 2003 Microchip Technology Inc.

DS21160F-page 9

24LC21A

4.0

WRITE OPERATION

4.1

Byte Write

Following the start signal from the master, the slave

address (four bits), three zero bits (000) and the R/W bit

which is a logic low are placed onto the bus by the

master transmitter. This indicates to the addressed

slave receiver that a byte with a word address will

follow after it has generated an Acknowledge bit during

the ninth clock cycle. Therefore, the next byte

transmitted by the master is the word address and will

be written into the address pointer of the 24LC21A.

After receiving another acknowledge signal from the

24LC21A the master device will transmit the data word

to be written into the addressed memory location. The

24LC21A

acknowledges

again

and

the

master

generates a Stop condition. This initiates the internal

write cycle, and during this time the 24LC21A will not

generate acknowledge signals (Figure 4-1).

It is required that VCLK be held at a logic high level

during command and data transfer in order to program

the device. This applies to both byte write and page

write operation. Note, however, that the VCLK is

ignored during the self-timed program operation.

Changing VCLK from high-to-low during the self-timed

program operation will not halt programming of the

device.

4.2

Page Write

The write control byte, word address and the first data

byte are transmitted to the 24LC21A in the same way

as in a byte write. But instead of generating a Stop

condition the master transmits up to eight data bytes to

the 24LC21A which are temporarily stored in the on-

chip page buffer and will be written into the memory

after the master has transmitted a Stop condition. After

the receipt of each word, the three lower order address

pointer bits are internally incremented by one. The

higher order five bits of the word address remains

constant. If the master should transmit more than eight

words 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 4-3).

It is required that VCLK be held at a logic high level

during command and data transfer in order to program

the device. This applies to both byte write and page

write operation. Note, however, that the VCLK is

ignored during the self-timed program operation.

Changing VCLK from high-to-low during the self-timed

program operation will not halt programming of the

device.

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 multi-

ples 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.