2013-2016 Microchip Technology Inc. DS20005215C-page 9
24AA256UID
6.0 WRITE OPERATIONS
6.1 Byte Write
Following the Start condition from the master, the
control code (four bits), the Chip Select (three bits) and
the R/W bit (which is a logic low) are clocked onto the
bus by the master transmitter. This indicates to the
addressed slave receiver that the address high byte will
follow after it has generated an Acknowledge bit during
the ninth clock cycle. Therefore, the next byte
transmitted by the master is the high-order byte of the
word address and will be written into the Address
Pointer of the 24AA256UID. The next byte is the Least
Significant Address Byte. After receiving another
Acknowledge signal from the 24AA256UID, the master
device will transmit the data word to be written into the
addressed memory location. The 24AA256UID
acknowledges again and the master generates a Stop
condition. This initiates the internal write cycle and
during this time, the 24AA256UID will not generate
Acknowledge signals (Figure 6-1).
6.2 Page Write
The write control byte, word address and the first data
byte are transmitted to the 24AA256UID in much the
same way as in a byte write. The exception is that
instead of generating a Stop condition, the master
transmits up to 63 additional bytes, which are
temporarily stored in the on-chip page buffer, and will
be written into memory once the master has
transmitted a Stop condition. Upon receipt of each
word, the six lower Address Pointer bits are internally
incremented by one. If the master should transmit more
than 64 bytes prior to generating the Stop condition, the
address counter will roll over and the previously
received data will be overwritten. As with the byte write
operation, once the Stop condition is received, an
internal write cycle will begin (Figure 6-2).
6.3 Write Protection
The upper eighth of the array (7000h-7FFFh) is
permanently write-protected. Write operations to this
address range are inhibited. Read operations are not
affected.
The remainder of the array (0000h-6FFFh) can be
written to and read from normally.
Note: When doing a write of less than 64 bytes,
the data in the rest of the page is
refreshed along with the data bytes being
written. This will force the entire page to
endure a write cycle, and for this reason
endurance is specified per page.
Note: Page write operations are limited to
writing bytes within a single physical page,
regardless of the number of bytes
actually being written. Physical page
boundaries start at addresses that are
integer multiples of the page buffer size
(or ‘page size’) and end at addresses that
are integer multiples of [page size – 1]. If
a Page Write command attempts to write
across a physical page boundary, the
result is that the data wraps around to the
beginning of the current page (overwriting
data previously stored there), instead of
being written to the next page, as might be
expected. It is, therefore, necessary for
the application software to prevent page
write operations that would attempt to
cross a page boundary.
