FM25640B
64Kb Serial 5V F-RAM Memory
Features
64K bit Ferroelectric Nonvolatile RAM
Organized as 8,192 x 8 bits
High Endurance 1 Trillion (10
12
) Read/Writes
38 Year Data Retention
NoDelay™ Writes
Advanced high-reliability ferroelectric process
Very Fast Serial Peripheral Interface - SPI
Up to 20 MHz maximum bus frequency
Direct hardware replacement for EEPROM
SPI Mode 0 & 3 (CPOL, CPHA=0,0 & 1,1)
Sophisticated Write Protection Scheme
Hardware Protection
Software Protection
Low Power Consumption
250
A
Active Current (1 MHz)
4
A
(typ.) Standby Current
Industry Standard Configuration
Industrial Temperature -40C to +85C
8-pin “Green”/RoHS SOIC (-G)
Description
The FM25640B is a 64-kilobit nonvolatile memory
employing an advanced ferroelectric process. A
ferroelectric random access memory or F-RAM is
nonvolatile but operates in other respects as a RAM.
It provides reliable data retention for 38 years while
eliminating the complexities, overhead, and system
level reliability problems caused by EEPROM and
other nonvolatile memories.
The FM25640B performs write operations at bus
speed. No write delays are incurred. Data is written to
the memory array immediately after it has been
successfully transferred to the device. The next bus
cycle may commence immediately without the need
for data polling. The FM25640B is capable of
supporting up to 10
12
read/write cycles, or a million
times more write cycles than EEPROM.
These capabilities make the FM25640B ideal for
nonvolatile memory applications requiring frequent
or rapid writes. Examples range from data collection,
where the number of write cycles may be critical, to
demanding industrial controls where the long write
time of EEPROM can cause data loss.
The FM25640B provides substantial benefits to users
of serial EEPROM, in a hardware drop-in
replacement. The FM25640B uses the high-speed SPI
bus, which enhances the high-speed write capability
of F-RAM technology. The specifications are
guaranteed over an industrial temperature range of
-40°C to +85°C.
Pin Configuration
CS
SO
WP
VSS
1
2
3
4
8
7
6
5
VDD
HOLD
SCK
SI
Pin Names
/CS
/HOLD
/WP
SCK
SI
SO
VDD
VSS
Function
Chip Select
Hold
Write Protect
Serial Clock
Serial Data Input
Serial Data Output
5V
Ground
Ordering Information
FM25640B-G
FM25640B-GTR
“Green”/RoHS 8-pin SOIC
“Green”/RoHS 8-pin SOIC,
Tape & Reel
This product conforms to specifications per the terms of the Ramtron
standard warranty. The product has completed Ramtron’s internal
qualification testing and has reached production status.
Rev. 3.0
Jan. 2012
Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 545-FRAM, (719) 481-7000
www.ramtron.com
Page 1 of 13
FM25640B - 64Kb 5V SPI F-RAM
WP
CS
HOLD
SCK
Instruction Decode
Clock Generator
Control Logic
Write Protect
1,024 x 64
FRAM Array
Instruction Register
Address Register
Counter
13
8
Data I/O Register
3
Nonvolatile Status
Register
SO
Figure 1. Block Diagram
Pin Description
Pin Name
/CS
I/O
Input
Pin Description
Chip Select: Enables and disables the device. When /CS is high, the output pin SO is hi-
Z, all other inputs are ignored, and the device remains in a low-power standby mode.
When /CS is low, the part will respond to the SCK signal. A falling edge on /CS must
occur for every op-code.
Serial Clock: All I/O activity is synchronized to the serial clock. Inputs are latched on the
rising edge and outputs occur on the falling edge. The device is static so the clock
frequency may be any value between 0 and 20 MHz and may be interrupted at any time.
Hold: The /HOLD signal is used when the host CPU must interrupt a memory operation
for another task. Asserting the /HOLD signal low pauses the current operation. The
device ignores SCK and /CS. All transitions on /HOLD must occur while SCK is low.
