FM25V20
2Mb Serial 3V F-RAM Memory
Features
2M bit Ferroelectric Nonvolatile RAM
Organized as 256K x 8 bits
High Endurance 100 Trillion (10
14
) Read/Writes
10 Year Data Retention
NoDelay™ Writes
Advanced High-Reliability Ferroelectric Process
Very Fast Serial Peripheral Interface - SPI
Up to 40 MHz Frequency
Direct Hardware Replacement for Serial Flash
SPI Mode 0 & 3 (CPOL, CPHA=0,0 & 1,1)
Write Protection Scheme
Hardware Protection
Software Protection
Device ID
Device ID reads out Manufacturer ID & Part ID
Low Voltage, Low Power
Low Voltage Operation 2.0V – 3.6V
100
A
Standby Current (typ.)
3
A
Sleep Mode Current (typ.)
Industry Standard Configurations
Industrial Temperature -40C to +85C
8-pin “Green”/RoHS EIAJ SOIC Package
8-pin “Green”/RoHS TDFN Package
8-pin “Green”/POHS PDIP Package
Description
The FM25V20 is a 2-megabit nonvolatile memory
employing an advanced ferroelectric process. A
ferroelectric random access memory or F-RAM is
nonvolatile and performs reads and writes like a
RAM. It provides reliable data retention for 10 years
while eliminating the complexities, overhead, and
system level reliability problems caused by Serial
Flash and other nonvolatile memories.
Unlike Serial Flash, the FM25V20 performs write
operations at bus speed. No write delays are incurred.
Data is written to the memory array immediately
after it has been transferred to the device. The next
bus cycle may commence without the need for data
polling. The product offers very high write
endurance, orders of magnitude more endurance than
Serial Flash. Also, F-RAM exhibits lower power
consumption than Serial Flash.
These capabilities make the FM25V20 ideal for
nonvolatile memory applications requiring frequent
or rapid writes or low power operation. Examples
range from data collection, where the number of
write cycles may be critical, to demanding controls
where the long write time of Serial Flash can cause
data loss.
The FM25V20 provides substantial benefits to users
of Serial Flash as a hardware drop-in replacement.
The device uses the high-speed SPI bus, which
enhances the high-speed write capability of F-RAM
technology. The device incorporates a read-only
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.
Device ID that allows the host to determine the
manufacturer, product density, and product revision.
The device is guaranteed over an industrial
temperature range of -40°C to +85°C.
Pin Configuration
Top View
/S
Q
/W
VSS
1
2
3
4
8
7
6
5
VDD
/HOLD
C
D
S
Q
W
VSS
1
2
3
4
8
7
6
5
VDD
HOLD
C
D
Pin Name
/S
/W
/HOLD
C
D
Q
VDD
VSS
Function
Chip Select
Write Protect
Hold
Serial Clock
Serial Data Input
Serial Data Output
Supply Voltage
Ground
Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 545-FRAM, (719) 481-7000
http://www.ramtron.com
Rev. 3.0
August 2012
Page 1 of 17
FM25V20 – 2Mb SPI F-RAM
W
S
HOLD
C
Instruction Decode
Clock Generator
Control Logic
Write Protect
32K x 64
FRAM Array
Instruction Register
Address Register
Counter
D
18
8
Q
Data I/O Register
3
Nonvolatile Status
Register
Figure 1. Block Diagram
Pin Descriptions
Pin Name
/S
I/O
Input
Description
Chip Select: This active-low input activates the device. When high, the device enters
low-power standby mode, ignores other inputs, and all outputs are tri-stated. When
low, the device internally activates the C signal. A falling edge on /S must occur prior
to 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. Since the device is static, the
clock frequency may be any value between 0 and 40 MHz and may be interrupted at
any time.
Hold: The /HOLD pin is used when the host CPU must interrupt a memory operation
for another task. When /HOLD is low, the current operation is suspended. The device
ignores any transition on C or /S. All transitions on /HOLD must occur while C is low.
If it is not used, the /HOLD pin should be tied to V
DD
.
Write Protect: This active-low pin prevents write operations to the Status Register
only. A complete explanation of write protection is provided on pages 6 and 7. If it is
not used, the /W pin should be tied to V
DD
.
Serial Input: All data is input to the device on this pin. The pin is sampled on the
rising edge of C and is ignored at other times. It should always be driven to a valid
logic level to meet I
DD
specifications.
* D may be connected to Q for a single pin data interface.
Serial Output: This is the data output pin. It is driven during a read and remains tri-
stated at all other times including when /HOLD is low. Data transitions are driven on
the falling edge of the serial clock.
* Q may be connected to D for a single pin data interface.
Power Supply
Ground
C
Input
/HOLD
Input
/W
Input
D
Input
Q
Output
VDD
VSS
Supply
Supply
Rev. 3.0
August 2012
Page 2 of 17
FM25V20 – 2Mb SPI F-RAM
Overview
The FM25V20 is a serial F-RAM memory. The
memory array is logically organized as 262,144 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 Flash. The major
differences between the FM25V20 and a Serial Flash
with the same pinout are the F-RAM’s superior write
performance, very high endurance, and lower power
consumption.
Memory Architecture
When accessing the FM25V20, the user addresses
256K locations of 8 data bits each. These data bits are
shifted 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 three-
byte address. The complete address of 18-bits
specifies each byte address uniquely.
