After each byte is transfered, the internal byte

address counter (3 least significant bits only) is

incremented. The transfer is terminated by the

master generating a STOP condition. Care must be

taken to avoid address counter ’roll-over’ which

could result in data being overwritten. Note that, for

any write mode, the generation by the master of the

STOP condition starts the internal memory pro-

gram cycle. All inputs are disabled until the comple-

tion of this cycle and the memory will not respond

to any request.

Minimizing System Delays by Polling On ACK.

During the internal write cycle, the memory discon-

nects itself from the bus in order to copy the data

from the internal latches to the memory cells. The

AC Characteristics table, since the typical time is

shorter, the time seen by the system may be re-

duced by an ACK polling sequence issued by the

master. The sequence is as follows:

– Initial condition: a Write is in progress (see Figure

9).

– Step 1: the Master issues a START condition

followed by a Device Select byte (1st byte of the

new instruction).

– Step 2: if the memory is busy with the internal

write cycle, no ACK will be returned and the

master goes back to Step 1. If the memory has

terminated the internal write cycle, it will respond

with an ACK, indicating that the memory is ready

to receive the second part of the next instruction

(the first byte of this instruction was already sent

during Step 1).

WRITE Cycle

in Progress

AI01099B

Next

Operation is

Addressing the

Memory

START Condition

DEVICE SELECT

with RW = 0

ACK

Returned

YES

NO

YES

NO

ReSTART

STOP

Proceed

WRITE Operation

Proceed

Random Address

READ Operation

Send

Byte Address

First byte of instruction

with RW = 0 already

decoded by ST24xxx

Figure 9. Write Cycle Polling using ACK

DEVICE OPERATIONS (cont’d)

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ST24LC21