Datasheet

Note 1: Forparasite-powereddevices,themastermustenableastrongpulluponthe1-Wirebusduringtemperatureconversions
andcopiesfromthescratchpadtoEEPROM.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: All 3 bytes must be written before a reset is issued.
MAX31820 Function Commands
After the bus master has used a ROM command to
address the device with which it wishes to communi-
cate, the master can issue one of the device function
commands. These commands allow the master to write
to and read from the device’s scratchpad memory,
initiate temperature conversions, and determine the
power-supply mode. Table 3 summarizes the device
function commands, and
Figure8 illustrates the function
commands.
Convert T [44h]
This command initiates a single temperature conversion.
Following the conversion, the resulting thermal data is
stored in the 2-byte temperature register in the scratch-
pad memory and the device returns to its low-power
idle state. If the device is being used in parasite-power
mode, within 10µs (max) after this command is issued
the master must enable a strong pullup on the 1-Wire bus
for the duration of the conversion (t
CONV
), as described
in the Powering the MAX31820 section. If the device is
powered by an external supply, the master can issue
read time slots after the Convert T command and the
device responds by transmitting a 0 while the temperature
conversion is in progress and a 1 when the conversion is
done. In parasite-power mode, this notification technique
cannot be used since the bus is pulled high by the strong
pullup during the conversion.
Write Scratchpad [4Eh]
This command allows the master to write 3 bytes of data
to the device’s scratchpad. The first data byte is written
into the T
H
register (byte 2 of the scratchpad), the second
byte is written into the T
L
register (byte 3), and the third
byteiswrittenintotheconfigurationregister(byte4).Data
must be transmitted least significant bit first. All three
bytes must be written before the master issues a reset,
or the data may be corrupted.
Read Scratchpad [BEh]
This command allows the master to read the contents
of the scratchpad. The data transfer starts with the least
significant bit of byte 0 and continues through the scratch-
pad until the 9th byte (byte 8 – CRC) is read. The master
can issue a reset to terminate reading at any time if only
part of the scratchpad data is needed.
Copy Scratchpad [48h]
This command copies the contents of the scratchpad
T
H
, T
L
and configuration registers (bytes 2, 3, and 4) to
EEPROM.Ifthedeviceisbeingusedinparasitepower
mode, within 10µs (max) after this command is issued
the master must enable a strong pullup on the 1-Wire
bus for at least 10ms, as described in the Powering the
MAX31820 section.
Table 3. MAX31820 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 entire scratchpad
including the CRC byte.
BEh The device transmits up to 9 data bytes to master.
WriteScratchpad
(Note3)
Writes to scratchpad bytes 2, 3,
and 4 (T
H
, T
L
,andconguration
registers).
4Eh The master transmits 3 data bytes to the device.
CopyScratchpad
(Note1)
Copies T
H
, T
L
,andconguration
register data from the scratchpad to
EEPROM.
48h None.
Recall E
2
Recalls T
H
, T
L
,andconguration
registerdatafromEEPROMtothe
scratchpad.
B8h The device transmits recall status to the master.
ReadPowerSupply
Signalsthedevice’spower-supply
mode to the master.
B4h The device transmits supply status to the master.
MAX31820 1-Wire Ambient Temperature Sensor
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