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

sion cycle is truncated, and the data from that conversion

is not latched into either temperature reading register. The

previous data is not changed and remains available.

In standby mode, supply current drops to about 3µA.

At very low supply voltages (under the power-on-reset

threshold), the supply current is higher due to the

address pin bias currents. It can be as high as 100µA,

depending on ADD0 and ADD1 settings.

SMBus Digital Interface

From a software perspective, the MAX1668/MAX1805

appear as a set of byte-wide registers that contain tem-

perature data, alarm threshold values, or control bits. A

standard SMBus 2-wire serial interface is used to read

temperature data and write control bits and alarm thresh-

old data. Each A/D channel within the devices responds

to the same SMBus slave address for normal reads and

writes.

The MAX1668/MAX1805 employ four standard SMBus

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

Byte (Figure 3). The shorter Receive Byte protocol allows

quicker transfers, provided that the correct data register

was previously selected by a Read Byte instruction. Use

caution with the shorter protocols in multimaster systems,

since a second master could overwrite the command

byte without informing the first master.

The temperature data format is 7 bits plus sign in two’s-com-

plement form for each channel, with each data bit represent-

ing 1°C (Table 2), transmitted MSB first. Measurements are

offset by +1/2°C to minimize internal rounding errors; for

example, +99.6°C is reported as +100°C.

Alarm Threshold Registers

Ten (six for MAX1805) registers store alarm threshold

measured temperature equals or exceeds the corre-

sponding alarm threshold value, an ALERT interrupt is

asserted.

full scale (0111 1111, or +127°C). The POR state of all

Multichannel Remote/Local

Temperature Sensor

10

______________________________________________________________________________________

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

S

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

S

ACK

7 bits

ADDRESS

RD

8 bits

DATA

///

P

S

Command Byte: sends com-

mand with no data

Data Byte: This command only

works immediately following a

Read Byte. Reads data from the

register commanded by that last

Read Byte; also used for SMBus

Alert Response return address

S = Start condition

Shaded = Slave transmission

P = Stop condition

/// = Not acknowledged

Figure 3. SMBus Protocols