Datasheet

DS1822-PAR
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learns the ROM codes through a process of elimination that requires the master to perform a Search ROM
cycle (i.e., Search ROM command followed by data exchange) as many times as necessary to identify all
of the slave devices. If there is only one slave on the bus, the simpler Read ROM command (see below)
can be used in place of the Search ROM process. For a detailed explanation of the Search ROM
procedure, refer to the iButton
®
Book of Standards at www.ibutton.com/ibuttons/standard.pdf. After
every Search ROM cycle, the bus master must return to Step 1 (Initialization) in the transaction sequence.
READ ROM [33h]
This command can only be used 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 procedure. If this command is used when there
is more than one slave present on the bus, a data collision will occur 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 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 DS1822-PARs 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 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 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 DS1822-PARs 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 transaction sequence. Refer to the OPERATION—ALARM
SIGNALING section for an explanation of alarm flag operation.
DS1822-PAR FUNCTION COMMANDS
After the bus master has used a ROM command to address the DS1822-PAR with which it wishes to
communicate, the master can issue one of the DS1822-PAR function commands. These commands allow
the master to write to and read from the DS1822-PAR’s scratchpad memory, initiate temperature
conversions and determine the power supply mode. The DS1822-PAR function commands, which are
described below, are summarized in Table 4 and illustrated by the flowchart in Figure 10.
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 scratchpad memory and the DS1822-PAR returns to
its low-power idle state. Within 10 μs (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 PARASITE
POWER section.
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