X1243
SCL
SDA
Start
Stop
FIGURE 4. VALID START AND STOP CONDITIONS
In the read mode, the device will transmit eight bits of data,
release the SDA line, then monitor the line for an
Clock and Data
Data states on the SDA line can change only during SCL
LOW. SDA state changes during SCL HIGH are reserved for
indicating start and stop conditions. See Figure 3.
acknowledge. If an acknowledge is detected and no stop
condition is generated by the master, the device will continue
to transmit data. The device will terminate further data
transmissions if an acknowledge is not detected. The master
must then issue a stop condition to return the device to
Standby mode and place the device into a known state.
Start Condition
All commands are preceded by the start condition, which is a
HIGH to LOW transition of SDA when SCL is HIGH. The
device continuously monitors the SDA and
Write Operations
Byte Write
SCL lines for the start condition and will not respond to any
command until this condition has been met. See Figure 4.
For a byte write operation (Refer to Figure 13), the device
requires the Slave Address Byte and the Word Address
Bytes. This gives the master access to any one of the words
in the array or CCR. (Note: Prior to writing to the CCR, the
master must write a 02h, then 06h to the status register in
preceding operations to enable the write operation. See
“Writing to the Clock/ Control Registers” on page 6.) Upon
receipt of each address byte, the X1243 responds with an
acknowl-edge. After receiving both address bytes the X1243
awaits the eight bits of data. After receiving the 8 data bits,
the X1243 again responds with an acknowledge. The master
then terminates the transfer by generating a stop condition.
The X1243 then begins an internal write cycle of the data to
the nonvolatile memory. Dur-ing the internal write cycle, the
device inputs are dis-abled, so the device will not respond to
any requests from the master. The SDA output is at high
impedance. See Figure 6.
Stop Condition
All communications must be terminated by a stop con-dition,
which is a LOW to HIGH transition of SDA when SCL is
HIGH. The stop condition is also used to place the device
into the Standby power mode after a read sequence. A stop
condition can only be issued after the transmitting device
has released the bus Refer to Figure 4.
Acknowledge
Acknowledge is a software convention used to indicate
successful data transfer. The transmitting device, either
master or slave, will release the bus after transmitting eight
bits. During the ninth clock cycle, the receiver will pull the
SDA line LOW to acknowledge that it received the eight bits
of data. Refer to Figure 5.
The device will respond with an acknowledge after rec-
ognition of a start condition and if the correct Device Identier
and Select bits are contained in the Slave Address Byte. If a
write operation is selected, the device will respond with an
acknowledge after the receipt of each subsequent eight bit
word. The device will acknowledge all incoming data and
address bytes, except for:
A write to a protected block of memory is ignored, but will still
receive an acknowledge. At the end of the write command,
the X1243 will not initiate an internal write cycle, and will
continue to ACK commands.
- The Slave Address Byte when the Device Identier
and/or Select bits are incorrect
- All Data Bytes of a write when the WEL in the Write
Protect Register is LOW
- The 2nd Data Byte of a Register Write Operation (when
only 1 data byte is allowed)
FN8249.0
7
April 28, 2005
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