DS1922E-F5 MAXIM [Maxim Integrated Products], DS1922E-F5 Datasheet - Page 30

• Manufacturer Part Number: DS1922E-F5
• Description: High-Temperature Logger iButton with 8KB Data-Log Memory
• Manufacturer: MAXIM [Maxim Integrated Products]

All transactions on the 1-Wire bus begin with an initial-
ization 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
presence pulse lets the bus master know that the
DS1922E is on the bus and is ready to operate. For
more details, see the 1-Wire Signaling section.
Once the bus master has detected a presence, it can
issue one of the eight ROM function commands that the
DS1922E supports. All ROM function commands are 8
bits long. A list of these commands follows (see the
flowchart in Figure 11).
This command allows the bus master to read the
DS1922E’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 on the bus. If more than one slave is pre-
sent 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.
The Match ROM command, followed by a 64-bit ROM
sequence, allows the bus master to address a specific
DS1922E on a multidrop bus. Only the DS1922E that
exactly matches the 64-bit ROM sequence responds to
the following memory 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.
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
High-Temperature Logger iButton with 8KB
Data-Log Memory
30
1-Wire ROM Function Commands
______________________________________________________________________________________
Search ROM [F0h]
Match ROM [55h]
Read ROM [33h]
Initialization
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 Conditional Search ROM command operates simi-
larly to the Search ROM command except that only
those devices that 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 indi-
vidually 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 DS1922E responds to the Conditional Search ROM
command if one of the three alarm flags of the Alarm
Status register (address 0214h) reads 1. The tempera-
ture alarm only occurs if enabled (see the Temperature
Sensor Alarm section). The BOR alarm is always
enabled. The first alarm that occurs makes the device
respond to the Conditional Search ROM command.
Conditional Search ROM [ECh]