Ramtron
FM1608
Overview
The FM1608 is a bytewide FRAM memory. The
memory array is logically organized as 8,192 x 8 and is
accessed using an industry standard parallel
interface. The FM1608 is inherently nonvolatile via its
unique ferroelectric process. All data written to the
part is immediately nonvolatile with no delay.
Functional operation of the FRAM memory is similar
to SRAM type devices. The major operating
difference between the FM1608 and an SRAM
(beside nonvolatile storage) is that the FM1608
latches the address on the falling edge of /CE.
cycle is initiated. Once started, a complete memory
cycle must be completed internally regardless of the
state of /CE. Data becomes available on the bus after
the access time has been satisfied.
After the address has been latched, the address value
may change upon satisfying the hold time parameter.
Unlike an SRAM, changing address values will have
no effect on the memory operation after the address is
latched.
The FM1608 will drive the data bus when /OE is
asserted to a low state. If /OE is asserted after the
memory access time has been satisfied, the data bus
will be driven with valid data. If /OE is asserted prior
to completion of the memory access, the data bus will
not be driven until valid data is available. This feature
minimizes supply current in the system by eliminating
transients due to invalid data. When /OE is inactive
the data bus will remain tri-stated.
Memory Architecture
Users access 8,192 memory locations each with 8 data
bits through a parallel interface. The complete address
of 13-bits specifies each of the 8,192 bytes uniquely.
Internally, the memory array is organized into 8 blocks
of 1Kb each. The 3 most-significant address lines
decode one of 8 blocks. This block segmentation has
no effect on operation, however the user may wish to
group data into blocks by its endurance requirements
as explained in a later section.
Write Operation
Writes occur in the FM1608 in the same time interval
as reads. The FM1608 supports both /CE and /WE
controlled write cycles. In all cases, the address is
latched on the falling edge of /CE.
The access and cycle time are the same for read and
write memory operations. Writes occur immediately at
the end of the access with no delay. Unlike an
EEPROM, it is not necessary to poll the device for a
ready condition since writes occur at bus speed. A
pre-charge operation, where /CE goes inactive, is a
part of every memory cycle. Thus unlike SRAM, the
FM1608 access and cycle times are not equal.
In a /CE controlled write, the /WE signal is asserted
prior to beginning the memory cycle. That is, /WE is
low when /CE falls. In this case, the part begins the
memory cycle as a write. The FM1608 will not drive
the data bus regardless of the state of /OE.
Note that the FM1608 has no special power-down
demands. It will not block user access and it contains
no power-management circuits other than a simple
internal power-on reset. It is the user’s responsibility
to ensure that VDD is within data sheet tolerances to
prevent incorrect operation.
In a /WE controlled write, the memory cycle begins on
the falling edge of /CE. The /WE signal falls after the
falling edge of /CE. Therefore the memory cycle
begins as a read. The data bus will be driven
according to the state of /OE until /WE falls. The
timing of both /CE and /WE controlled write cycles is
shown in the electrical specifications.
Memory Operation
Write access to the array begins asynchronously
after the memory cycle is initiated. The write access
terminates on the rising edge of /WE or /CE,
whichever is first. Data set-up time, as shown in the
electrical specifications, indicates the interval during
which data cannot change prior to the end of the write
access.
The FM1608 is designed to operate in a manner very
similar to other bytewide memory products. For users
familiar with BBSRAM, the performance is comparable
but the bytewide interface operates in a slightly
different manner as described below. For users
familiar with EEPROM, the obvious differences result
from the higher write performance of FRAM
technology including NoDelay writes and much
higher write endurance.
Unlike other truly nonvolatile memory technologies,
there is no write delay with FRAM. Since the read and
write access times of the underlying memory are the
same, the user experiences no delay through the bus.
The entire memory operation occurs in a single bus
Read Operation
A read operation begins on the falling edge of /CE. At
this time, the address bits are latched and a memory
28 July 2000
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