7

LTC1623

OPERATIO

SMBus Operation

SMBus is a serial bus interface that uses only two bus

lines, DATA and CLK, to control low power peripheral

devices in portable equipment. It consists of masters, also

known as hosts, and slave devices. The master of the

SMBus is always the one to initiate communications to its

slave devices by varying the status of the DATA and CLK

lines. The SMBus specification establishes a set of proto-

cols that devices on the bus must follow during commu-

nications.

The protocol that the LTC1623 uses is the Send Byte

Protocol. In this protocol, the master first sends out a Start

signal by switching the DATA line from high to low while

CLK is high. (Because there may be more than one master

on the same bus, an arbitration process takes place if two

masters attempt to take control of the DATA line simulta-

neously; the first master that outputs a one while the other

master is zero loses the arbitration and becomes a slave

itself.) Upon detecting this Start signal, all slave devices on

the bus wake up and get ready to shift in the next byte of

data.

The master then sends out the first byte. The first seven

bits of this byte consist of the address of the device that the

master wishes to communicate with. The last bit indicates

whether the command will be a read (logic one) or write

(logic zero). Because the LTC1623 is a slave device that

can only be written to by a master, it will ignore the ensuing

commands of the master if it wants to read from the

LTC1623, even if the address sent by the master matches

that of the LTC1623. After reception of the first byte, the

slave device (LTC1623) with the matching address then

acknowledges the master by pulling the data line low

before the rising edge of the ninth clock cycle.

By now, all other nonmatching slave devices will have

gone back to their original standby states to wait for the

next start signal. Meanwhile, upon receiving the acknowl-

edge from the matching slave, the master then sends out

the command byte. In the case of the LTC1623, the two

LSBs of this second byte from the master are the signals

controlling the status of the external switches; a digital

“one” turns on the charge pump to drive up the output gate

voltage while a digital “zero” shuts down the charge pump

and discharges the output gate voltage to zero.

After receiving the command byte, the slave device

(LTC1623) needs to again acknowledge the master by

pulling the DATA line low on the following clock cycle. The

master then ends this Send Byte Protocol by sending the

Stop signal, which is a transition from low to high on the

DATA line while the CLK line is high. Valid data is shifted

into the output latch on the last acknowledge signal; the

external switch will not be enabled, however, until the Stop

signal is detected. This double-buffering feature allows

the user to daisy-chain several differently addressed SMBus

devices such that their output executions are synchronous

to the Stop signal even though valid data were loaded into

their output latches at different times. Figure 1 shows an

example of this special protocol. If somehow either the

Start or the Stop signal is detected in the middle of a byte,

the slave device (LTC1623) will regard this as an error and

reject all previous data. Other than the Stop and Start

conditions, DATA must be stable during CLK high; DATA

can change state only during CLK low.

CLK

START

DATA

1

11

0

000

0

0000

0

01

(GB ON)(GA ON)

1623 TD02

COMMAND BYTE

ADDRESS BYTE

1

ACK

STOP

ACK

(PROGRAMMABLE)

(WRITE)

Example of Send Byte Protocol to Slave Address 1011000 Turning GA and GB On

COMMAND

ADD1 A

A

STOP

ADD2 A

START

START

COMMAND A

COMMAND A

ADD3 A

START

1623 F01

Figure 1. Daisy-Chaining Multiple SMBus Devices