DS28EA00 Datasheet, PDF (17/29 Pages) Dallas Semiconductor – 1-Wire Digital Thermometer with Sequence Detect and PIO

1-Wire Digital Thermometer with
Sequence Detect and PIO
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 DS28EA00
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,
which 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 DS28EA00 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. The DS28EA00 supports both a stan-
dard and overdrive communication speed of 15.3kbps
(max) and 125kbps (max), respectively. Note that lega-
cy 1-Wire products support a standard communication
speed of 16.3kbps and overdrive of 142kbps. The
slightly reduced rates for the DS28EA00 are a result of
additional recovery times, which in turn are driven by a
1-Wire physical interface enhancement to improve
noise immunity. The value of the pullup resistor primari-
ly depends on the network size and load conditions.
The DS28EA00 requires a pullup resistor of 2.2kΩ
(max) 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 could be reset.
Transaction Sequence
The protocol for accessing the DS28EA00 through the
1-Wire port is as follows:
• Initialization
• ROM Function Command
• Control Function Command
• Transaction/Data
Initialization
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
DS28EA00 is on the bus and is ready to operate. For
more details, see the 1-Wire Signaling section.
BUS MASTER
Rx
Tx
OPEN-DRAIN
PORT PIN
V_PUP
R_PUP
Rx = RECEIVE
Tx = TRANSMIT
DS28EA00 1-Wire PORT
DATA
Rx
I_L
Tx
100Ω MOSFET
Figure 10. Hardware Configuration
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