uration byte register. Hardware and software standbys

are very similar; all data is retained in memory, and the

SMBus interface is alive and listening for SMBus com-

mands but the SMBus timeout is disabled. The only dif-

ference is that in software standby mode, the one-shot

command initiates a conversion. With hardware stand-

by, the one-shot command is ignored. Activity on the

SMBus causes the device to draw extra supply current.

Driving STBY low overrides any software conversion

command. If a hardware or software standby command

is received while a conversion is in progress, the con-

version cycle is interrupted, and the temperature regis-

ters are not updated. The previous data is not changed

and remains available.

SMBus Digital Interface

From a software perspective, the MAX6695/MAX6696

appear as a series of 8-bit registers that contain tem-

perature data, alarm threshold values, and control bits.

A standard SMBus-compatible 2-wire serial interface is

used to read temperature data and write control bits

and alarm threshold data. The same SMBus slave

address provides access to all functions.

The MAX6695/MAX6696 employ four standard SMBus

protocols: Write Byte, Read Byte, Send Byte, and

Receive Byte (Figure 2). The shorter Receive Byte proto-

col allows quicker transfers, provided that the correct

data register was previously selected by a Read Byte

instruction. Use caution with the shorter protocols in mul-

timaster systems, since a second master could overwrite

the command byte without informing the first master.

When the conversion rate control register is set

≥ 06h,

temperature data can be read from the read internal

temperature (00h) and read external temperature (01h)

registers. The temperature data format in these regis-

ters is 7 bits + sign in two’s-complement form for each

channel, with the LSB representing +1°C (Table 2). The

MSB is transmitted first. Use bit 3 of the configuration

register to select the registers corresponding to remote

1 or remote 2.

When the conversion rate control register is set

≤ 05h,

temperature data can be read from the read internal

temperature (00h) and read external temperature (01h)

registers, the same as for faster conversion rates. An

additional 3 bits can be read from the read external

extended temperature register (10h) and read internal

Dual Remote/Local Temperature Sensors with

SMBus Serial Interface

_______________________________________________________________________________________

9

Figure 2. SMBus Protocols

ACK

7 bits

ADDRESS

ACK

WR

8 bits

DATA

ACK

1

P

8 bits

S

COMMAND

Write Byte Format

Read Byte Format

Send Byte Format

Receive Byte Format

Slave Address: equiva-

lent to chip-select line of

a 3-wire interface

Command Byte: selects which

register you are writing to

Data Byte: data goes into the register

set by the command byte (to set

thresholds, configuration masks, and

sampling rate)

ACK

7 bits

ADDRESS

ACK

WR

S

ACK

8 bits

DATA

7 bits

ADDRESS

RD

8 bits

///

P

COMMAND

Slave Address: equiva-

lent to chip-select line

Command Byte: selects

which register you are

reading from

Slave Address: repeated

due to change in data-

flow direction

Data Byte: reads from

the register set by the

command byte

ACK

7 bits

ADDRESS

WR

8 bits

COMMAND

ACK

P

ACK

7 bits

ADDRESS

RD

8 bits

DATA

///

P

S

Command Byte: sends com-

mand with no data, usually

used for one-shot command

Data Byte: reads data from

the register commanded

by the last Read Byte or

Write Byte transmission;

also used for SMBus Alert

Response return address

S = Start condition

Shaded = Slave transmission

P = Stop condition

/// = Not acknowledged