DS1820
1–Wire
TM
DS1820
Digital Thermometer
FEATURES
PIN ASSIGNMENT
DALLAS
DALLAS
DS2434
DS1820
1
2
3
•
Unique 1–Wire
TM
interface requires only one port pin
for communication
•
Multidrop capability simplifies distributed temperature
sensing applications
•
Requires no external components
•
Can be powered from data line
•
Zero standby power required
•
Measures
temperatures from –55°C to +125°C in
0.5°C increments. Fahrenheit equivalent is –67°F to
+257°F in 0.9°F increments
BOTTOM VIEW
1
2 3
•
Temperature is read as a 9–bit digital value.
•
Converts temperature to digital word in 200 ms (typ.)
•
User–definable,
tings
nonvolatile temperature alarm set-
DS1820
PR35 PACKAGE
See Mech. Drawings
Section
GND
DQ
VDD
NC
NC
NC
NC
NC
NC
VDD
DQ
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
NC
NC
NC
NC
NC
NC
NC
GND
DS1820S
16–PIN SSOP
See Mech. Drawings
Section
•
Alarm
search command identifies and addresses
devices whose temperature is outside of pro-
grammed limits (temperature alarm condition)
systems, consumer products, thermometers, or any
thermally sensitive system
PIN DESCRIPTION
GND
DQ
V
DD
NC
–
–
–
–
Ground
Data In/Out
Optional V
DD
No Connect
•
Applications include thermostatic controls, industrial
DESCRIPTION
The DS1820 Digital Thermometer provides 9–bit tem-
perature readings which indicate the temperature of the
device.
Information is sent to/from the DS1820 over a 1–Wire
interface, so that only one wire (and ground) needs to be
connected from a central microprocessor to a DS1820.
Power for reading, writing, and performing temperature
conversions can be derived from the data line itself with
no need for an external power source.
Because each DS1820 contains a unique silicon serial
number, multiple DS1820s can exist on the same
1–Wire bus. This allows for placing temperature sen-
sors in many different places. Applications where this
feature is useful include HVAC environmental controls,
sensing temperatures inside buildings, equipment or
machinery, and in process monitoring and control.
030598 1/27
DS1820
DETAILED PIN DESCRIPTION
PIN
16–PIN SSOP
9
8
7
PIN
PR35
1
2
3
SYMBOL
GND
DQ
V
DD
Ground.
Data Input/Output pin.
For 1–Wire operation: Open drain. (See
“Parasite Power” section.)
Optional V
DD
pin.
See “Parasite Power” section for details of
connection.
DESCRIPTION
DS1820S (16–pin SSOP): All pins not specified in this table are not to be connected.
OVERVIEW
The block diagram of Figure 1 shows the major compo-
nents of the DS1820. The DS1820 has three main data
components: 1) 64–bit lasered ROM, 2) temperature
sensor, and 3) nonvolatile temperature alarm triggers
TH and TL. The device derives its power from the
1–Wire communication line by storing energy on an
internal capacitor during periods of time when the signal
line is high and continues to operate off this power
source during the low times of the 1–Wire line until it
returns high to replenish the parasite (capacitor) supply.
As an alternative, the DS1820 may also be powered
from an external 5 volts supply.
Communication to the DS1820 is via a 1–Wire port. With
the 1–Wire port, the memory and control functions will
not be available before the ROM function protocol has
been established. The master must first provide one of
five ROM function commands: 1) Read ROM, 2) Match
ROM, 3) Search ROM, 4) Skip ROM, or 5) Alarm
Search. These commands operate on the 64–bit
lasered ROM portion of each device and can single out
a specific device if many are present on the 1–Wire line
as well as indicate to the Bus Master how many and
what types of devices are present. After a ROM function
sequence has been successfully executed, the memory
and control functions are accessible and the master
may then provide any one of the six memory and control
function commands.
One control function command instructs the DS1820 to
perform a temperature measurement. The result of this
measurement will be placed in the DS1820’s scratch-
pad memory, and may be read by issuing a memory
function command which reads the contents of the
scratchpad memory. The temperature alarm triggers
TH and TL consist of one byte EEPROM each. If the
alarm search command is not applied to the DS1820,
these registers may be used as general purpose user
memory. Writing TH and TL is done using a memory
function command. Read access to these registers is
through the scratchpad. All data is read and written
least significant bit first.
