FM24C64B
Figure 3. Data Transfer Protocol
Stop Condition
A Stop condition is indicated when the bus master
drives SDA from low to high while the SCL signal is
high. All operations must end with a Stop condition.
If an operation is pending when a stop is asserted, the
operation will be aborted. The master must have
control of SDA (not a memory read) in order to assert
a Stop condition.
Start Condition
A Start condition is indicated when the bus master
drives SDA from high to low while the SCL signal is
high. All read and write transactions begin with a
Start condition. An operation in progress can be
aborted by asserting a Start condition at any time.
Aborting an operation using the Start condition will
ready the FM24C64B for a new operation.
If during operation the power supply drops below the
specified V
DD
minimum, the system should issue a
Start condition prior to performing another operation
Data/Address Transfer
All data transfers (including addresses) take place
while the SCL signal is high. Except under the two
conditions described above, the SDA signal should
not change while SCL is high.
Acknowledge
The Acknowledge takes place after the 8
th
data bit
has been transferred in any transaction. During this
state the transmitter should release the SDA bus to
allow the receiver to drive it. The receiver drives the
SDA signal low to acknowledge receipt of the byte.
If the receiver does not drive SDA low, the condition
is a No-Acknowledge and the operation is aborted.
The receiver could fail to acknowledge for two
distinct reasons. First, if a byte transfer fails, the No-
Acknowledge ends the current operation so that the
device can be addressed again. This allows the last
byte to be recovered in the event of a
communication error. Second and most common,
the receiver does not acknowledge the data to
deliberately end an operation. For example, during a
read operation, the FM24C64B will continue to
place data onto the bus as long as the receiver sends
acknowledges (and clocks). When a read operation
is complete and no more data is needed, the receiver
must not acknowledge the last byte. If the receiver
acknowledges the last byte, this will cause the
FM24C64B to attempt to drive the bus on the next
clock while the master is sending a new command
such as a Stop command.
Slave Address
The first byte that the FM24C64B expects after a
start condition is the slave address. As shown in
Figure 4, the slave address contains the Slave ID
(device type), the device select address bits, and a
bit that specifies if the transaction is a read or a
write. Bits 7-4 define the device type and must be
set to 1010b for the FM24C64B. These bits allow
other types of function types to reside on the 2-wire
bus within an identical address range. Bits 3-1 are
the select bits which are equivalent to chip select
bits. They must match the corresponding value on
the external address pins to select the device. Up to
eight FM24C64Bs can reside on the same two-wire
bus by assigning a different address to each. Bit 0 is
the read/write bit. A 1 indicates a read operation,
and a 0 indicates a write.
Rev. 1.3
Feb. 2011
4 of 12
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
