FM1808
256Kb Bytewide FRAM Memory
Features
256Kbit Ferroelectric Nonvolatile RAM
•
Organized as 32,768 x 8 bits
•
High Endurance 10 Billion (10
10
) Read/Writes
•
10 year Data Retention
•
NoDelay™ Writes
•
Advanced High-Reliability Ferroelectric Process
Superior to BBSRAM Modules
•
No Battery Concerns
•
Monolithic Reliability
•
True Surface Mount Solution, No Rework Steps
•
Superior for Moisture, Shock, and Vibration
•
Resistant to Negative Voltage Undershoots
SRAM & EEPROM Compatible
•
JEDEC 32Kx8 SRAM & EEPROM pinout
•
70 ns Access Time
•
130 ns Cycle Time
Low Power Operation
•
25 mA Active Current
•
20
µA
Standby Current
Industry Standard Configuration
•
Industrial Temperature -40° C to +85° C
•
28-pin SOIC or DIP
Description
The FM1808 is a 256-kilobit nonvolatile memory
employing an advanced ferroelectric process. A
ferroelectric random access memory or FRAM is
nonvolatile but operates in other respects as a RAM.
It provides data retention for 10 years while
eliminating the reliability concerns, functional
disadvantages and system design complexities of
battery-backed SRAM (BBSRAM). Fast write timing
and high write endurance make FRAM superior to
other types of nonvolatile memory.
In-system operation of the FM1808 is very similar to
other RAM devices. Minimum read- and write-cycle
times are equal. The FRAM memory, however, is
nonvolatile due to its unique ferroelectric memory
process. Unlike BBSRAM, the FM1808 is a truly
monolithic nonvolatile memory. It provides the same
functional benefits of a fast write without the
disadvantages associated with modules and batteries
or hybrid memory solutions.
These capabilities make the FM1808 ideal for
nonvolatile memory applications requiring frequent
or rapid writes in a bytewide environment. The
availability of a true surface-mount package improves
the manufacturability of new designs, while the DIP
package facilitates simple design retrofits. Device
specifications are guaranteed over an industrial
temperature range of -40°C to +85°C.
Pin Configuration
A14
A12
A7
A6
A5
A4
A3
A2
A1
A0
DQ0
DQ1
DQ2
VSS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
VDD
WE
A13
A8
A9
A11
OE
A10
CE
DQ7
DQ6
DQ5
DQ4
DQ3
Ordering Information
FM1808-70-P
70 ns access, 28-pin plastic DIP
FM1808-70-S
70 ns access, 28-pin SOIC
FM1808-120-P 120 ns access, 28-pin plastic DIP
FM1808-120-S 120 ns access, 28-pin SOIC
This product conforms to specifications per the terms of the Ramtron
standard warranty. Production processing does not necessarily in-
clude testing of all parameters.
Rev. 2.0
Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 545-FRAM, (719) 481-7000, Fax (719) 481-7058
www.ramtron.com
Jan. 2002
Page 1 of 12
FM1808
A10-A14
Block Decoder
A0-A14
Address
Latch
A0-A7
Row
Decoder
32,768 x 8 FRAM Array
CE
A8-A9
Column Decoder
WE
OE
Control
Logic
I/O Latch
Bus Driver
DQ0-7
Figure 1. Block Diagram
Pin Description
Pin Name
A0-A14
DQ0-7
/CE
Type
Input
I/O
Input
/OE
/WE
VDD
VSS
Input
Input
Supply
Supply
Description
Address: The 15 address lines select one of 32,768 bytes in the FRAM array. The
address value is latched on the falling edge of /CE.
Data: 8-bit bi-directional data bus for accessing the FRAM array.
Chip Enable: /CE selects the device when low. Asserting /CE low causes the
address to be latched internally. Address changes that occur after /CE goes low
will be ignored until the next falling edge occurs.
Output Enable: Asserting /OE low causes the FM1808 to drive the data bus when
valid data is available. Deasserting /OE high causes the DQ pins to be tri-stated.
Write Enable: Asserting /WE low causes the FM1808 to write the contents of the
data bus to the address location latched by the falling edge of /CE.
Supply Voltage: 5V
Ground
Functional Truth Table
/CE
/WE
H
X
X
↓
L
H
L
↓
Function
Standby/Precharge
Latch Address (and Begin Write if /WE=low)
Read
Write
Note: The /OE pin controls only the DQ output buffers.
