6

LTC1710

OPERATIO

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 LTC1710 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

LTC1710, even if the address sent by the master matches

that of the LTC1710. After reception of the first byte, the

slave device (LTC1710) with the matching address then

acknowledges the master by pulling the DATA line low

before the next rising clock edge.

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 acknowledge

from the matching slave, the master then sends out the

command byte (see Table 1).

Table 1. Switch Control Table

COMMAND

XXXXXX00

XXXXXX01

XXXXXX10

XXXXXX11

Switch 0

SW0 Off

SW0 On

SW0 Off

SW0 On

Switch 1

SW1 Off

SW1 Off

SW1 On

SW1 On

After receiving the command byte, the slave device

(LTC1710) needs to acknowledge the master again 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

output switch will not turn on, however, until the Stop

signal is detected. This double buffering feature of the

output latch allows the user to “daisy-chain” multiple

SMBus devices such that their outputs are synchronously

executed on the Stop signal despite the fact that valid data

were loaded into their output latches at different times. An

example is shown in Figure 1. If somehow either the Start

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

slave device (LTC1710) will regard this as an error and

reject all previous data.

Address

The LTC1710 has an address of 10110XX; the five MSBs

are hardwired, but the two LSBs are programmable by the

user with the help of a three-state address pin. Refer to

Table 2 for the pin configurations and their corresponding

addresses.

Table 2. Address Pin Truth Table

AD1

ADDRESS

GND

1011000

1011001

1011010

To conserve standby current, it is preferable to tie the

needed, then the address pin can be tied to the third state

Figure 2.

CLK

DATA

ACK

1710 TA03

1

(SW0

ON)

1

(SW1

ON)

0

0

0

0

0

0

0

0

1

1

0

1

START

0

(PROGRAMMABLE)

ACK

0

(WRITE)

STOP

ADDRESS BYTE

COMMAND BYTE

Figure 2. The LTC1710 Programmed with Address 1011001

START ADDR1

ADDR2

COMMAND

COMMAND

A

START

START

A

A

A

A

A

ADDR3

COMMAND

EXECUTION OF DATA STORED IN

OUTPUT LATCH OF DEVICES WITH

ADDR1, ADDR2 AND ADDR3

STOP

1710 F01

Figure 1. Daisy-Chain Example

5

3

6

4

1

2

1M

5V

8

7

1710 F02

DATA

CLK

AD1

SW0D

OUT0

LTC1710

OUT1

GND

LOAD 1

LOAD 2

1M

Example of Send Byte Protocol to Slave Address 1011000 Turning SW0 and SW1 On