Datasheet

Note 1: For parasite-powered DS18S20s, the master must enable a strong pullup on the 1-Wire bus during temperature conver-
sions and copies from the scratchpad to EEPROM. No other bus activity may take place during this time.
Note 2: The master can interrupt the transmission of data at any time by issuing a reset.
Note 3: Both bytes must be written before a reset is issued.
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 will respond to the function command issued
by the master; all other slaves on the bus will 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
DS18S20s on the bus perform simultaneous temperature
conversions by issuing a Skip ROM command followed by
a Convert T [44h] command.
Note that the Read Scratchpad [BEh] command can fol-
low 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 will cause a data collision
on the bus if there is more than one slave since multiple
devices will attempt to transmit data simultaneously.
Alarm Search [ECh]
The operation of this command is identical to the operation
of the Search ROM command except that only slaves with
a set alarm flag will respond. This command allows the
master device to determine if any DS18S20s experienced
an alarm condition during the most recent temperature
conversion. After every Alarm Search cycle (i.e., Alarm
Search command followed by data exchange), the bus
master must return to Step 1 (Initialization) in the transac-
tion sequence. See the Operation—Alarm Signaling sec-
tion for an explanation of alarm flag operation.
DS18S20 Function Commands
After the bus master has used a ROM command to
address the DS18S20 with which it wishes to communi-
cate, the master can issue one of the DS18S20 function
commands. These commands allow the master to write
to and read from the DS18S20’s scratchpad memory,
initiate temperature conversions and determine the power
supply mode. The DS18S20 function commands, which
are described below, are summarized in Table 2 and illus-
trated by the flowchart in Figure 17.
Table 2. DS18S20 Function Command Set
COMMAND DESCRIPTION PROTOCOL
1-Wire BUS ACTIVITY AFTER
COMMAND IS ISSUED
NOTES
TEMPERATURE CONVERSION COMMANDS
Convert T
Initiates temperature
conversion.
44h
DS18S20 transmits conversion status
to master (not applicable for parasite-
powered DS18S20s).
1
MEMORY COMMANDS
Read Scratchpad
Reads the entire scratchpad
including the CRC byte.
BEh
DS18S20 transmits up to 9 data
bytes to master.
2
Write Scratchpad
Writes data into scratchpad
bytes 2 and 3 (T
H
and T
L
).
4Eh
Master transmits 2 data bytes to
DS18S20.
3
Copy Scratchpad
Copies T
H
and T
L
data from the
scratchpad to EEPROM.
48h None 1
Recall E
2
Recalls T
H
and T
L
data from
EEPROM to the scratchpad.
B8h
DS18S20 transmits recall status to
master.
Read Power
Supply
Signals DS18S20 power supply
mode to the master.
B4h
DS18S20 transmits supply status to
master.
DS18S20 High-Precision 1-Wire Digital Thermometer
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