Ramtron
cycle. Therefore, any operation including read or write
can occur immediately following a write. Data polling,
a technique used with EEPROMs to determine if a
write is complete, is unnecessary.
Pre-charge Operation
The pre-charge operation is an internal condition
where the state of the memory is prepared for a new
access. All memory cycles consist of a memory
access and a pre-charge. The pre-charge is user
initiated by taking the /CE signal high or inactive. It
must remain high for at least the minimum pre-charge
timing specification.
The user dictates the beginning of this operation
since a pre-charge will not begin until /CE rises.
However the device has a maximum /CE low time
specification that must be satisfied.
FM1608
flexibility, the FM1608 employs a unique memory
organization as described below.
The memory array is divided into 8 blocks, each 1Kx8.
The 3-upper address lines decode the block selection
as shown in Figure 2. Data targeted for significantly
different numbers of cycles should be located in
separate blocks since memory rows do not extend
across block boundaries.
Figure 2. Address Blocks
Endurance and Memory Architecture
Data retention is specified in the electrical
specifications below. This section elaborates on the
relationship between data retention and endurance.
FRAM offers substantially higher write endurance
than other nonvolatile memories. Above a certain
level, however, the effect of increasing memory
accesses on FRAM produces an increase in the soft
error rate. There is a higher likelihood of data loss but
the memory continues to function properly. This
effect becomes significant only after 100 million (1E8)
read/write cycles, far more than allowed by other
nonvolatile memory technologies.
Endurance is a soft specification. Therefore, the user
may operate the device with different levels of cycling
for different portions of the memory. For example,
critical data needing the highest reliability level could
be stored in memory locations that receive
comparatively few cycles. Data with frequent changes
or shorter-term use could be located in an area
receiving many more cycles. A scratchpad area,
needing little if any retention can be cycled virtually
without limit.
Internally, a FRAM operates with a read and restore
mechanism similar to a DRAM. Therefore, each cycle,
be it read or write, involves a change of state. The
memory architecture is based on an array of rows and
columns. Each access causes an endurance cycle for
an entire row. Therefore, data locations targeted for
substantially differing numbers of cycles should not
be located within the same row. To balance the
endurance cycles and allow the user the maximum
Each block of 1Kx8 consists of 256 rows and 4
columns. The address lines A0-A7 decode row
selection and A8-A9 lines decode column selection.
This scheme facilitates a relatively uniform
distribution of cycles across the rows of a block. By
allowing the address LSBs to decode row selection,
the user avoids applying multiple cycles to the same
row when accessing sequential data. For example, 256
bytes can be accessed sequentially without accessing
the same row twice. In this example, one cycle would
be applied to each row. An entire block of 1Kx8 can
be read or written with only four cycles applied to
each row. Figure 3 illustrates the organization within a
memory block.
28 July 2000
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RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
