DS1921G-F5# Maxim Integrated Products, DS1921G-F5# Datasheet - Page 28

• Manufacturer Part Number: DS1921G-F5#
• Description: IBUTTON THERMOCHRON F5
• Manufacturer: Maxim Integrated Products
• Series: iButton®r

Specifications of DS1921G-F5#

• Rohs Information: IButton RoHS Compliance Plan
• Memory Size: 512B
• Memory Type: NVSRAM (Non-Volatile SRAM)
• Ic Output Type: Digital
• Sensing Accuracy Range: ± 1.3°C
• Supply Current: 10µA
• Supply Voltage Range: 2.8V To 5.25V
• Termination Type: Quick Connect
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Min: 2.8V
• Rohs Compliant: Yes
• Filter Terminals: Quick Connect
• Accuracy %: 1°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Thermochron iButton
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
DS1921G 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 seven ROM function commands. All
ROM function commands are 8 bits long. A list of these
commands follows (see the flowchart in Figure 13).
This command allows the bus master to read the
DS1921G’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.
The Match ROM command, followed by a 64-bit ROM
sequence, allows the bus master to address a specific
DS1921G on a multidrop bus. Only the DS1921G that
exactly matches the 64-bit ROM sequence responds to
the 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
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
28
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ROM Function Commands
Search ROM [F0h]
Match ROM [55h]
Read ROM [33h]
Initialization
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
devices fulfilling the specified condition participate in
the search. The condition is specified by the bit func-
tions TAS, THS, and TLS in the Control register,
address 20Eh. The Conditional Search ROM provides
an efficient means for the bus master to determine
devices on a multidrop system that have to signal an
important event, such as a temperature leaving the tol-
erance band. 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
command had been issued, since all other devices
have dropped out of the search process and are wait-
ing for a reset pulse.
For the conditional search, one can select any combi-
nation of the three search conditions by writing the
associated bit to a logical 1. These bits correspond
directly to the flags in the Status register of the device.
If the flag in the Status register reads 1 and the corre-
sponding bit in the Control register is a logical 1 too,
the device responds to the Conditional Search ROM
command. If more than one bit search condition is
selected, the first event that occurs makes the device
respond to the Conditional Search ROM command.
This command can save time in a single-drop bus sys-
tem by allowing the bus master to access the memory
functions without providing the 64-bit ROM code. If
more than one slave is present on the bus and, for
example, a read command is issued following the Skip
ROM command, data collision occurs on the bus as
multiple slaves transmit simultaneously (open-drain
pulldowns produce a wired-AND result).
Conditional Search ROM [ECh]
Skip ROM [CCh]