DS18S20
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DS18S20 BLOCK DIAGRAM Figure 1
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power
whenever the DQ or V
DD
pins are high. DQ will provide sufficient power as long as the specified timing
and voltage requirements are met (see the section titled “1–Wire Bus System”). The advantages of
parasite power are twofold: 1) by parasiting off this pin, no local power source is needed for remote
sensing of temperature, and 2) the ROM may be read in absence of normal power.
In order for the DS18S20 to be able to perform accurate temperature conversions, sufficient power must
be provided over the DQ line when a temperature conversion is taking place. Since the operating current
of the DS18S20 is up to 1.5 mA, the DQ line will not have sufficient drive due to the 5k pullup resistor.
This problem is particularly acute if several DS18S20s are on the same DQ and attempting to convert
simultaneously.
There are two ways to assure that the DS18S20 has sufficient supply current during its active conversion
cycle. The first is to provide a strong pullup on the DQ line whenever temperature conversions or copies
to the E
2
memory are taking place. This may be accomplished by using a MOSFET to pull the DQ line
directly to the power supply as shown in Figure 2. The DQ line must be switched over to the strong
pull-up within 10 µs maximum after issuing any protocol that involves copying to the E
2
memory or
initiates temperature conversions. When using the parasite power mode, the V
DD
pin must be tied to
ground.
Another method of supplying current to the DS18S20 is through the use of an external power supply tied
to the V
DD
pin, as shown in Figure 3. The advantage to this is that the strong pullup is not required on the
DQ line, and the bus master need not be tied up holding that line high during temperature conversions.
This allows other data traffic on the 1–Wire bus during the conversion time. In addition, any number of
DS18S20s may be placed on the 1–Wire bus, and if they all use external power, they may all
simultaneously perform temperature conversions by issuing the Skip ROM command and then issuing the
Convert T command. Note that as long as the external power supply is active, the GND pin may not be
floating.
The use of parasite power is not recommended above 100°C, since it may not be able to sustain
communications given the higher leakage currents the DS18S20 exhibits at these temperatures. For
applications in which such temperatures are likely, it is strongly recommended that V
DD
be applied to the
DS18S20.
POWER
SUPPLY
SENSE
64-BIT ROM
AND
1-WIRE PORT
MEMORY AND
CONTROL LOGIC
SCRATCHPAD
8-BIT CRC
GENERATOR
TEMPERATURE
SENSOR
HIGH TEMPERATURE
TRIGGER, TH
LOW TEMPERATURE
TRIGGER, TH
DQ
V
DD
INTERNAL V
