Ramtron
FM1608
FRAM Design Considerations
The main design issue is to create a decoder scheme
that will drive /CE active, then inactive for each
address. This accomplishes the two goals of latching
the new address and creating the precharge period.
When designing with FRAM for the first time, users
of SRAM will recognize a few minor differences. First,
bytewide FRAM memories latch each address on the
falling edge of chip enable. This allows the address
bus to change after starting the memory access. Since
every access latches the memory address on the
falling edge of /CE, users should not ground it as they
might with SRAM.
A second design consideration relates to the level of
VDD during operation. Battery-backed SRAMs are
forced to monitor VDD in order to switch to battery
backup. They typically block user access below a
certain VDD level in order to prevent loading the
battery with current demand from an active SRAM.
The user can be abruptly cut off from access to the
nonvolatile memory in a power down situation with
no warning or indication.
Users that are modifying existing designs to use
FRAM should examine the hardware address
decoders. Decoders should be modified to qualify
addresses with an address valid signal if they do not
already. In many cases, this is the only change
required. Systems that drive chip enable active, then
inactive for each valid address may need no
modifications. An example of the target signal
relationships is shown in Figure 4. Also shown is a
common SRAM signal relationship that will not work
for the FM1608.
FRAM memories do not need this system overhead.
The memory will not block access at any VDD level.
The user, however, should prevent the processor
from accessing memory when VDD is out-of-
tolerance. The common design practice of holding a
processor in reset when VDD drops is adequate; no
special provisions must be taken for FRAM design.
Figure 4. Memory Address Relationships
28 July 2000
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RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