Rev. 2.0
Jan. 2002
Page 2 of 12
FM1808
Overview
The FM1808 is a bytewide FRAM memory. The
memory array is logically organized as 32,768 x 8
and is accessed using an industry standard parallel
interface. All data written to the part is immediately
nonvolatile with no delay. Functional operation of the
FRAM memory is the same as SRAM type devices,
except the FM1808 requires a falling edge of /CE to
start each memory cycle.
Read Operation
A read operation begins on the falling edge of /CE.
At this time, the address bits are latched and a
memory cycle is initiated. Once started, a full
memory cycle must be completed internally even if
/CE goes inactive. Data becomes available on the bus
after the access time has been satisfied.
After the address has been latched, the address value
may be changed 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 FM1808 will drive the data bus when /OE is
asserted low. 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 caused by invalid data being driven onto
the bus. When /OE is inactive the data bus will
remain tri-stated.
Write Operation
Writes occur in the FM1808 in the same time interval
as reads. The FM1808 supports both /CE- and /WE-
controlled write cycles. In all cases, the address is
latched on the falling edge of /CE.
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 FM1808 will not drive
the data bus regardless of the state of /OE.
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.
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.
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
Page 3 of 12
Memory Architecture
Users access 32,768 memory locations each with 8
data bits through a parallel interface. The complete
15-bit address specifies each of the 32,768 bytes
uniquely. Internally, the memory array is organized
into 32 blocks of 8Kb each. The 5 most-significant
address lines decode one of 32 blocks. This block
segmentation has no effect on operation, however the
user may wish to group data into blocks by its
endurance characteristics as explained on page 4.
The cycle time is the same for read and write memory
operations. This simplifies memory controller logic
and timing circuits. Likewise the access time is the
same for read and write memory operations. When
/CE is deasserted high, a precharge operation begins,
and is required of every memory cycle. Thus unlike
SRAM, the access and cycle times are not equal.
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.
Note that the FM1808 has no special power-down
requirements. 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 remains within
datasheet tolerances to prevent incorrect operation.
Also proper voltage level and timing relationships
between VDD and /CE must be maintained in power-
up and power-down events.
Memory Operation
The FM1808 is designed to operate in a manner
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.
Rev. 2.0
Jan. 2002
FM1808
the same, the user experiences no delay through the
bus. The entire memory operation occurs in a single
bus 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.
Precharge Operation
The precharge operation is an internal condition that
prepares the memory for a new access. All memory
cycles consist of a memory access and a precharge.
The precharge is initiated by deasserting the /CE pin
high. It must remain high for at least the minimum
precharge time t
PC
.
The user determines the beginning of this operation
since a precharge will not begin until /CE rises.
However, the device has a maximum /CE low time
specification that must be satisfied.
eliminates
these
weaknesses.
The
considerations in upgrading to FRAM follow:
major
Endurance
Internally, a FRAM operates with a read and restore
mechanism. Therefore, each read and write cycle
involves a change of state. The memory architecture
is based on an array of rows and columns. Each read
or write access causes an endurance cycle for an
entire row. In the FM1808, a row is 32 bits wide.
Every 4-byte boundary marks the beginning of a new
row. Endurance can be optimized by ensuring
frequently accessed data is located in different rows.
Regardless, FRAM offers substantially higher write
endurance than other nonvolatile memories. The
rated endurance limit of 10
10
cycles will allow 30
accesses per second to the same row for over 10
years.
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, but under no bias and no pressure.
These conditions are chosen because multi-
component assemblies are vulnerable to moisture and
dirt. FRAM is qualified using HAST – highly
accelerated stress test. This requires 120º C at 85%
Rh, 24.4 psia at 5.5V.
3. System reliability
Data integrity must be questioned 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, height, and dictates through-
hole assembly. FRAM offers a true surface-mount
solution that uses 25% of the board space.
Applications
As the first truly nonvolatile RAM, the FM1808 fits
into many diverse applications. Clearly, its monolithic
nature and high performance make it superior to
battery-backed SRAM in many applications. 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
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 many 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
Rev. 2.0
Direct Battery Issues
5. Field maintenance
No matter how mature batteries become, they are a
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.
Page 4 of 12
Jan. 2002
FM1808
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.
Rev. 2.0
Jan. 2002
Page 5 of 12
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