DS1922T-F5# Maxim Integrated Products, DS1922T-F5# Datasheet - Page 37

• Manufacturer Part Number: DS1922T-F5#
• Description: IBUTTON TEMP LOGGER
• Manufacturer: Maxim Integrated Products
• Series: iButton®r

Specifications of DS1922T-F5#

• Rohs Information: IButton RoHS Compliance Plan
• Memory Size: 512B
• Memory Type: NVSRAM (Non-Volatile SRAM)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

All transactions on the 1-Wire bus begin with an initializa-
tion 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 pres-
ence pulse lets the bus master know that the
DS1922L/DS1922T are on the bus and are ready to oper-
ate. 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
DS1922L/DS1922T support. All ROM function com-
mands 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
DS1922L/DS1922T’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 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 resul-
tant 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
DS1922L/DS1922T on a multidrop bus. Only the
DS1922L/DS1922T that exactly matches the 64-bit
ROM sequence responds to the following memory func-
tion 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
1-Wire ROM Function Commands
______________________________________________________________________________________
Temperature Logger iButton with 8KB
Search ROM [F0h]
Match ROM [55h]
Read ROM [33h]
Initialization
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 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 DS1922L/DS1922T respond to the conditional
search ROM command if one of the three alarm flags of
the Alarm Status register (address 0214h) reads 1. The
temperature 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 com-
mand.
Data-Log Memory
Conditional Search ROM [ECh]
37