Write Protect: This pin prevents write operations to the status register. This is critical
since other write protection features are controlled through the status register. A
complete explanation of write protection is provided below. *Note that the function of
/WP is different from the FM25040 where it prevents all writes to the part.
Serial Input: SI is the data input pin. It is sampled on the rising edge of SCK and is
ignored otherwise. It should always be driven to a valid logic level to meet IDD
specifications.
* SI may be connected to SO for a single pin data interface.
Serial Output: SO is the data output pin. It is driven during read cycles and remains hi-Z
at all other times including when HOLD\ is low. Data transitions are driven on the falling
edge of the serial clock.
* SO can be connected to SI for a single pin data interface since the part communicates
in half-duplex.
Supply Voltage: 5V
Ground
SCK
Input
/HOLD
Input
/WP
Input
SI
Input
SO
Output
VDD
VSS
Supply
Supply
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Jan. 2012
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FM25640B - 64Kb 5V SPI F-RAM
Overview
The FM25640B is a serial F-RAM memory. The
memory array is logically organized as 8,192 x 8 and
is accessed using an industry standard Serial
Peripheral Interface or SPI bus. Functional operation
of the F-RAM is similar to serial EEPROMs. The
major difference between the FM25640B and a serial
EEPROM with the same pinout relates to its superior
write performance.
Serial Peripheral Interface – SPI Bus
The FM25640B employs a Serial Peripheral Interface
(SPI) bus. It is specified to operate at speeds up to 20
MHz. This high-speed serial bus provides high
performance serial communication to a host
microcontroller. Many common microcontrollers
have hardware SPI ports allowing a direct interface.
It is quite simple to emulate the port using ordinary
port pins for microcontrollers that do not. The
FM25640B operates in SPI Mode 0 and 3.
The SPI interface uses a total of four pins: clock,
data-in, data-out, and chip select. It is possible to
connect the two data lines together. Figure 2
illustrates a typical system configuration using the
FM25640B with a microcontroller that offers an SPI
port. Figure 3 shows a similar configuration for a
microcontroller that has no hardware support for the
SPI bus.
Protocol Overview
The SPI interface is a synchronous serial interface
using clock and data lines. It is intended to support
multiple devices on the bus. Each device is activated
using a chip select. Once chip select is activated by
the bus master, the FM25640B will begin monitoring
the clock and data lines. The relationship between the
falling edge of /CS, the clock, and data is dictated by
the SPI mode. The device will make a determination
of the SPI mode on the falling edge of each chip
select. While there are four such modes, the
FM25640B supports modes 0 and 3. Figure 4 shows
the required signal relationships for modes 0 and 3.
In both cases, data is clocked into the FM25640B on
the rising edge of SCK and data is expected on the
first rising edge after /CS goes active. If the clock
begins from a high state, it will fall prior to beginning
data transfer in order to create the first rising edge.
The FM25640B is controlled by SPI op-codes. These
op-codes specify the commands to the part. After /CS
is asserted, the first byte transferred from the bus
master is the op-code. Following the op-code,
addresses and data are then transferred. Note that the
WREN and WRDI op-codes are commands with no
subsequent data transfer.
Important: The /CS must go inactive after an
operation is complete and before a new op-code
can be issued. There is one valid op-code only per
active chip select.
Memory Architecture
When accessing the FM25640B, the user addresses
8,192 locations of 8 data bits each. These data bits
are shifted in and out serially. The addresses are
accessed using the SPI protocol, which includes a
chip select (to permit multiple devices on the bus), an
op-code and a two-byte address. The upper 3 bits of
the address range are ignored by the device. The
complete address of 13-bits specifies each byte
address uniquely.
Most functions of the FM25640B either are
controlled by the SPI interface or are handled
automatically by on-board circuitry. The access time
for memory operation essentially is zero, beyond the
time needed for the serial protocol. That is, the
memory is read or written at the speed of the SPI bus.
