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DS1923
iButton Hygrochron Temperature/Humidity
Logger with 8KB Data-Log Memory
1-Wire Bus System
The 1-Wire bus is a system that has a single bus mas-
ter and one or more slaves. In all instances the DS1923
is a slave device. The bus master is typically a micro-
controller. The discussion of this bus system is broken
down into three topics: hardware configuration, trans-
action sequence, and 1-Wire signaling (signal types
and timing). The 1-Wire protocol defines bus transac-
tions in terms of the bus state during specific 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 DS1923 is
open drain with an internal circuit equivalent 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 DS1923 is not guaranteed to be fully compliant to
the iButton device standard. Its maximum data rate in
standard speed is 15.4kbps and 125kbps in overdrive
V PUP
BUS MASTER
R PUP
speed. The value of the pullup resistor primarily
depends on the network size and load conditions. The
DS1923 requires 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 DS1923 does not quite meet the
full 16μs maximum low time of the normal 1-Wire bus
overdrive timing. With the DS1923 the bus must be left
low for no longer than 12μs at overdrive to ensure that
no DS1923 on the 1-Wire bus performs a reset. The
DS1923 communicates 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 DS1923 through the
1-Wire port is as follows:
? Initialization
? ROM Function Command
? Memory/Control Function Command
? Transaction/Data
DS1923 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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