Most functions of the FM25V20 either are controlled
by the SPI interface or are handled automatically by
on-board circuitry. The access time for memory
operation is essentially 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 Serial
Flash, it is not necessary to poll the device for a ready
condition since writes occur at bus speed. So, by the
time a new bus transaction can be shifted into the
device, a write operation will be complete. This is
explained in more detail in the interface section.
Users expect several obvious system benefits from
the FM25V20 due to its fast write cycle and high
endurance as compared to Serial Flash. In addition
there are less obvious benefits as well. For example
in a high noise environment, the fast-write operation
is less susceptible to corruption than Serial Flash
since it is completed quickly. By contrast, Serial
Flash requiring milliseconds to write is vulnerable to
noise during much of the cycle.
Protocol Overview
The SPI interface is a synchronous serial interface
using clock and data pins. 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 FM25V20 will begin monitoring
the clock and data lines. The relationship between the
falling edge of /S, 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
FM25V20 supports only modes 0 and 3. Figure 2
shows the required signal relationships for modes 0
and 3. For both modes, data is clocked into the
FM25V20 on the rising edge of C and data is
expected on the first rising edge after /S goes active.
If the clock starts from a high state, it will fall prior to
the first data transfer in order to create the first rising
edge.
The SPI protocol is controlled by op-codes. These
op-codes specify the commands to the device. After
/S is activated the first byte transferred from the bus
master is the op-code. Following the op-code, any
addresses and data are then transferred.
Certain op-codes are commands with no subsequent
data transfer. The /S 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.
SPI Mode 0: CPOL=0, CPHA=0
S
C
D
7
MSB
6
5
4
3
2
1
0
LSB
Serial Peripheral Interface – SPI Bus
The FM25V20 employs a Serial Peripheral Interface
(SPI) bus. It is specified to operate at speeds up to
40MHz. 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
FM25V20 operates in SPI Mode 0 and 3.
SPI Mode 3: CPOL=1, CPHA=1
S
C
D
7
MSB
6
5
4
3
2
1
0
LSB
Figure 2. SPI Modes 0 & 3
Rev. 3.0
August 2012
Page 3 of 17
FM25V20 – 2Mb SPI F-RAM
System Hookup
The SPI interface uses a total of four pins: clock,
data-in, data-out, and chip select. A typical system
configuration uses one or more FM25V20 devices
with a microcontroller that has a dedicated SPI port,
as Figure 3 illustrates. Note that the clock, data-in,
and data-out pins are common among all devices.
The Chip Select and Hold pins must be driven
separately for each FM25V20 device.
For a microcontroller that has no dedicated SPI bus, a
general purpose port may be used. To reduce
hardware resources on the controller, it is possible to
connect the two data pins together and tie off the
Hold pin. Figure 4 shows a configuration that uses
only three pins.
SCK
MOSI
MISO
Q
D
C
Q
D
C
SPI
Microcontroller
SS1
SS2
HOLD1
HOLD2
FM25V20
S
HOLD
FM25V20
S
HOLD
MOSI : Master Out Slave In
MISO : Master In Slave Out
SS : Slave Select
Figure 3. 4Mbit (512KB) System Configuration with SPI port
P1.0
P1.1
Microcontroller
Q
D
C
FM25V20
S
P1.2
HOLD
VDD
Figure 4. System Configuration without SPI port
Rev. 3.0
August 2012
Page 4 of 17
FM25V20 – 2Mb SPI F-RAM
Power Up to First Access
The FM25V20 is not accessible for a period of time
(t
PU
) after power up. Users must comply with the
timing parameter t
PU
, which is the minimum time
from V
DD
(min) to the first /S low.
Data Transfer
All data transfers to and from the FM25V20 occur in
8-bit groups. They are synchronized to the clock
signal (C), and they transfer most significant bit
(MSB) first. Serial inputs are registered on the rising
edge of C. Outputs are driven from the falling edge of
clock C.
Command Structure
There are nine commands called op-codes that can be
issued by the bus master to the FM25V20. 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 address and one or
more bytes of data.
Table 1. Op-code Commands
Name
Description
Set Write Enable Latch
WREN
Write Disable
WRDI
Read Status Register
RDSR
Write Status Register
WRSR
Read Memory Data
READ
FSTRD
Fast Read Memory Data
WRITE
Write Memory Data
Enter Sleep Mode
SLEEP
Read Device ID
RDID
without another WREN command. Figure 5 below
illustrates the WREN command bus configuration.
S
0
C
1
2
3
4
5
6
7
D
Q
0
0
0
0
0
Hi-Z
1
1
0
Figure 5. WREN 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.
S
0
C
1
2
3
4
5
6
7
Op-code
0000
0000
0000
0000
0000
0000
0000
1011
1001
0110b
0100b
0101b
0001b
0011b
1011b
0010b
1001b
1111b
D
Q
0
0
0
0
0
Hi-Z
1
0
0
Figure 6. WRDI Bus Configuration
RDSR – Read Status Register
The RDSR command allows the bus master to
verify the contents of the Status Register. Reading
Status provides information about the current state
of the write protection features. Following the
RDSR op-code, the FM25V20 will return one byte
with the contents of the Status Register. The Status
Register is described in detail in the section below.
WREN – Set Write Enable Latch
The FM25V20 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
(WRSR) and writing the memory (WRITE).
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 on the state of this bit.
Completing any write operation will automatically
clear the write-enable latch and prevent further writes
Rev. 3.0
August 2012
Page 5 of 17
Embedded System
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