© 2007 Microchip Technology Inc.

DS21189L-page 7

24AA64/24LC64/24FC64

4.0

WRITE OPERATIONS

4.1

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 once it has generated an Acknowledge bit during

the ninth clock cycle. Therefore, the next byte transmit-

ted by the master is the high-order byte of the word

address and will be written into the Address Pointer of

the 24XX64. The next byte is the Least Significant

Address Byte. After receiving another Acknowledge

signal from the 24XX64, the master device will transmit

the data word to be written into the addressed memory

location. The 24XX64 acknowledges again and the

master generates a Stop condition. This initiates the

internal write cycle and, during this time, the 24XX64

will not generate Acknowledge signals (Figure 4-1). 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.

4.2

Page Write

The write control byte, word address and the first data

byte are transmitted to the 24XX64 in the same way as

in a byte write. However, instead of generating a Stop

condition, the master transmits up to 31 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 five lower Address Pointer bits are internally

incremented by one. If the master should transmit more

than 32 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 inter-

nal write cycle will begin (Figure 4-2). 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.

4.3

Write Protection

The WP pin allows the user to write-protect the entire

sampled at the Stop bit for every Write command

(Figure 3-1). 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.