Datasheet

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 request-
ed data. In addition, the master can generate read time
slots after issuing Convert T [44h] or Recall E
2
[B8h] com-
mands 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 60µs in duration
with a minimum of a 1µs recovery time between slots. A
read time slot is initiated by the master device pulling the
1-Wire bus low for a minimum of 1µs and then releasing
the bus (see Figure 16). 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 data from
the DS18B20 is valid for 15µs after the falling edge that
initiated the read time slot. Therefore, the master must
release the bus and then sample the bus state within
15µs from the start of the slot.
Figure 17 illustrates that the sum of T
INIT
, T
RC
, and
T
SAMPLE
must be less than 15µs for a read time slot.
Figure 18 shows that system timing margin is maximized
by keeping T
INIT
and T
RC
as short as possible and by
locating the master sample time during read time slots
towards the end of the 15µs period.
Figure 16. Read/Write Time Slot Timing Diagram
MASTER WRITE “0” SLOT
MASTER WRITE “1” SLOT
MASTER READ “0” SLOT MASTER READ “1” SLOT
60µs < Tx “0” < 120µs
START
OF SLOT
START
OF SLOT
1µs < T
REC
< ∞
1µs
MIN TYP MAX
DS18B20 SAMPLES
15µs 15µs 30µs
MIN TYP MAX
DS18B20 SAMPLES
15µs 15µs 30µs
V
PU
1-Wire BUS
GND
V
PU
1-Wire BUS
GND
1µs < T
REC
< ∞
15µs
> 1µs
MASTER SAMPLES
45
µs
MASTER SAMPLES
>
1µs
15µs
LINE TYPE LEGEND
RESISTOR PULLUPDS18B20 PULLING LOWBUS MASTER PULLING LOW
DS18B20 Programmable Resolution
1-Wire Digital Thermometer
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