12

SMS44

2047 2.3 10/23/00

SUMMIT MICROELECTRONICS, Inc.

START and STOP Conditions

When both the data and clock lines are HIGH the bus is

said to be not busy. A High-to-Low transition on the data

line, while the clock is HIGH, is defined as the “START”

condition. A Low-to-High transition on the data line, while

the clock is HIGH, is defined as the “STOP” condition. See

Figure 9.

Acknowledge (ACK)

Acknowledge is a software convention used to indicate

successful data transfers. The transmitting device, either

the master or the slave, will release the bus after transmit-

ting eight bits. During the ninth clock cycle the receiver will

pull the SDA line low to ACKnowledge that it received the

eight bits of data.

The SMS44 will respond with an ACKnowledge after

recognition of a START condition and its slave address

byte. If both the device and a write operation are selected

the SMS44 will respond with an ACKnowledge after the

receipt of each subsequent 8-bit word. In the READ mode

the SMS44 transmits eight bits of data, then releases the

SDA line, and monitors the line for an ACKnowledge

signal. If an ACKnowledge is detected and no STOP

condition is generated by the master, the SMS44 will

continue to transmit data. If an ACKnowledge is not

detected the SMS44 will terminate further data transmis-

sions and awaits a STOP condition before returning to the

standby power mode.

Device Addressing

Following a start condition the master must output the

address of the slave it is accessing. The most significant

four bits of the slave address are the device type identifier/

two bits are the Bus Address. The next bit (the 7th) is the

MSB of the memory address.

Figure 9. START and STOP Conditions

Read/Write Bit

The last bit of the data stream defines the operation to be

performed. When set to “1” a read operation is selected;

when set to “0” a write operation is selected.

WRITE OPERATIONS

The SMS44 allows two types of write operations: byte

write and page write. A byte write operation writes a single

write operation, limited to the memory array, allows up to

Byte Write

After the slave address is sent (to identify the slave device

and select either a read or write operation), a second byte

is transmitted which contains the low order 8 bit address

of any one of the 512 words in the array. Upon receipt of

the word address the SMS44 responds with an ACKnowl-

edge.

After receiving the next byte of data it again

responds with an ACKnowledge. The master then termi-

nates the transfer by generating a STOP condition, at

which time the SMS44 begins the internal write cycle.

While the internal write cycle is in progress the SMS44

inputs are disabled and the device will not respond to any

requests from the master.

Page Write (memory only)

The SMS44 is capable of a 16-byte page write operation.

It is initiated in the same manner as the byte-write opera-

tion, but instead of terminating the write cycle after the first

data word the master can transmit up to 15 more bytes of

data. After the receipt of each byte the SMS44 will respond

with an ACKnowledge.

The SMS44 automatically increments the address for

subsequent data words. After the receipt of each word, the

low order address bits are internally incremented by one.

7

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2047 Table09 1.0

Table 9. Slave Addresses

2047 Fig09 1.0

SCL

SDA In

START

Condition

STOP

Condition