Datasheet

MAX31850/MAX31851
Cold-Junction Compensated, 1-Wire
Thermocouple-to-Digital Converters
12Maxim Integrated
Figure 3. 64-Bit ROM Code
Figure 4. Memory Map
In some situations the bus master might not know whether
the devices on the bus are parasite powered or powered
by external supplies. The master needs this information
to determine if the strong bus pullup should be used dur-
ing temperature conversions. To get this information, the
master can issue a Skip ROM [CCh] command, followed
by a Read Power Supply [B4h] command, followed by a
read time slot. During the read time slot, parasite-pow-
ered devices pull the bus low, and externally powered
devices let the bus remain high. If the bus is pulled low,
the master knows that it must supply the strong pullup on
the 1-Wire bus during temperature conversions.
64-Bit ROM Code
Each device contains a unique 64-bit code stored in ROM
(Figure 3). The least significant 8 bits of the ROM code
contain the device’s 1-Wire family code, 3Bh. The next 48
bits contain a unique serial number. The most significant
8 bits contain a cyclic redundancy check (CRC) byte that
is calculated from the first 56 bits of the ROM code. See
the CRC Generation section for a detailed explanation
of the CRC bits. The 64-bit ROM code and associated
ROM function control logic allow the device to operate as
a 1-Wire device using the protocol detailed in the 1-Wire
Bus System section.
Scratchpad
The device’s scratchpad is organized as shown
in Figure 4. All memory commands are described
in detail in the MAX31850/MAX31851 Function
Commands section.
Byte 0 and byte 1 of Scratchpad contain the least sig-
nificant byte and the most significant byte of the thermo-
couple temperature register, respectively. Bytes 2 and 3
contain the LSB and MSB of the internal (cold-junction)
temperature value, as well as fault status. Byte 4 con-
tains the configuration information. Bytes 5, 6, and 7 are
reserved for internal use by the device and cannot be
overwritten; these bytes return all ones when read.
Byte 8 of Scratchpad is read-only and contains the CRC
code for bytes 0–7 of the scratchpad. The device gener-
ates this CRC using the method described in the CRC
Generation section.
MSb
8-BIT
CRC CODE
48-BIT SERIAL NUMBER
MSb MSbLSb
LSb
LSb
8-BIT FAMILY CODE
(3Bh)
MSbLSb
COLD-JUNCTION-COMPENSATED THERMOCOUPLE TEMPERATURE LSB AND FAULT STATUS (00h)BYTE 0
COLD-JUNCTION-COMPENSATED THERMOCOUPLE TEMPERATURE MSB (00h)BYTE 1
INTERNAL (COLD JUNCTION) TEMPERATURE AND FAULT STATUS LSB (00h)BYTE 2
INTERNAL (COLD JUNCTION) TEMPERATURE MSB (00h)BYTE 3
CONFIGURATION REGISTER*BYTE 4
RESERVED (FFh)BYTE 5
RESERVED (FFh)BYTE 6
RESERVED (FFh)BYTE 7
CRCBYTE 8
*THE LOWER 4 BITS (AD[3:0]) OF THE CONFIGURATION REGISTER ARE HARDWIRED THROUGH AD0–AD3.
SCRATCHPAD (POWER-UP STATE SHOWN IN PARENTHESES)