1-Wire ROM Function Commands
Once the bus master has detected a presence, it can
issue one of the eight ROM function commands that the
DS28EA00 supports. All ROM function commands are 8
bits long. A list of these commands follows (refer to the
flowchart in Figure 11).
Read ROM [33h]
This command allows the bus master to read the
DS28EA00’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 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 fami-
ly code and 48-bit serial number result in a mismatch of
the CRC.
Match ROM [55h]
The Match ROM command, followed by a 64-bit ROM
sequence, allows the bus master to address a specific
DS28EA00 on a multidrop bus. Only the DS28EA00 that
exactly matches the 64-bit ROM sequence responds to
the following Control Function command. All other
slaves wait for a reset pulse. This command can be
used with a single device or multiple devices on the
bus.
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. The Search
ROM command does not reveal any information about
the location of a device in a network. If multiple
DS28EA00 are wired as a linear network (“chain”), the
device location can be detected using Conditional
Read ROM in conjunction with the Chain function.
Conditional Search ROM [ECh]
The Conditional Search ROM command operates similar-
ly to the Search ROM command except that only those
devices which fulfill certain conditions, participate in the
search. This function provides an efficient means for the
bus master to identify devices on a multidrop system that
have to signal an important event. After each pass of the
conditional search that successfully determined the
64-bit ROM code for a specific device on the multidrop
bus, that particular device can be individually accessed
as if a Match ROM had been issued, since all other
devices have dropped out of the search process and are
waiting for a reset pulse. The DS28EA00 responds to the
Conditional Search ROM command if a temperature
alarm condition exists. For more details see the
Temperature Alarm Registers Bitmap
section.
Conditional Read ROM [0Fh]
This command is used in conjunction with the Chain
function to detect the physical sequence of devices in a
linear network (chain). A DS28EA00 responds to
Conditional Read ROM if two conditions are met: a) the
device is in chain ON state, and b) the EN input (PIOB)
is at logic 0. This condition is met by exactly one device
during the sequence discovery process. Upon receiv-
ing the Conditional Read ROM command, this particu-
lar device transmits its 64-bit registration number. A
device in chain ON state, but with a logic 1 level at EN
does not respond to Conditional Read ROM. See the
Sequence Discovery Procedure
section for more details
on the use of Conditional Read ROM and the Chain
commands.
1-Wire Digital Thermometer with
Sequence Detect and PIO
DS28EA00
18 Maxim Integrated
