DS18S20-PAR
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It is very important to follow the transaction sequence every time the DS18S20-PAR is accessed, as the
DS18S20-PAR will not respond if any steps in the sequence are missing or out of order. Exceptions to
this rule are the Search ROM [F0h] and Alarm Search [ECh] commands. After issuing either of these
ROM commands, the master must return to Step 1 in the sequence.
INITIALIZATION
All transactions on the 1-wire bus begin with an initialization sequence. The initialization sequence
consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the
slave(s). The pr esence p ulse lets the bus master k now that sl ave devices (such as t he DS18S 20-PAR) are
on the bus and are ready to operate. Timing for the reset and presence pulses is detailed in the
1-WIRE SIGNALING section.
ROM COMMANDS
After the bus master has detected a presence pulse, it can issue a ROM command. These commands
operate on the unique 64–bit ROM codes of each slave device and allow the master to single out a
specific device if many are present on the 1-wire bus. These commands also allow the master to
determine how man y and what types of devices are present on the bus or if an y device has experienc ed an
alarm condition. There are fiv e ROM commands, and each command is 8 bits long. The master device
must issue an appropriate ROM command before issuing a DS18S20-PAR function command. A
flowchart for operation of the ROM commands is shown in Figure 9.
SEARCH ROM [F0h]
When a system is initially powered up, the master must identify the ROM codes of all slave devices on
the bus, which allows the master to determine the number of slaves and their device types. The master
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 t he bus, the simpler Read ROM command (see b elow)
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 onl y be used when t here is one slave on the bus. It allows the bus master t o read the
slave’s 64-bit ROM code without using the Sear ch 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 b y a 64–bit ROM code sequence allows the bus master to address a
specific slave device on a multi-drop 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 DS18S20-PARs on the bus perform
simultaneous temperature conversions by issuing a Skip ROM command followed by a Convert T [44h]
command. Note, however, that the Skip ROM command can only be followed by the Read Scratchpad
[BEh] command when there is one slave on the bus. This sequence saves time by allowing the master to