Datasheet

Read-Time Slots
The DS18S20 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 DS18S20 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 DS18S20 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 13). After the master initiates the
read-time slot, the DS18S20 will begin transmitting a 1
or 0 on bus. The DS18S20 transmits a 1 by leaving the
bus high and transmits a 0 by pulling the bus low. When
transmitting a 0, the DS18S20 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 DS18S20 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 14 illustrates that the sum of T
INIT
, T
RC
, and
T
SAMPLE
must be less than 15µs for a read-time slot.
Figure 15 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 14. Detailed Master Read 1 Timing
Figure 15. Recommended Master Read 1 Timing
V
PU
GND
1-Wire BUS
MASTER SAMPLES
VIH OF MASTER
T
INT
> 1µs
T
RC
15µs
LINE TYPE LEGEND
BUS MASTER PULLING LOW
RESISTOR PULLUP
V
PU
GND
1-Wire BUS
VIH OF MASTER
T
RC
=
SMALL
MASTER SAMPLES
T
INT
=
SMALL
15µs
DS18S20 High-Precision 1-Wire Digital Thermometer
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