DS1820
PARASITE POWER
The block diagram (Figure 1) shows the parasite pow-
ered circuitry. This circuitry “steals” power whenever the
I/O or V
DD
pins are high. I/O will provide sufficient power
as long as the specified timing and voltage require-
ments are met (see the section titled “1–Wire Bus Sys-
tem”). The advantages of parasite power are two–fold:
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 DS1820 to be able to perform accurate
temperature conversions, sufficient power must be pro-
vided over the I/O line when a temperature conversion is
taking place. Since the operating current of the DS1820
is up to 1 mA, the I/O line will not have sufficient drive
due to the 5K pull–up resistor. This problem is particu-
larly acute if several DS1820’s are on the same I/O and
attempting to convert simultaneously.
There are two ways to assure that the DS1820 has suffi-
cient supply current during its active conversion cycle.
The first is to provide a strong pull–up on the I/O 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 I/O line directly to the power
supply as shown in Figure 2. The I/O line must be
switched over to the strong pull–up within 10
µs
maxi-
mum 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 DS1820 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 pull–up is not required on the I/O 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 DS1820’s 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 communica-
tions given the higher leakage currents the DS1820
exhibits at these temperatures. For applications in
which such temperatures are likely, it is strongly recom-
mended that V
DD
be applied to the DS1820.
For situations where the bus master does not know
whether the DS1820’s on the bus are parasite powered
or supplied with external V
DD
, a provision is made in the
DS1820 to signal the power supply scheme used. The
bus master can determine if any DS1820’s are on the
bus which require the strong pull–up by sending a Skip
ROM protocol, then issuing the read power supply com-
mand. After this command is issued, the master then
issues read time slots. The DS1820 will send back “0”
on the 1–Wire bus if it is parasite powered; it will send
back a “1” if it is powered from the V
DD
pin. If the master
receives a “0”, it knows that it must supply the strong
pull–up on the I/O line during temperature conversions.
See “Memory Command Functions” section for more
detail on this command protocol.
STRONG PULL–UP FOR SUPPLYING DS1820 DURING TEMPERATURE CONVERSION
Figure 2
+5V
DS1820
+5V
GND
4.7K
V
DD
µP
I/O
030598 3/27
DALLAS [ DALLAS SEMICONDUCTOR ]
