The DS4550’s slave address of the DS4550 is deter-

mined by the state of the A0, A1, and A2 address pins

result in a ‘0’ in the corresponding bit position in the

slave address. Conversely, address pins connected to

When the R/W bit is 0 (such as in A0h), the master is

indicating it will write data to the slave. If R/W = 1, (A1h

in this case), the master is indicating it wants to read

from the slave.

If an incorrect slave address is written, the DS4550

device and ignores the communication until the next

start condition is sent.

master must transmit a memory address to identify the

memory location where the slave is to store the data.

The memory address is always the second byte trans-

mitted during a write operation following the slave

address byte.

Writing a Single Byte to a Slave: The master must

generate a start condition, write the slave address byte

(R/W = 0), write the memory address, write the byte of

data, and generate a stop condition. Remember the

master must read the slave’s acknowledgement during

all byte write operations.

Writing Multiple Bytes to a Slave: To write multiple

bytes to a slave, the master generates a start condition,

writes the slave address byte (R/W = 0), writes the

memory address, writes up to 8 data bytes, and gener-

ates a stop condition.

The DS4550 is capable of writing up to 8 bytes (1 page

nally controlled by an address counter that allows data

to be written to consecutive addresses without transmit-

ting a memory address before each data byte is sent.

The address counter limits the write to one 8-byte

page. Attempts to write to additional pages of memory

without sending a stop condition between pages

results in the address counter wrapping around to the

beginning of the present row. The first row begins at

address 00h and subsequent rows begin at multiples of

8 there on (08h, 10h, 18h, 20h, etc).

To prevent address wrapping from occurring, the mas-

ter must send a stop condition at the end of the page,

and then wait for the bus free or EEPROM write time to

elapse. Then the master can generate a new start con-

dition, write the slave address byte (R/W = 0), and the

first memory address of the next memory row before

continuing to write data.

Acknowledge Polling: Any time an EEPROM page is

written, the DS4550 requires the EEPROM write time

page to EEPROM. During the EEPROM write time, the

device does not acknowledge its slave address

because it is busy. It is possible to take advantage of

this phenomenon by repeatedly addressing the

DS4550, which allows communication to continue as

soon as the DS4550 is ready. The alternative to

acknowledge polling is to wait for a maximum period of

EEPROM Write Cycles: When EEPROM writes occur

EEPROM memory page even if only a single byte on a

page was modified. Writes that do not modify all 8

bytes on the page are valid and do not corrupt any

other bytes on the same page. Because the whole

page is written, even bytes on the page that were not

modified during the transaction are still subject to a

write cycle. The DS4550’s EEPROM write cycles are

specified in the Nonvolatile Memory Characteristics

perature. It is capable of handling many more writes at

room temperature.

Reading a Single Byte from a Slave: Unlike the write

operation that uses the specified memory address byte

to define where the data is to be written, the read oper-

ation occurs at the present value of the memory

address counter. To read a single byte from the slave,

the master generates a start condition, writes the slave

address byte with R/W = 1, reads the data byte with a

NACK to indicate the end of the transfer, and generates

a stop condition. However, since requiring the master

to keep track of the memory address counter is imprac-

tical, the following method should be used to perform

reads from a specified memory location.

Manipulating the Address Counter for Reads: A

dummy write cycle can be used to force the address

counter to a particular value. To do this, the master gen-

erates a start condition, writes the slave address byte

(R/W = 0), writes the memory address where it desires

to read, generates a repeated start condition, writes the

slave address byte (R/W = 1), reads data with ACK or

NACK as applicable, and generates a stop condition.

Expander Plus Memory

16

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