FM1608 datasheet(6/12 Pages) RAMTRON

6 / 12 page

Ramtron

FM1608

28 July 2000

6/12

FRAM Design Considerations

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.

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.

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.

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.

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

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