FM24C64
Device
Select
Memory Operation
The FM24C64 is designed to operate in a manner
very similar to other 2-wire interface memory
products. The major differences result from the
higher performance write capability of FRAM
technology. These improvements result in some
differences between the FM24C64 and a similar
configuration EEPROM during writes. The
complete operation for both writes and reads is
explained below.
Write Operation
All writes begin with a device address, then a
memory address. The bus master indicates a write
operation by setting the LSB of the device address
to a 0. After addressing, the bus master sends each
byte of data to the memory and the memory
generates an acknowledge condition. Any number of
sequential bytes may be written. If the end of the
address range is reached internally, the address
counter will wrap from 1FFFh to 0000h.
Unlike other nonvolatile memory technologies,
there is no write delay with FRAM. The entire
memory cycle occurs in less time than a single bus
clock. Therefore, any operation including a read or
write can occur immediately following a write.
Acknowledge polling, a technique used with
EEPROMs to determine if a write is complete is
unnecessary and will always return a ready
condition.
Internally, the actual memory write occurs after the
8
th
data bit is transferred. It will be complete before
the Acknowledge is sent. Therefore, if the user
desires to abort a write without altering the memory
contents, this should be done using a Start or Stop
condition prior to the 8
th
data bit. The FM24C64
uses no page buffering.
Portions of the memory array can be write protected
using the WP pin. Pulling the WP pin high (V
DD
)
will write-protect addresses in the upper quadrant
from 1800h to 1FFFh. The FM24C64 will not
acknowledge data bytes that are written to protected
addresses. In addition, the address counter will not
increment if writes are attempted to these addresses.
Pulling WP low (V
SS
) will deactivate this feature.
WP should not be left floating.
Figures 5 and 6 illustrate both a single-byte write
and multiple-byte write.
Slave
ID
1
7
0
6
1
5
0
4
A2
3
A1 A0 R/W
2
1
0
Figure 4. Slave Address
Addressing Overview
After the FM24C64 (as receiver) acknowledges the
device address, the master can place the memory
address on the bus for a write operation. The address
requires two bytes. The first is the MSB (upper byte).
Since the device uses only 13 address bits, the value
of the upper three bits are don’t care. Following the
MSB is the LSB (lower byte) with the remaining
eight address bits. The address value is latched
internally. Each access causes the latched address
value to be incremented automatically. The current
address is the value that is held in the latch, either a
newly written value or the address following the last
access. The current address will be held as long as
power remains or until a new value is written. Reads
always use the current address. A random read
address can be loaded by beginning a write operation
as explained below.
After transmission of each data byte and just prior to
the acknowledge, the FM24C64 increments the
internal address latch. This allows the next sequential
byte to be accessed with no additional addressing
externally. After the last address (1FFFh) is reached,
the address latch will roll over to 0000h. There is no
limit to the number of bytes that can be accessed with
a single read or write operation.
Data Transfer
After the address information has been transmitted,
data transfer between the bus master and the
FM24C64 can begin. For a read operation, the
FM24C64 will place 8 data bits on the bus then wait
for an Acknowledge from the master. If the
Acknowledge occurs, the FM24C64 will transfer the
next sequential byte. If the Acknowledge is not sent,
the FM24C64 will end the read operation. For a write
operation, the FM24C64 will accept 8 data bits from
the master and then send an Acknowledge. All data
transfer occurs MSB (most significant bit) first.
Rev. 3.0
Mar. 2005
5 of 12
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
