CS5535 datasheet(118/555 Pages) NSC

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SMB Controller Functional Description (Continued)

Figure 4-26. SMB Acknowledge Cycle

4.10.1.3 Acknowledge After Every Byte Rule

According to this rule, the master generates an acknowl-

edge clock pulse after each byte transfer, and the receiver

sends an acknowledge signal after every byte received.

There are two exceptions to this rule:

• When the master is the receiver, it must indicate to the

transmitter the end of data by not acknowledging (nega-

tive acknowledge) the last byte clocked out of the slave.

This negative acknowledge still includes the acknowl-

edge clock pulse (generated by the master), but the SDA

line is not pulled down.

• When the receiver is full, otherwise occupied, or a

problem has occurred, it sends a negative acknowledge

to indicate that it cannot accept additional data bytes.

4.10.1.4 Addressing Transfer Formats

Each device on the bus has a unique address. Before any

data is transmitted, the master transmits the address of the

slave being addressed. The slave device should send an

acknowledge signal on the SDA line, once it recognizes its

address.

The address consists of the first seven bits after a START

condition. The direction of the data transfer (R/W) depends

on the bit sent after the address, the eighth bit. A low-to-

high transition during an SCL high period indicates the

STOP condition, and ends the transaction of SDA (see Fig-

ure 4-27).

When the address is sent, each device in the system com-

pares this address with its own. If there is a match, the

device considers itself addressed and sends an acknowl-

edge signal. Depending on the state of the R/W bit (1 =

Read, 0 = Write), the device acts either as a transmitter or

a receiver.

The SMB protocol allows a general call address to be sent

to all slaves connected to the bus. The first byte sent speci-

fies the general call address (00h) and the second byte

specifies the meaning of the general call (for example, write

slave address by software only). Those slaves that require

data acknowledge the call, and become slave receivers;

other slaves ignore the call.

Figure 4-27. SMB Complete Data Transaction

START

Condition

SCL

12 3 - 6

Transmitter Stays Off Bus

During Acknowledge Clock

Acknowledge

Signal From Receiver

Data Output

by Transmitter

Data Output

by Receiver

START

Condition

STOP

Condition

SDA

SCL

1 - 7

Address R/W ACK

Data

ACK

Data

ACK

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