FM24CL16
SCL
SDA
Stop
(Master)
Start
(Master)
7
6
0
Data bit Acknowledge
(Transmitter) (Receiver)
Data bits
(Transmitter)
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 using the FM24CL16 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
prepare the FM24CL16 for a new operation.
If during operation the power supply drops below the
specified VDD 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. For system design
considerations, keeping SCL in a low state while idle
improves robustness.
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 would fail to acknowledge for two
distinct reasons. First is that a byte transfer fails. In
this case, the No-Acknowledge ends the current
operation so that the part 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 to deliberately end an operation. For
example, during a read operation, the FM24CL16
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 FM24CL16 to attempt to drive the
bus on the next clock while the master is sending a
new command such as a Stop.
Slave Address
The first byte that the FM24CL16 expects after a
Start condition is the slave address. As shown in
Figure 4, the slave address contains the device type,
the page of memory to be accessed, and a bit that
specifies if the transaction is a read or a write.
Bits 7-4 are the device type and should be set to
1010b for the FM24CL16. The device type allows
other types of functions to reside on the 2-wire bus
within an identical address range. Bits 3-1 are used
for page select. They specify the 256-byte block of
memory that is targeted for the current operation. Bit
0 is the read/write bit. R/W=1 indicates a read
operation and R/W=0 indicates a write operation.
Rev 3.3
Nov. 2005
Page 4 of 13
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
