FM25L256 Extended Temp.
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
FM25L256 operates in SPI Mode 0 and 3.
Overview
The FM25L256 is a serial FRAM memory. The
memory array is logically organized as 32,768 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 FM25L256 and a serial
EEPROM with the same pinout is the FRAM’s
superior write performance and power consumption.
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 FM25L256 devices
with a microcontroller that has a dedicated SPI port,
as Figure 2 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 FM25L256 device.
Memory Architecture
When accessing the FM25L256, the user addresses
32K 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 bit of the address range is a
“don’t care” value. The complete address of 15-bits
specifies each byte address uniquely.
For a microcontroller that has no dedicated SPI bus, a
general purpose port may be used. To reduce
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hardware resources on the controller, it is possible to
connect the two data pins together and tie off the
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Hold pin. Figure 3 shows a configuration Sthat uses
only three pins.
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Most functions of the FM25L256 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
Protocol Overview
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The SPI interface is a synchronous serial interface
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using clock and data pins. It is intended to support
multiple devices on the bus. Each device is activated
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using a chip select. Once chip select is activated by
ME
I
the bus master, the FM25L256 will begin monitoring
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the clock and data lines. The relationship between the
S
falling edge of /CS, the clock and da6ta is dictated by
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5
the SPI mode. The device will make a determination
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2
of the SPI mode on the falling edge of each chip
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complete. This is explained in more detail in the
interface section.
select. While there are Lfour such modes, the
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5
FM25L256 supports only modes 0 and 3. Figure 4
C
2
shows the required signal relationships for modes 0
and 3. For both modes, data is clocked into the
FM25L256 on the rising edge of SCK and data is
expected on the first rising edge after /CS goes
Users expect several obvious system benefits from
the FM25L256 due to its fast write cycle and high
endurance as compared to EEPROM. In addition
E
M
W
F
there are less obvious benefits Ras well. For example
:
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active. If the clock starts from a high state, it will fall
e
in a high noise environment, the fast-write operation
is less susceptible to corruption than an EEPROM
pri
v
or to the first data transfer in order to create the
i
t
T
N
first rising edge.
since it is completed quickly. By contrast, an
a
EEPROM requiring milliseconds to write is
vulnerable to noise during much of the cycle.
O
n
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.
r
R
e
N
t
Note that the FM25L256 contains no power
l
O
management circuits other than a simple internal
A
power-on reset circuit. It is the user’s
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responsibility to ensure that VDD is within
datasheet tolerances to prevent incorrect
operation. It is recommended that the part is not
powered down with chip select active.
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.
Serial Peripheral Interface – SPI Bus
The FM25L256 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
Rev. 2.3
March 2007
Page 3 of 14
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
