command byte) that occurs immediately after POR

returns the current local temperature data.

Single-Shot

The single-shot command immediately forces a new

conversion cycle to begin. If the single-shot command

is received while the MAX6642 is in standby mode

(RUN bit = 1), a new conversion begins, after which the

device returns to standby mode. If a single-shot con-

version is in progress when a single-shot command is

received, the command is ignored. If a single-shot

command is received in autonomous mode (RUN bit =

0), the command is ignored.

Configuration Byte Functions

The configuration byte register (Table 4) is a read-write

register with several functions. Bit 7 is used to mask

(disable) interrupts. Bit 6 puts the MAX6642 into stand-

by mode (STOP) or autonomous (RUN) mode. Bit 5 dis-

ables local temperature conversions for faster (8Hz)

remote temperature monitoring. Bit 4 prevents setting

the ALERT output until two consecutive measurements

result in fault conditions.

Status Byte Functions

The status byte register (Table 5) indicates which (if

any) temperature thresholds have been exceeded. This

byte also indicates whether the ADC is converting and

whether there is an open-circuit fault detected on the

external sense junction. After POR, the normal state of

all flag bits is zero, assuming no alarm conditions are

present. The status byte is cleared by any successful

read of the status byte after the overtemperature fault

condition no longer exists.

Slave Addresses

The MAX6642 has eight fixed addresses available.

These are shown in Table 6.

The MAX6642 also responds to the SMBus alert

response slave address (see the Alert Response

Address section).

POR and UVLO

To prevent ambiguous power-supply conditions from

corrupting the data in memory and causing erratic

the logic blocks begin operating, although reads and

(UVLO) comparator prevents the ADC from converting

Power-Up Defaults

Power-up defaults include:

• ALERT output is cleared.

• ADC begins autoconverting at a 4Hz rate.

• Command byte is set to 00h to facilitate quick

local Receive Byte queries.

Applications Information

Remote-Diode Selection

The MAX6642 can directly measure the die temperature

of CPUs and other ICs that have on-board temperature-

sensing diodes (see the Typical Operating Circuit) or

they can measure the temperature of a discrete diode-

connected transistor.

Effect of Ideality Factor

The accuracy of the remote temperature measurements

depends on the ideality factor (n) of the remote “diode”

(actually a transistor). The MAX6642 is optimized for n

= 1.008, which is the typical value for the Intel Pentium

III. A thermal diode on the substrate of an IC is normally

a PNP with its collector grounded. DXP should be con-

nected to the anode (emitter) and the cathode should

be connected at GND of the MAX6642.

If a sense transistor with an ideality factor other than

1.008 is used, the output data is different from the data

obtained with the optimum ideality factor. Fortunately,

the difference is predictable.

Assume a remote-diode sensor designed for a nominal

TT

n

n

M

ACTUAL

NOMINAL

=













1

±1°C, SMBus-Compatible Remote/Local

Temperature Sensor with Overtemperature Alarm

_______________________________________________________________________________________

9

PART NO. SUFFIX

ADDRESS

MAX6642ATT90

1001 000

MAX6642ATT92

1001 001

MAX6642ATT94

1001 010

MAX6642ATT96

1001 011

MAX6642ATT98

1001 100

MAX6642ATT9A

1001 101

MAX6642ATT9C

1001 110

MAX6642ATT9E

1001 111

Table 6. Slave Address