Datasheet

or equal to T
L
or higher than or equal to T
H
, an alarm con-
dition exists and an alarm flag is set inside the DS18B20.
This flag is updated after every temperature measure-
ment; therefore, if the alarm condition goes away, the flag
will be turned off after the next temperature conversion.
The master device can check the alarm flag status of
all DS18B20s on the bus by issuing an Alarm Search
[ECh] command. Any DS18B20s with a set alarm flag will
respond to the command, so the master can determine
exactly which DS18B20s have experienced an alarm
condition. If an alarm condition exists and the T
H
or T
L
settings have changed, another temperature conversion
should be done to validate the alarm condition.
Powering the DS18B20
The DS18B20 can be powered by an external supply on
the V
DD
pin, or it can operate in “parasite power” mode,
which allows the DS18B20 to function without a local
external supply. Parasite power is very useful for applica-
tions that require remote temperature sensing or that are
very space constrained. Figure 3 shows the DS18B20’s
parasite-power control circuitry, which “steals” power from
the 1-Wire bus via the DQ pin when the bus is high. The
stolen charge powers the DS18B20 while the bus is high,
and some of the charge is stored on the parasite power
capacitor (C
PP
) to provide power when the bus is low.
When the DS18B20 is used in parasite power mode, the
V
DD
pin must be connected to ground.
In parasite power mode, the 1-Wire bus and CPP can pro-
vide sufficient current to the DS18B20 for most operations
as long as the specified timing and voltage requirements
are met (see the DC Electrical Characteristics and AC
Electrical Characteristics). However, when the DS18B20
is performing temperature conversions or copying data
from the scratchpad memory to EEPROM, the operating
current can be as high as 1.5mA. This current can cause
an unacceptable voltage drop across the weak 1-Wire
pullup resistor and is more current than can be supplied
by C
PP
. To assure that the DS18B20 has sufficient supply
current, it is necessary to provide a strong pullup on the
1-Wire bus whenever temperature conversions are tak-
ing place or data is being copied from the scratchpad to
EEPROM. This can be accomplished by using a MOSFET
to pull the bus directly to the rail as shown in Figure 6. The
1-Wire bus must be switched to the strong pullup within
10µs (max) after a Convert T [44h] or Copy Scratchpad
[48h] command is issued, and the bus must be held high
by the pullup for the duration of the conversion (t
CONV
)
or data transfer (t
WR
= 10ms). No other activity can take
place on the 1-Wire bus while the pullup is enabled.
The DS18B20 can also be powered by the conventional
method of connecting an external power supply to the
V
DD
pin, as shown in Figure 7. The advantage of this
method is that the MOSFET pullup is not required, and
the 1-Wire bus is free to carry other traffic during the tem-
perature conversion time.
The use of parasite power is not recommended for tem-
peratures above +100°C since the DS18B20 may not be
able to sustain communications due to the higher leak-
age currents that can exist at these temperatures. For
applications in which such temperatures are likely, it is
strongly recommended that the DS18B20 be powered by
an external power supply.
In some situations the bus master may not know whether
the DS18B20s on the bus are parasite powered or pow-
ered by external supplies. The master needs this informa-
tion to determine if the strong bus pullup should be used
during temperature conversions. To get this information,
the master can issue a Skip ROM [CCh] command fol-
lowed by a Read Power Supply [B4h] command followed
by a “read time slot”. During the read time slot, parasite
powered DS18B20s will pull the bus low, and externally
powered DS18B20s will let the bus remain high. If the
bus is pulled low, the master knows that it must supply
the strong pullup on the 1-Wire bus during temperature
conversions.
Figure 6. Supplying the Parasite-Powered DS18B20 During
Temperature Conversions
Figure 7. Powering the DS18B20 with an External Supply
V
PU
4.7kΩ
V
PU
1-Wire BUS
DS18B20
GND DQ V
DD
TO OTHER
1-Wire DEVICES
µP
V
PU
4.7kΩ
1-Wire BUS
DS18B20
GND DQ V
DD
TO OTHER
1-Wire DEVICES
µP
V
DD
(EXTERNAL
SUPPLY)
DS18B20 Programmable Resolution
1-Wire Digital Thermometer
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