FM24C64
Overview
The FM24C64 is a serial FRAM memory. The
memory array is logically organized as a 8,192 x 8 bit
memory array and is accessed using an industry
standard two-wire interface. Functional operation of
the FRAM is similar to serial EEPROMs. The major
difference between the FM24C64 and a serial
EEPROM with the same pinout relates to its superior
write performance.
Two-wire Interface
The FM24C64 employs a bi-directional two-wire bus
protocol using few pins and little board space. Figure
2 illustrates a typical system configuration using the
FM24C64 in a microcontroller-based system. The
industry standard two-wire bus is familiar to many
users but is described in this section.
By convention, any device that is sending data onto
the bus is the transmitter while the target device for
this data is the receiver. The device that is controlling
the bus is the master. The master is responsible for
generating the clock signal for all operations. Any
device on the bus that is being controlled is a slave.
The FM24C64 always is a slave device.
The bus protocol is controlled by transition states in
the SDA and SCL signals. There are four conditions:
Start, Stop, Data bit, and Acknowledge. Figure 3
illustrates the signal conditions that specify the four
states. Detailed timing diagrams are shown in the
Electrical Specifications section.
Memory Architecture
When accessing the FM24C64, the user addresses
8,192 locations each with 8 data bits. These data bits
are shifted serially. The 8,192 addresses are accessed
using the two-wire protocol, which includes a slave
address (to distinguish from other non-memory
devices), and an extended 16-bit address. Only the
lower 13 bits are used by the decoder for accessing
the memory. The upper three address bits should be
set to 0 for compatibility with larger devices in the
future.
The memory is read or written at the speed of the
two-wire bus. Unlike an EEPROM, it is not
necessary to poll the device for a ready condition
since writes occur at bus speed. That is, by the time a
new bus transaction can be shifted into the part, a
write operation is complete. This is explained in more
detail in the interface section below.
Users can expect several obvious system benefits
from the FM24C64 due to its fast write cycle and
high endurance as compared with EEPROM.
However 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 FM24C64 contains no power
management circuits other than a simple internal
power-on reset. It is the user’s responsibility to
ensure that V
DD
is within datasheet tolerances to
prevent incorrect operation.
VDD
Microcontroller
Rmin = 1.8 KΩ
Rmax = tR/Cbus
SDA
SCL
SDA
SCL
FM24C64
A0 A1 A2
FM24C64
A0 A1 A2
Figure 2. Typical System Configuration
Rev. 3.0
Mar. 2005
3 of 12
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