Unlike an EEPROM, it is not necessary to poll the
device for a ready condition since writes occur at bus
speed. That is, by the time a new bus transaction can
be shifted into the part, a write operation will be
complete. This is explained in more detail in the
interface section.
Users expect several obvious system benefits from
the FM25640B due to its fast write cycle and high
endurance as compared with EEPROM. However
there are less obvious benefits as well. For example
in a high noise environment, the fast-write operation
is less susceptible to corruption than an EEPROM
since it is completed quickly. By contrast, an
EEPROM requiring milliseconds to write is
vulnerable to noise during much of the cycle.
Note that the FM25640B contains no power
management circuits other than a simple internal
power-on reset. It is the user’s responsibility to
ensure that V
DD
is within datasheet tolerances to
prevent incorrect operation. It is recommended
that the part is not powered down with chip
enable active.
Rev. 3.0
Jan. 2012
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FM25640B - 64Kb 5V SPI F-RAM
SCK
MOSI
MISO
SO
SI SCK
SO
SI SCK
SPI
Microcontroller
SS1
SS2
HOLD1
HOLD2
MOSI: Master Out, Slave In
MISO: Master In, Slave Out
SS: Slave Select
FM25640B
CS
HOLD
FM25640B
CS
HOLD
Figure 2. System Configuration with SPI port
Microcontroller
SO
SI SCK
FM25640B
CS
HOLD
Figure 3. System Configuration without SPI port
SPI Mode 0: CPOL=0, CPHA=0
7
6
5
4
3
2
1
0
SPI Mode 3: CPOL=1, CPHA=1
7
6
5
4
3
2
1
0
Figure 4. SPI Modes 0 & 3
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Jan. 2012
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FM25640B - 64Kb 5V SPI F-RAM
Data Transfer
All data transfers to and from the FM25640B occur
in 8-bit groups. They are synchronized to the clock
signal (SCK) and they transfer most significant bit
(MSB) first. Serial inputs are registered on the rising
edge of SCK. The SO output is driven from the
falling edge of SCK.
Command Structure
There are six commands called op-codes that can be
issued by the bus master to the FM25640B. They are
listed in the table below. These op-codes control the
functions performed by the memory. They can be
divided into three categories. First, there are
commands that have no subsequent operations. They
perform a single function such as to enable a write
operation. Second are commands followed by one
byte, either in or out. They operate on the status
register. The third group includes commands for
memory transactions followed by an address and one
or more bytes of data.
Table 1. Op-code Commands
Name
Description
WREN
WRDI
RDSR
WRSR
READ
WRITE
Set Write Enable Latch
Write Disable
Read Status Register
Write Status Register
Read Memory Data
Write Memory Data
WREN - Set Write Enable Latch
The FM25640B will power up with writes disabled.
The WREN command must be issued prior to any
write operation. Sending the WREN op-code will
allow the user to issue subsequent op-codes for
write operations. These include writing the status
register and writing the memory.
Sending the WREN op-code causes the internal
Write Enable Latch to be set. A flag bit in the status
register, called WEL, indicates the state of the latch.
WEL=1 indicates that writes are permitted.
Attempting to write the WEL bit in the status
register has no effect. Completing any write
operation will automatically clear the write-enable
latch and prevent further writes without another
WREN command. Figure 5 illustrates the WREN
command bus configuration.
WRDI - Write Disable
The WRDI command disables all write activity by
clearing the Write Enable Latch. The user can verify
that writes are disabled by reading the WEL bit in
the status register and verifying that WEL=0. Figure
6 illustrates the WRDI command bus configuration.
Op-code value
0000_0110b
0000_0100b
0000_0101b
0000_0001b
0000_0011b
0000_0010b
Figure 5. WREN Bus Configuration
Figure 6. WRDI Bus Configuration
Rev. 3.0
Jan. 2012
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