Preliminary
FM25L16
16Kb FRAM Serial 3V Memory
Features
16K bit Ferroelectric Nonvolatile RAM
•
Organized as 2,048 x 8 bits
•
Unlimited Read/Write Cycles
•
10 Year Data Retention
•
NoDelay™ Writes
•
Advanced High-Reliability Ferroelectric Process
Very Fast Serial Peripheral Interface - SPI
•
Up to 20 MHz 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
•
Low Voltage Operation 2.7-3.6V
•
1
µA
Standby Current
Industry Standard Configuration
•
Industrial Temperature -40°C to +85°C
•
“Green” 8-pin SOIC and 8-pin DFN Packages
•
DFN Footprint Conforms to TSSOP-8
Description
The FM25L16 is a 16-kilobit nonvolatile memory
employing an advanced ferroelectric process. A
ferroelectric random access memory or FRAM 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
EEPROM and other nonvolatile memories.
Unlike serial EEPROMs, the FM25L16 performs
write operations at bus speed. No write delays are
incurred. The next bus cycle may commence
immediately without the need for data polling. The
next bus cycle may start immediately. In addition, the
product offers virtually unlimited write endurance,
orders of magnitude more endurance than EEPROM.
Also, FRAM exhibits much lower power during
writes than EEPROM since write operations do not
require an internally elevated power supply voltage
for write circuits.
These capabilities make the FM25L16 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 FM25L16 provides substantial benefits to users
of serial EEPROM as a hardware drop-in
replacement. The FM25L16 uses the high-speed SPI
bus, which enhances the high-speed write capability
of FRAM technology. Device specifications are
guaranteed over an industrial temperature range of
-40°C to +85°C.
This is a product that has fixed target specifications but are
subject to change pending characterization results.
Pin Configuration
CS
SO
WP
VSS
1
2
3
4
8
7
6
5
VDD
HOLD
SCK
SI
/CS
SO
/WP
VSS
1
2
3
4
Top View
8
7
6
5
VDD
/HOLD
SCK
SI
Pin Name
/CS
/WP
/HOLD
SCK
SI
SO
VDD
VSS
Function
Chip Select
Write Protect
Hold
Serial Clock
Serial Data Input
Serial Data Output
Supply Voltage
Ground
Ordering Information
FM25L16-G
FM25L16-DG
“Green” 8-pin SOIC
“Green” 8-pin DFN
Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 545-FRAM, (719) 481-7000
www.ramtron.com
Page 1 of 14
Rev. 1.0
Oct 2004
FM25L16
WP
CS
HOLD
SCK
Instruction Decode
Clock Generator
Control Logic
Write Protect
512 x 32
FRAM Array
Instruction Register
Address Register
Counter
SI
11
8
Data I/O Register
3
Nonvolatile Status
Register
SO
Figure 1. Block Diagram
Pin Descriptions
Pin Name
/CS
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 SCK signal. A falling edge on /CS 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 20 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 SCK or /CS. All transitions on /HOLD must occur while
SCK is low.
Write Protect: This active low 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 on pages 6 and 7.
Serial Input: All data is input to the device on this pin. The pin is sampled on the
rising edge of SCK and is ignored at other times. It should always be driven to a valid
logic level to meet I
DD
specifications.
* SI may be connected to SO 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.
* SO may be connected to SI for a single pin data interface.
Power Supply (2.7V to 3.6V)
Ground
SCK
Input
/HOLD
Input
/WP
Input
SI
Input
SO
Output
VDD
VSS
Supply
Supply
Rev. 1.0
Oct 2004
Page 2 of 14
FM25L16
Overview
The FM25L16 is a serial FRAM memory. The
memory array is logically organized as 2,048 x 8 and
is accessed using an industry standard Serial
Peripheral Interface or SPI bus. Functional operation
of the FRAM is similar to serial EEPROMs. The
major difference between the FM25L16 and a serial
EEPROM with the same pinout is the FRAM’s
superior write performance.
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
FM25L16 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 pins together. Figure 2
illustrates a typical system configuration using the
FM25L16 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 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 FM25L16 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
FM25L16 supports modes 0 and 3. Figure 4 shows
the required signal relationships for modes 0 and 3.
For both modes, data is clocked into the FM25L16 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 SPI protocol is controlled by op-codes. These
op-codes specify the commands to the device. After
/CS 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. Note that the
WREN and WRDI op-codes are commands with no
subsequent data transfer.
Important: The /CS must go inactive (high) 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 FM25L16, the user addresses
2,048 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 two-byte address. The upper 5 bits of the address
range are ‘don’t care’ values. The complete address
of 11-bits specifies each byte address uniquely.
Most functions of the FM25L16 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 an
EEPROM, 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 FM25L16 due to its fast write cycle and high
endurance as compared with EEPROM. 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 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 FM25L16 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 datasheet tolerances to
prevent incorrect operation.
Serial Peripheral Interface – SPI Bus
The FM25L16 employs a Serial Peripheral Interface
(SPI) bus. It is specified to operate at speeds up to 20
Rev. 1.0
Oct 2004
Page 3 of 14
FM25L16
SPI
Microcontroller
FM25640
FM25L16
FM25640
FM25L16
MOSI : Master Out Slave In
MISO : Master In Slave Out
SS : Slave Select
Figure 2. System Configuration with SPI port
P1.0
P1.1
Microcontroller
FM25640
FM25L16
P1.2
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
Rev. 1.0
Oct 2004
Page 4 of 14
FM25L16
Data Transfer
All data transfers to and from the FM25L16 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. Outputs are 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 FM25L16. 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
Set Write Enable Latch
WREN
Write Disable
WRDI
Read Status Register
RDSR
Write Status Register
WRSR
Read Memory Data
READ
WRITE
Write Memory Data
WREN - Set Write Enable Latch
The FM25L16 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 below 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
0000
0000
0000
0000
0000
0000
0110b
0100b
0101b
0001b
0011b
0010b
Figure 5. WREN Bus Configuration
Figure 6. WRDI Bus Configuration
Rev. 1.0
Oct 2004
Page 5 of 14
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