAPHAT™ - BR1225,
48 mAh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Data from M48Z08/18, M48Z58, and M48Z58Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Data from M48Z35/Y/AV devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Data from M48T02/12 devices (available only in CAPHAT™ - BR1632,
120 mAh) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Data from M48T08/Y/18 and M48T58/Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Data from M48T35/Y/AV and M48T37V/Y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Data from M41T56/94, M41ST85W, M41ST87W/Y, and M41ST95W ind. temp. (MH6)
devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Data from M41T00/S, M41T11, and M41T81/S industrial temperature (MH6) devices . . . 31
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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AN1012
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Standard ZEROPOWER, TIMEKEEPER, supervisor, and serial RTC packages. . . . . . . . . 1
Four-transistor (4T) SRAM cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
(A) BR1225 discharge rate and (B) BR1632 discharge rate. . . . . . . . . . . . . . . . . . . . . . . . . 7
Predicted battery storage life versus temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Block diagram of a TIMEKEEPER
®
device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
M48T02/12 data retention lifetime vs. temperature (120 mAh, 100% battery backup). . . . 14
M48T08/18 data retention lifetime vs. temperature (120 mAh, 100% battery backup). . . . 15
M48T58 data retention lifetime vs. temperature (48 mAh, 100% battery backup) . . . . . . . 16
M48T58 data retention lifetime vs. temperature (120 mAh, 100% battery backup) . . . . . . 16
M48T35/37V/37Y data retention lifetime vs. temperature (48 mAh, 100% battery backup) 17
M48T35/37V/37Y data retention lifetime vs. temperature (120 mAh, 100% battery
backup) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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