Datasheet

1-Wire ROM Function Commands
Once the bus master has detected a presence, it can issue
one of the ROM function commands the DS1990A sup-
ports. All ROM function commands are 8 bits long. A list of
these commands follows. (See Figure 5 for a flowchart.)
Read ROM [33h]
This command allows the bus master to read the
DS1990A’s 8-bit family code, unique 48-bit serial number,
and 8-bit CRC. This command can only be used if there is
a single slave device on the bus. If more than one slave is
present on the bus, a data collision occurs when all slaves
try to transmit at the same time (open drain produces a
wired-AND result). The resultant family code and 48-bit
serial number results in a mismatch of the CRC.
Search ROM [F0h]
When a system is initially brought up, the bus master
might not know the number of devices on the 1-Wire
bus or their registration numbers. By taking advantage
of the wired-AND property of the bus, the master can
use a process of elimination to identify the registration
numbers of all slave devices. For each bit of the registra-
tion number, starting with the least significant bit, the bus
master issues a triplet of time slots. On the first slot, each
slave device participating in the search outputs the true
value of its registration number bit. On the second slot,
each slave device participating in the search outputs the
complemented value of its registration number bit. On
the third slot, the master writes the true value of the bit
to be selected. All slave devices that do not match the
bit written by the master stop participating in the search.
If both of the read bits are zero, the master knows that
slave devices exist with both states of the bit. By choosing
which state to write, the bus master branches in the ROM
code tree. After one complete pass, the bus master knows
the registration number of a single device. Additional
passes identify the registration numbers of the remain-
ing devices. Refer to
Application Note 187: 1-Wire Search
Algorithm
for a detailed discussion, including an example.
Match ROM [55h]/Skip ROM [CCh]
The minimum set of 1-Wire ROM function commands
includes a Match ROM and a Skip ROM command.
Because the DS1990A contains only the 64-bit ROM with-
out any additional data fields, Match ROM and Skip ROM
are not applicable. The DS1990A remains silent (inactive)
upon receiving a ROM function command that it does not
support. This allows the DS1990A to coexist on a multidrop
bus with other 1-Wire devices that do respond to Match
ROM or Skip ROM. DS1990AA will return a response after
MatchROM/Skip ROM. If a multidrop configuration exists
in the application, this should be taken into account.
Figure 5. ROM Functions Flowchart
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Analog Devices │ 6
DS1990A Serial Number iButton
DS1990A Tx
PRESENCE PULSE
BUS MASTER Tx
RESET PULSE
BUS MASTER Tx ROM
FUNCTION COMMAND
DS1990A Tx
CRC BYTE
DS1990A Tx
FAMILY CODE
(1 BYTE)
DS1990A Tx
SERIAL NUMBER
(6 BYTES)
Y
33h
READ ROM
COMMAND?
N
BIT 0 MATCH?
N
N
N
F0h
SEARCH ROM
COMMAND?
N
Y
Y
DS1990A Tx BIT 0
DS1990A Tx BIT 0
MASTER Tx BIT 0
Y
BIT 1 MATCH?
BIT 63 MATCH?
DS1990A Tx BIT 1
DS1990A Tx BIT 1
MASTER Tx BIT 1
DS1990A Tx BIT 63
DS1990A Tx BIT 63
MASTER Tx BIT 63
Y