FM25L04B - 4Kb 3V SPI F-RAM
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
FM25L04B operates in SPI Mode 0 and 3.
Overview
The FM25L04B is a serial F-RAM memory. The
memory array is logically organized as 512 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 FM25L04B and a serial
EEPROM with the same pinout is the F-RAM’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 FM25L04B 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 FM25L04B device.
Memory Architecture
When accessing the FM25L04B, the user addresses
512 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 an
address. The upper address bit is included in the op-
code. The complete address of 9-bits specifies each
byte address uniquely.
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 (SI, SO) together and tie
off (high) the /HOLD pin. Figure 3 shows a
configuration that uses only three pins.
Most functions of the FM25L04B 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.
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 FM25L04B 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
FM25L04B 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 FM25L04B
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.
Users expect several obvious system benefits from
the FM25L04B 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.
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.
Note that the FM25L04B 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. It is recommended
that the part is not powered down with chip
enable active.
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.
Serial Peripheral Interface – SPI Bus
The FM25L04B employs
a
Serial Peripheral
Interface (SPI) bus. It is specified to operate at speeds
up to 20 MHz. This high-speed serial bus provides
Rev. 1.3
Feb. 2011
Page 3 of 14
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
