Following a start condition the master must output the
address of the slave it is accessing. The most significant
four bits of the slave are the device type identifier (see
Figure 4). For the X24C04 this is fixed as 1010[B].
Figure 4. Slave Address
1 0 1 0 A2 A1 A0 R/W
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The next two significant bits address a particular device.
A system could have up to four X24C04 devices on the
bus (see Figure 10). The four addresses are defined by
the state of the A1 and A2 inputs.
The next bit of the slave address is an extension of the
array’s address and is concatenated with the eight bits
of address in the word address field, providing direct
access to the whole 512 x 8 array.
Figure 5. Byte Write
The last bit of the slave address defines the operation to
be performed. When set to one a read operation is
selected, when set to zero a write operation is selected.
Following the start condition, the X24C04 monitors the
SDA bus comparing the slave address being transmit-
ted with its slave address (device type and state of A1
and A2 inputs). Upon a correct compare the X24C04
outputs an acknowledge on the SDA line. Depending on
the state of the R/W bit, the X24C04 will execute a read
or write operation.
For a write operation, the X24C04 requires a second
address field. This address field is the word address,
comprised of eight bits, providing access to any one of the
512 words of memory. Upon receipt of the word address
the X24C04 responds with an acknowledge, and awaits
the next eight bits of data, again responding with an
acknowledge. The master then terminates the transfer by
generating a stop condition, at which time the X24C04
begins the internal write cycle to the nonvolatile memory.
While the internal write cycle is in progress the X24C04
inputs are disabled, and the device will not respond to any
requests from the master. Refer to Figure 5 for the
address, acknowledge and data transfer sequence.
BUS ACTIVITY: A
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