DS18B20
16 of 22
The DS18B20 samples the 1-Wire bus during a window that lasts from 15祍 to 60祍 after the master
initiates the write time slot. If the bus is high during the sampling window, a 1 is written to the DS18B20.
If the line is low, a 0 is written to the DS18B20.
Figure 14. Read/Write Time Slot Timing Diagram
READ TIME SLOTS
The DS18B20 can only transmit data to the master when the master issues read time slots. Therefore, the
master must generate read time slots immediately after issuing a Read Scratchpad [BEh] or Read Power
Supply [B4h] command, so that the DS18B20 can provide the requested data. In addition, the master can
generate read time slots after issuing Convert T [44h] or Recall E
2
[B8h] commands to find out the status
of the operation as explained in the DS18B20 Function Commands section.
All read time slots must be a minimum of 60祍 in duration with a minimum of a 1祍 recovery time
between slots. A read time slot is initiated by the master device pulling the 1-Wire bus low for a
minimum of 1祍 and then releasing the bus (see Figure 14). After the master initiates the read time slot,
the DS18B20 will begin transmitting a 1 or 0 on bus. The DS18B20 transmits a 1 by leaving the bus high
and transmits a 0 by pulling the bus low. When transmitting a 0, the DS18B20 will release the bus by the
end of the time slot, and the bus will be pulled back to its high idle state by the pullup resister. Output
45?/SPAN>s
15?/SPAN>s
V
PU
GND
1-WIRE BUS
60 s < T
X
0 < 120 s
1?/SPAN>s < T
REC
<
DS18B20 Samples
MIN
TYP
MAX
15 s
30 s
> 1?/SPAN>s
MASTER WRITE 0 SLOT
MASTER WRITE 1 SLOT
V
PU
GND
1-WIRE BUS
15?/SPAN>s
MASTER READ 0 SLOT
MASTER READ 1 SLOT
Master samples
Master samples
START
OF SLOT
START
OF SLOT
> 1 ?/SPAN>s
1?/SPAN>s < T
REC
<
15 s
15?/SPAN>s
30?/SPAN>s
15?/SPAN>s
DS18B20 Samples
MIN TYP MAX
LINE TYPE LEGEND
Bus master pulling low
DS18B20 pulling low
Resistor pullup
> 1?/SPAN>s
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