DS1822.PDF

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PRELIMINARY
DS1822
Econo-MicroLAN Digital Thermometer
www.dalsemi.com
FEATURES
Unique 1–Wire 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. Power supply
range is 3.0V to 5.5V
Zero standby power required
Measures temperatures from –55°C to
+125°C. Fahrenheit equivalent is –67°F to
+257°F
±0.5°C
accuracy from –10°C to +85°C
Thermometer resolution is programmable
from 9 to 12 bits
Converts 12-bit temperature to digital word in
500 ms (max.)
User–definable, temperature alarm settings
Alarm search command identifies and
addresses devices whose temperature is
outside of programmed limits (temperature
alarm condition)
Software compatible with DS18B20 if
temperature alarm setting volatility is not a
necessity
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system
TM
PIN ASSIGNMENT
BOTTOM VIEW
DS1822 TO-92 PACKAGE
See Mech Drawings Section
GND
DQ
VDD
NC
NC
VDD
DQ
1
2
3
4
8
7
6
5
NC
NC
NC
GND
DS1822Z
8-PIN SOIC (150-MIL)
PIN DESCRIPTION
GND
DQ
V
DD
NC
- Ground
- Data In/Out
- Power Supply Voltage
- No Connect
DESCRIPTION
The DS1822 Digital Thermometer provides 9 to 12 –bit temperature readings which indicate the
temperature of the device.
Information is sent to/from the DS1822 over a 1–Wire interface, so that only one wire (and ground) needs
to be connected from a central microprocessor to a DS1822. 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 DS1822 contains a unique silicon serial number, multiple DS1822s can exist on the same
1–Wire bus. This allows for placing temperature sensors in many different places. Applications where
this feature is useful include HVAC environmental controls, sensing temperatures inside buildings,
equipment or machinery, and process monitoring and control.
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112099
DS1822
DETAILED PIN DESCRIPTION
DESCRIPTION
Ground.
Data Input/Output pin.
For 1-Wire operation: Open drain.
(See “Parasite Power” section.)
3
3
V
DD
Optional V
DD
pin.
See “Parasite Power” section for details of
connection.
DS1822Z (8-pin SOIC): All pins not specified in this table are not to be connected.
PIN
8PIN SOIC
5
4
PIN
TO92
1
2
SYMBOL
GND
DQ
OVERVIEW
The block diagram of Figure 1 shows the major components of the DS1822. The DS1822 has four main
data components: 1) 64–bit lasered ROM, 2) temperature sensor, 3) temperature alarm triggers TH and
TL, and a configuration register. 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 DS1822 may also be powered from an external 3V -
5V supply.
Communication to the DS1822 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 DS1822 to perform a temperature measurement. The result
of this measurement will be placed in the DS1822’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 1 byte SRAM each. If the alarm search command is not applied to the
DS1822, these registers may be used as general purpose user memory. The scratchpad also contains a
configuration byte to set the desired resolution of the temperature to digital conversion. Writing TH, TL,
and the configuration byte 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.
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DS1822
DS1822 BLOCK DIAGRAM
Figure 1
MEMORY AND
CONTROL LOGIC
DQ
INTERNAL V
DD
64-BIT ROM
AND
1-WIRE PORT
SCRATCHPAD
TEMPERATURE
SENSOR
HIGH TEMPERATURE
TRIGGER, TH
POWER
SUPPLY
SENSE
LOW TEMPERATURE
TRIGGER, TH
V
DD
8-BIT CRC
GENERATOR
CONFIGURATION
REGISTER
PARASITE POWER
The block diagram (Figure 1) shows the parasite-powered 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 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 DS1822 to be able to perform accurate temperature conversions, sufficient power must be
provided over the I/O line when a temperature conversion is taking place. Since the operating current of
the DS1822 is up to 1.5 mA, the I/O line will not have sufficient drive due to the 5K pullup resistor. This
problem is particularly acute if several DS1822s are on the same I/O and attempting to convert
simultaneously.
There are two ways to assure that the DS1822 has sufficient supply current during its active conversion
cycle. The first is to provide a strong pullup on the I/O line whenever temperature conversions 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
maximum after
issuing any protocol that involves initiating temperature conversions. When using the parasite power
mode, the V
DD
pin must be tied to ground.
Another method of supplying current to the DS1822 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
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
DS1822s 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 DS1822 exhibits at these temperatures. For
applications in which such temperatures are likely, it is strongly recommended that V
DD
be applied to the
DS1822.
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DS1822
For situations where the bus master does not know whether the DS1822s on the bus are parasite powered
or supplied with external V
DD
, a provision is made in the DS1822 to signal the power supply scheme
used. The bus master can determine if any DS1822s are on the bus which require the strong pullup by
sending a Skip ROM protocol, then issuing the read power supply command. After this command is
issued, the master then issues read time slots. The DS1822 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 pullup 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 DS1822 DURING TEMPERATURE
CONVERSION
Figure 2
+3V - +5V
DS1822
+3V - +5V
µ
P
GND
4.7K
I/O
V
DD
USING V
DD
TO SUPPLY TEMPERATURE CONVERSION CURRENT
Figure 3
TO OTHER
1-WIRE DEVICES
DS1822
+3V - +5V
GND
4.7K
V
DD
EXTERNAL +3V - +5V
SUPPLY
µ
P
I/O
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DS1822
OPERATION – MEASURING TEMPERATURE
The core functionality of the DS1822 is its direct-to-digital temperature sensor. The resolution of the
DS1822 is configurable (9, 10, 11, or 12 bits), with 12-bit readings the power-up default state. This
equates to a temperature resolution of 0.5°C, 0.25°C, 0.125°C, or 0.0625°C. Following the issuance of
the Convert T [44h] command, a temperature conversion is performed and the thermal data is stored in
the scratchpad memory in a 16-bit, sign-extended two’s complement format. The temperature
information can be retrieved over the 1-Wire™ interface by issuing a Read Scratchpad [BEh] command
once the conversion has been performed. The data is transferred over the 1-Wire™ bus, LSB first. The
MSB of the temperature register contains the “sign” (S) bit, denoting whether the temperature is positive
or negative.
Table 2 describes the exact relationship of output data to measured. The table assumes 12-bit resolution.
If the DS1822 is configured for a lower resolution, insignificant bits will contain zeros. For Fahrenheit
usage, a lookup table or conversion routine must be used.
Temperature/Data Relationships
Table 2
2
3
MSb
S
S
S
2
2
2
1
2
0
2
-1
2
-2
2
-3
2
-4
(unit =
°C)
S
S
2
6
2
5
LSb
2
4
MSB
LSB
TEMPERATURE
DIGITAL OUTPUT
(Binary)
0000 0111 1101 0000
0000 0001 1001 0001
0000 0000 1010 0010
0000 0000 0000 1000
0000 0000 0000 0000
1111 1111 1111 1000
1111 1111 0101 1110
1111 1110 0110 1111
1111 1100 1001 0000
+125°C
+25.0625°C
+10.125°C
+0.5°C
0°C
-0.5°C
-10.125°C
-25.0625°C
-55°C
DIGITAL
OUTPUT
(Hex)
07D0h
0191h
00A2h
0008h
0000h
FFF8h
FF5Eh
FF6Fh
FC90h
OPERATION – ALARM SIGNALING
After the DS1822 has performed a temperature conversion, the temperature value is compared to the
trigger values stored in TH and TL. Since these registers are 8–bit only, bits 9-12 are 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 measurement. As long as
the alarm flag is set, the DS1822 will respond to the alarm search command. This allows many DS1822s
to be connected in parallel doing simultaneous 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.
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