DS18B20
1-WIRE SIGNALING
The DS18B20 uses a strict 1-Wire communication protocol to ensure data integrity. Several signal types
are defined by this protocol: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. The bus
master initiates all these signals, with the exception of the presence pulse.
INITIALIZATION PROCEDURE—RESET AND PRESENCE PULSES
All communication with the DS18B20 begins with an initialization sequence that consists of a reset pulse
from the master followed by a presence pulse from the DS18B20. This is illustrated in Figure 13. When
the DS18B20 sends the presence pulse in response to the reset, it is indicating to the master that it is on
the bus and ready to operate.
During the initialization sequence the bus master transmits (TX) the reset pulse by pulling the 1-Wire bus
low for a minimum of 480μs. The bus master then releases the bus and goes into receive mode (RX).
When the bus is released, the 5kΩ pullup resistor pulls the 1-Wire bus high. When the DS18B20 detects
this rising edge, it waits 15μs to 60μs and then transmits a presence pulse by pulling the 1-Wire bus low
for 60μs to 240μs.
Figure 13. Initialization Timing
MASTER TX RESET PULSE
MASTER RX
480μs minimum
480μs minimum
DS18B20 TX
presence pulse
DS18B20
waits 15-60μs
60-240μs
VPU
1-WIRE BUS
GND
LINE TYPE LEGEND
Bus master pulling low
DS18B20 pulling low
Resistor pullup
READ/WRITE TIME SLOTS
The bus master writes data to the DS18B20 during write time slots and reads data from the DS18B20
during read time slots. One bit of data is transmitted over the 1-Wire bus per time slot.
WRITE TIME SLOTS
There are two types of write time slots: “Write 1” time slots and “Write 0” time slots. The bus master
uses a Write 1 time slot to write a logic 1 to the DS18B20 and a Write 0 time slot to write a logic 0 to the
DS18B20. All write time slots must be a minimum of 60μs in duration with a minimum of a 1μs recovery
time between individual write slots. Both types of write time slots are initiated by the master pulling the
1-Wire bus low (see Figure 14).
To generate a Write 1 time slot, after pulling the 1-Wire bus low, the bus master must release the 1-Wire
bus within 15μs. When the bus is released, the 5kΩ pullup resistor will pull the bus high. To generate a
Write 0 time slot, after pulling the 1-Wire bus low, the bus master must continue to hold the bus low for
the duration of the time slot (at least 60μs).
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DALLAS [ DALLAS SEMICONDUCTOR ]
