Ramtron
Figure 3. Row and Column Organization
FM1608
qualified using HAST – highly accelerated stress test.
This requires 120º C at 85% Rh, 24.4 psia at 5.5V bias.
3. System reliability
Data integrity must be in question when using a
battery-backed SRAM. They are inherently
vulnerable to shock and vibration. If the battery
contact comes loose, data will be lost. In addition a
negative voltage, even a momentary undershoot, on
any pin of a battery-backed SRAM can cause data
loss. The negative voltage causes current to be drawn
directly from the battery. These momentary short
circuits can greatly weaken a battery and reduce its
capacity over time. In general, there is no way to
monitor the lost battery capacity. Should an
undershoot occur in a battery backed system during a
power down, data can be lost immediately.
4. Space
Certain disadvantages of battery-backed, such as
susceptibility to shock, can be reduced by using the
old fashioned DIP module. However, this alternative
takes up board space, add height, and dictates
through-hole assembly. FRAM offers a true surface-
mount solution that uses 25% of the board space.
No multi-piece assemblies no c
onnectors, and no
modules. A real nonvolatile RAM is finally
available!
Direct Battery Issues
5. Field maintenance
Batteries, no matter how mature, are a built-in
maintenance problem. They eventually must be
replaced. Despite long life projections, it is impossible
to know if any individual battery will last considering
all of the factors that can degrade them.
6. Environmental
Lithium batteries are widely regarded as an
environmental problem. They are a potential fire
hazard and proper disposal can be a burden. In
addition, shipping of lithium batteries may be
restricted.
7. Style!
Backing up an SRAM with a battery is an old-
fashioned approach. In many cases, such modules are
the only through-hole component in sight. FRAM is
the latest memory technology and it is changing the
way systems are designed.
FRAM is nonvolatile and writes fast -- no battery
required!
Applications
As the first truly nonvolatile RAM, the FM1608 fits
into many diverse applications. Clearly, its monolithic
nature and high performance make it superior to
battery-backed SRAM in most every application. This
applications guide is intended to facilitate the
transition from BBSRAM to FRAM. It is divided into
two parts. First is a treatment of the advantages of
FRAM memory compared with battery-backed
SRAM. Second is a design guide, which highlights
the simple design considerations that should be
reviewed in both retrofit and new design situations.
FRAM Advantages
Although battery-backed SRAM is a mature and
established solution, it has numerous weaknesses.
These stem, directly or indirectly from the presence of
the battery. FRAM uses an inherently nonvolatile
storage mechanism that requires no battery. It
therefore eliminates these weaknesses. The major
considerations in upgrading to FRAM are as follows.
Construction Issues
1. Cost
The cost of both the component and the
manufacturing overhead of battery-backed SRAM is
high. FRAM, with its monolithic construction is
inherently a lower cost solution. In addition, there is
no ‘built-in’ rework step required for battery
attachment when using surface mount parts.
Therefore assembly is streamlined and more cost
effective. In the case of DIP battery-backed modules,
the user is constrained to through-hole assembly
techniques and a board wash using no water.
2. Humidity
A typical battery-backed SRAM module is qualified at
60º C, 90% Rh, no bias, and no pressure. This is
because the multi-component assemblies are
vulnerable to moisture, not to mention dirt. FRAM is
28 July 2000
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RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
