FM25V02 - 256Kb SPI FRAM
Protocol Overview
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 FM25V02 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
FM25V02 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
FM25V02 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 FM25V02 is a serial F-RAM 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 F-RAM is similar to Serial Flash. The major
differences between the FM25V02 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 FM25V02, 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 complete address of 15-bits
specifies each byte address uniquely.
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.
Most functions of the FM25V02 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.
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
Users expect several obvious system benefits from
the FM25V02 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.
Serial Peripheral Interface – SPI Bus
SPI Mode 3: CPOL=1, CPHA=1
The FM25V02 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
FM25V02 operates in SPI Mode 0 and 3.
Figure 2. SPI Modes 0 & 3
Rev. 2.0
May 2010
Page 3 of 17
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
