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DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
1-Wire Bus System
The 1-Wire bus is a system that has a single bus master
and one or more slaves. In all instances the
DS1922L/DS1922T are slave devices. The bus master
is typically a microcontroller. The discussion of this bus
system is broken down into three topics: hardware con-
figuration, transaction sequence, and 1-Wire signaling
(signal types and timing). The 1-Wire protocol defines
bus transactions in terms of the bus state during specif-
ic time slots that are initiated on the falling edge of sync
pulses from the bus master.
Hardware Configuration
The 1-Wire bus has only a single line by definition; it is
important that each device on the bus be able to drive
it at the appropriate time. To facilitate this, each device
attached to the 1-Wire bus must have open-drain or
three-state outputs. The 1-Wire port of the DS1922L/
DS1922T is open drain with an internal circuit equiva-
lent to that shown in Figure 10.
A multidrop bus consists of a 1-Wire bus with multiple
slaves attached. At standard speed the 1-Wire bus
has a maximum data rate of 16.3kbps. The speed can
be boosted to 142kbps by activating the overdrive
mode. The DS1922L/DS1922T are not guaranteed to
be fully compliant to the iButton device standard.
Their maximum data rate in standard speed is
15.4kbps and 125kbps in overdrive speed. The value
V PUP
BUS MASTER
R PUP
of the pullup resistor primarily depends on the network
size and load conditions. The DS1922L/DS1922T
require a pullup resistor of maximum 2.2k Ω at any
speed.
The idle state for the 1-Wire bus is high. If for any rea-
son a transaction needs to be suspended, the bus
must be left in the idle state if the transaction is to
resume. If this does not occur and the bus is left low for
more than 16μs (overdrive speed) or more than 120μs
(standard speed), one or more devices on the bus may
be reset. Note that the DS1922L/DS1922T do not quite
meet the full 16μs maximum low time of the normal
1-Wire bus overdrive timing. With the DS1922L/
DS1922T, the bus must be left low for no longer than
12μs at overdrive to ensure that no DS1922L/DS1922T
on the 1-Wire bus performs a reset. The DS1922L/
DS1922T communicate properly when used in conjunc-
tion with a DS2480B or DS2490 1-Wire driver and
adapters that are based on these driver chips.
Transaction Sequence
The protocol for accessing the DS1922L/DS1922T
through the 1-Wire port is as follows:
? Initialization
? ROM Function Command
? Memory/Control Function Command
? Transaction/Data
DS1922L/DS1922T 1-Wire PORT
Rx
DATA
Rx
Tx
Rx = RECEIVE
I L
Tx
Tx = TRANSMIT
Figure 10. Hardware Configuration
36
OPEN-DRAIN
PORT PIN
100 Ω MOSFET
Maxim Integrated
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