Data Sheet

September 2011 Doc ID 6395 Rev 4 1/33
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
SOIC and SNAPHAT
®
To p
CAPHAT™
Hybrid DIP
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