DS1820 BLOCK DIAGRAM
Figure 1
MEMORY AND
CONTROL LOGIC
DQ
64–BIT ROM
AND
1–WIRE PORT
TEMPERATURE SENSOR
INTERNAL V
DD
SCRATCHPAD
HIGH TEMPERATURE
TRIGGER, TH
LOW TEMPERATURE
TRIGGER, TL
V
DD
POWER
SUPPLY
SENSE
8–BIT CRC
GENERATOR
030598 2/27
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
DS1820
USING V
DD
TO SUPPLY TEMPERATURE CONVERSION CURRENT
Figure 3
TO OTHER 1–WIRE
DEVICES
DS1820
+5V
4.7K
V
DD
µP
I/O
EXTERNAL +5V SUPPLY
OPERATION – MEASURING TEMPERATURE
The DS1820 measures temperature through the use of
an on–board proprietary temperature measurement
technique. A block diagram of the temperature mea-
surement circuitry is shown in Figure 4.
The DS1820 measures temperature by counting the
number of clock cycles that an oscillator with a low tem-
perature coefficient goes through during a gate period
determined by a high temperature coefficient oscillator.
The counter is preset with a base count that corre-
sponds to –55°C. If the counter reaches zero before the
gate period is over, the temperature register, which is
also preset to the –55°C value, is incremented, indicat-
ing that the temperature is higher than –55°C.
At the same time, the counter is then preset with a value
determined by the slope accumulator circuitry. This cir-
cuitry is needed to compensate for the parabolic behav-
ior of the oscillators over temperature. The counter is
then clocked again until it reaches zero. If the gate
period is still not finished, then this process repeats.
The slope accumulator is used to compensate for the
non–linear behavior of the oscillators over temperature,
yielding a high resolution temperature measurement.
This is done by changing the number of counts neces-
sary for the counter to go through for each incremental
degree in temperature. To obtain the desired resolution,
therefore, both the value of the counter and the number
of counts per degree C (the value of the slope accumu-
lator) at a given temperature must be known.
Internally, this calculation is done inside the DS1820 to
provide 0.5°C resolution. The temperature reading is
provided in a 16–bit, sign–extended two’s complement
reading. Table 1 describes the exact relationship of out-
put data to measured temperature. The data is trans-
mitted serially over the 1–Wire interface. The DS1820
can measure temperature over the range of –55°C to
+125°C in 0.5°C increments. For Fahrenheit usage, a
lookup table or conversion factor must be used.
Note that temperature is represented in the DS1820 in
terms of a
1
/
2
°C
LSB, yielding the following 9–bit format:
MSB
1
1
1
0
0
1
1
1
LSB
0
= –25°C
The most significant (sign) bit is duplicated into all of the
bits in the upper MSB of the two–byte temperature reg-
ister in memory. This “sign–extension” yields the 16–bit
temperature readings as shown in Table 1.
Higher resolutions may be obtained by the following
procedure. First, read the temperature, and truncate
the 0.5°C bit (the LSB) from the read value. This value is
TEMP_READ. The value left in the counter may then be
read. This value is the count remaining
(COUNT_REMAIN) after the gate period has ceased.
The last value needed is the number of counts per
degree C (COUNT_PER_C) at that temperature. The
actual temperature may be then be calculated by the
user using the following:
TEMPERATURE = TEMP_READ – 0.25
)
(COUNT_PER_C – COUNT_REMAIN)
COUNT_PER_C
030598 4/27
DS1820
TEMPERATURE MEASURING CIRCUITRY
Figure 4
SLOPE ACCUMULATOR
PRESET
COMPARE
LOW TEMPERATURE
COEFFICIENT OSCILLATOR
COUNTER
PRESET
SET/CLEAR
LSB
INC
=0
TEMPERATURE REGISTER
HIGH TEMPERATURE
COEFFICIENT OSCILLATOR
COUNTER
=0
STOP
TEMPERATURE/DATA RELATIONSHIPS
Table 1
TEMPERATURE
+125°C
+25°C
+
1/2
°C
+0°C
–
1
/
2
°C
–25°C
–55°C
DIGITAL OUTPUT
(Binary)
00000000 11111010
00000000 00110010
00000000 00000001
00000000 00000000
11111111 11111111
11111111 11001110
11111111 10010010
DIGITAL OUTPUT
(Hex)
00FA
0032h
0001h
0000h
FFFFh
FFCEh
FF92h
OPERATION – ALARM SIGNALING
After the DS1820 has performed a temperature conver-
sion, the temperature value is compared to the trigger
values stored in TH and TL. Since these registers are
8–bit only, the 0.5°C bit is ignored for comparison. The
most significant bit of TH or TL directly corresponds to
the sign bit of the 16–bit temperature register. If the
result of a temperature measurement is higher than TH
or lower than TL, an alarm flag inside the device is set.
This flag is updated with every temperature measure-
ment. As long as the alarm flag is set, the DS1820 will
respond to the alarm search command. This allows
many DS1820s to be connected in parallel doing simul-
taneous temperature measurements. If somewhere the
temperature exceeds the limits, the alarming device(s)
can be identified and read immediately without having to
read non–alarming devices.
030598 5/27
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