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MAX31850KATB-T

Cold-Junction Compensated, 1-Wire Thermocouple-to-Digital Converters

Maxim Integrated 

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

Read ROM [33h]

This command can be used only when there is one slave

on the bus. It allows the bus master to read the slave’s

64-bit ROM code without using the Search ROM com-

mand procedure. If this command is used when there is

more than one slave present on the bus, a data collision

occurs when all the slaves attempt to respond at the

same time.

Match ROM [55h]

The Match ROM command followed by a 64-bit ROM

code sequence allows the bus master to address a

specific slave device on a multidrop or single-drop bus.

Only the slave that exactly matches the 64-bit ROM code

sequence responds to the function command issued by

the master; all other slaves on the bus wait for a reset

pulse.

Skip ROM [CCh]

The master can use this command to address all devices

on the bus simultaneously without sending out any ROM

code information. For example, the master can make all

devices on the bus perform simultaneous temperature

conversions by issuing a Skip ROM command followed

by a Convert T [44h] function command.

Note that the Read Scratchpad command can follow the

Skip ROM command only if there is a single slave device

on the bus. In this case, time is saved by allowing the

master to read from the slave without sending the device’s

64-bit ROM code. A Skip ROM command followed by a

Read Scratchpad command causes a data collision on

the bus if there is more than one slave because multiple

devices attempt to transmit data simultaneously.

MAX31850/MAX31851 Function Commands

After the bus master has used a ROM command

to address the MAX31850/MAX31851 with which it

wishes to communicate, the master can issue one of

the MAX31850/MAX31851 function commands. These

commands allow the master to read from the device’s

scratchpad memories, initiate temperature conversions,

and determine the power-supply mode. The MAX31850/

MAX31851 function commands are summarized in

Table 5 and illustrated by the flowchart in Figure 8.

Convert T [44h]

This command initiates a single thermocouple tempera-

ture conversion, which consists of measuring the internal

(cold junction) temperature, measuring the thermocouple

temperature, and detecting any faults. Following the con-

version, the resulting cold-junction-compensated thermo-

couple data, internal temperature data, and fault status

are stored in the two 2-byte temperature registers in the

scratchpad memory, and the MAX31850 returns to its low-

power idle state. If the device is being used in parasite-

power mode, within 10Fs (max) after this command is

issued the master must enable a strong pullup on the

1-Wire bus for the duration of the conversion (tCONV) as

described in the Powering the MAX31850/MAX31851 sec-

tion. If the device is powered by an external supply, the

master can issue read time slots after the Convert T com-

mand and the device responds by transmitting 0 while

the temperature conversion is in progress and 1 when the

conversion is done. In parasite-power mode, this notifica-

tion technique cannot be used because the bus is pulled

high by the strong pullup during the conversion.

Table 5. MAX31850/MAX31851 Function Command Set

COMMAND

DESCRIPTION

PROTOCOL

1-Wire BUS ACTIVITY AFTER COMMAND IS

ISSUED

Convert T

(Note 1)

Initiates temperature conversion.

44h

The device transmits conversion status to master

(not applicable for parasite-powered devices).

Read Scratchpad

(Note 2)

Reads the 9-byte scratchpad

including the CRC byte.

BEh

The device transmits up to 9 data bytes to master.

The 9th byte is the CRC byte.

Read Power Supply Signals the device’s power-supply

(Note 3)

mode to the master.

B4h

The device transmits supply status to the master.

Note 1: For parasite-powered devices, the master must enable a strong pullup on the 1-Wire bus during temperature conversions.

No other bus activity can take place during this time.

Note 2: The master can interrupt the transmission of data at any time by issuing a reset.

Note 3: During the read time slot, parasite-powered devices pull the DQ bus low, and externally powered units let the bus remain

high.

Maxim Integrated

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