24AA515/24LC515/24FC515
DS21673G-page 8 © 2008 Microchip Technology Inc.
6.0 WRITE OPERATIONS
6.1 Byte Write
Following the Start condition from the master, the
control code (four bits), the block select (one bit) the
Chip Select (two 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. There-
fore, 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 24XX515. The next byte
is the Least Significant Address Byte. After receiving
another Acknowledge signal from the 24XX515, the
master device will transmit the data word to be written
into the addressed memory location. The 24XX515
acknowledges again and the master generates a Stop
condition. This initiates the internal write cycle and
during this time, the 24XX515 will not generate
Acknowledge signals as long as the control byte being
polled matches the control byte that was used to initiate
the write (Figure 6-1). If an attempt is made to write to
the array with the WP pin held high, the device will
acknowledge the command but no write cycle will
occur, no data will be written, and the device will
immediately accept a new command. After a byte Write
command, the internal address counter will point to the
address location following the one that was just written.
6.2 Page Write
The write control byte, word address, and the first data
byte are transmitted to the 24XX515 in the same way
as in a byte write. But 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 after the master
has transmitted a Stop condition. After 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). If an attempt
is made to write to the array with the WP pin held high,
the device will acknowledge the command but no write
cycle will occur, no data will be written, and the device
will immediately accept a new command.
6.3 Write Protection
The WP pin allows the user to write-protect the entire
array (0000-FFFF) when the pin is tied to V
CC. If tied to
V
SS the write protection is disabled. The WP pin is
sampled at the Stop bit for every Write command
(Figure 1-1) Toggling the WP pin after the Stop bit will
have no effect on the execution of the write cycle.
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.
