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DS1990A
Serial Number iButton
1-Wire ROM Function Commands
BUS MASTER Tx
RESET PULSE
Once the bus master has detected a presence, it can
issue one of the ROM function commands the DS1990A
supports. All ROM function commands are 8 bits long.
A list of these commands follows. (See Figure 5 for a
BUS MASTER Tx ROM
FUNCTION COMMAND
DS1990A Tx
PRESENCE PULSE
flowchart.)
Read ROM [33h]
This command allows the bus master to read the
33h
READ ROM
COMMAND?
N
F0h
SEARCH ROM
COMMAND?
N
DS1990A’s 8-bit family code, unique 48-bit serial num-
ber, 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
Y
DS1990A Tx
FAMILY CODE
(1 BYTE)
DS1990A Tx
SERIAL NUMBER
(6 BYTES)
DS1990A Tx
CRC BYTE
Figure 5. ROM Functions Flowchart
6
N
N
N
Y
DS1990A Tx BIT 0
DS1990A Tx BIT 0
MASTER Tx BIT 0
BIT 0 MATCH?
Y
DS1990A Tx BIT 1
DS1990A Tx BIT 1
MASTER Tx BIT 1
BIT 1 MATCH?
Y
DS1990A Tx BIT 63
DS1990A Tx BIT 63
MASTER Tx BIT 63
BIT 63 MATCH?
Y
all slaves try to transmit at the same time (open drain pro-
duces 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 regis-
tration 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 sec-
ond slot, each slave device participating in the search
outputs the complemented value of its registration num-
ber 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 participat-
ing 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 com-
plete pass, the bus master knows the registration num-
ber of a single device. Additional passes identify the
registration numbers of the remaining 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
without 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.
Maxim Integrated
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