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061002
FEATURES
§ Unique 1-Wire
®
interface requires only one
port pin for communication.
§ Each device has a unique 64-bit serial code
stored in an on-board ROM.
§ 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.
§ Measures temperatures from -55°C to +125°C
(-67°F to +257°F).
§ ±2.0°C accuracy from -10°C to +85°C.
§ Thermometer resolution is user-selectable
from 9 to 12 bits.
§ Converts temperature to 12-bit digital word in
750ms (max.)
§ User-definable nonvolatile (NV) alarm
settings.
§ Alarm search command identifies and
addresses devices whose temperature is
outside of programmed limits (temperature
alarm condition).
§ Software compatible with the DS18B20.
§ Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system.
PIN ASSIGNMENT
PIN DESCRIPTION
GND - Ground
DQ
- Data In/Out
V
DD
- Power Supply Voltage
NC
- No Connect
DESCRIPTION
The DS1822 digital thermometer provides 9- to 12-bit centigrade temperature measurements and has an
alarm function with NV user-programmable upper and lower trigger points. The DS1822 communicates
over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a
central microprocessor. It has an operating temperature range of 55°C to +125°C and is accurate to
±2.0°C over the range of 10°C to +85°C. In addition, the DS1822 can derive power directly from the
data line ("parasite power"), eliminating the need for an external power supply.
Each DS1822 has a unique 64-bit serial code, which allows multiple DS1822s to function on the 1-Wire
bus; thus, it is simple to use one microprocessor to control many DS1822s distributed over a large area.
Applications that can benefit from this feature include HVAC environmental controls, temperature
monitoring systems inside buildings, equipment or machinery, and process monitoring and control
systems.
1
(BOTTOM VIEW)
2 3
DS1822
Econo 1-Wire Digital
Thermometer
www.maxim-ic.com
8-Pin 150mil SO
(DS1822Z)
TO-92
(DS1822)
DALLAS
1822
1
GND
DQ
V
DD
2 3
NC
NC
NC
NC
GND
DQ
V
DD
NC
6
8
7
5
3
1
2
4
D
S
182
2
1-Wire is a registered trademark of Dallas Semiconductor.
DS1822
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DETAILED PIN DESCRIPTIONS Table 1
8-PIN SO*
TO-92 SYMBOL DESCRIPTION
5
1
GND
Ground.
4
2
DQ
Data Input/Output pin. Open-drain 1-Wire interface pin. Also
provides power to the device when used in parasite power mode
(see "Parasite Power" section).
3
3
V
DD
Optional V
DD
pin. V
DD
must be grounded for operation in
parasite power mode.
*All pins not specified in this table are "No Connect" pins.
OVERVIEW
Figure 1 shows a block diagram of the DS1822, and pin descriptions are given in Table 1. The 64-bit
ROM stores the device's unique serial code. The scratchpad memory contains the 2-byte temperature
register that stores the digital output from the temperature sensor. In addition, the scratchpad provides
access to the 1-byte upper and lower alarm trigger registers (T
H
and T
L
), and the 1-byte configuration
register. The configuration register allows the user to set the resolution of the temperature-to-digital
conversion to 9, 10, 11, or 12 bits. The T
H
, T
L
and configuration
registers are NV (EEPROM), so they will
retain data when the device is powered down.
The DS1822 uses Dallas' exclusive 1-Wire bus protocol that implements bus communication using one
control signal. The control line requires a weak pullup resistor since all devices are linked to the bus via a
3-state or open-drain port (the DQ pin in the case of the DS1822). In this bus system, the microprocessor
(the master device) identifies and addresses devices on the bus using each device's unique 64-bit code.
Because each device has a unique code, the number of devices that can be addressed on one bus is
virtually unlimited. The 1-Wire bus protocol, including detailed explanations of the commands and "time
slots," is covered in the 1-WIRE BUS SYSTEM section of this data sheet.
Another feature of the DS1822 is the ability to operate without an external power supply. Power is instead
supplied through the 1-Wire pullup resistor via the DQ pin when the bus is high. The high bus signal also
charges an internal capacitor (C
PP
), which then supplies power to the device when the bus is low. This
method of deriving power from the 1-Wire bus is referred to as "parasite power." As an alternative, the
DS1822 may also be powered by an external supply on V
DD
.
DS1822 BLOCK DIAGRAM Figure 1
V
PU
4.7K
POWER
SUPPLY
SENSE
64-BIT ROM
AND
1-wire PORT
DQ
V
DD
INTERNAL V
DD
C
PP
PARASITE POWER
CIRCUIT
MEMORY CONTROL
LOGIC
SCRATCHPAD
8-BIT CRC GENERATOR
TEMPERATURE SENSOR
ALARM HIGH TRIGGER (T
H
)
REGISTER (EEPROM)
ALARM LOW TRIGGER (T
L
)
REGISTER (EEPROM)
CONFIGURATION REGISTER
(EEPROM)
GND
DS1822
DS1822
3 of 20
OPERATION--MEASURING TEMPERATURE
The core functionality of the DS1822 is its direct-to-digital temperature sensor. The resolution of the
temperature sensor is user-configurable to 9, 10, 11, or 12 bits, corresponding to increments of 0.5
°C,
0.25
°C, 0.125°C, and 0.0625°C, respectively. The default resolution at power-up is 12 bit. The DS1822
powers-up in a low-power idle state; to initiate a temperature measurement and A-to-D conversion, the
master must issue a Convert T [44h] command. Following the conversion, the resulting thermal data is
stored in the 2-byte temperature register in the scratchpad memory and the DS1822 returns to its idle
state. If the DS1822 is powered by an external supply, the master can issue "read-time slots" (see the 1-
WIRE BUS SYSTEM section) after the Convert T command and the DS1822 will respond by
transmitting 0 while the temperature conversion is in progress and 1 when the conversion is done. If the
DS1822 is powered with parasite power, this notification technique cannot be used since the bus must be
pulled high by a strong pullup during the entire temperature conversion. The bus requirements for parasite
power are explained in detail in the POWERING THE DS1822 section of this data sheet.
The DS1822 output temperature data is calibrated in degrees centigrade; for Fahrenheit applications, a
lookup table or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended
two's complement number in the temperature register (see Figure 2). The sign bits (S) indicate if the
temperature is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the
DS1822 is configured for 12-bit resolution, all bits in the temperature register will contain valid data. For
11-bit resolution, bit 0 is undefined. For 10-bit resolution, bits 1 and 0 are undefined, and for 9-bit
resolution bits 2, 1 and 0 are undefined. Table 2 gives examples of digital output data and the
corresponding temperature reading for 12-bit resolution conversions.
TEMPERATURE REGISTER FORMAT Figure 2
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
LS Byte
2
3
2
2
2
1
2
0
2
-1
2
-2
2
-3
2
-4
bit 15
bit 14
bit 13
bit 12
bit 11
bit 10
bit 9
bit 8
MS Byte
S
S
S
S
S
2
6
2
5
2
4
TEMPERATURE/DATA RELATIONSHIP Table 2
TEMPERATURE
DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex)
+125°C
0000 0111 1101 0000
07D0h
+85°C*
0000 0101 0101 0000
0550h
+25.0625°C
0000 0001 1001 0001
0191h
+10.125°C
0000 0000 1010 0010
00A2h
+0.5°C
0000 0000 0000 1000
0008h
0°C
0000 0000 0000 0000
0000h
-0.5°C
1111 1111 1111 1000
FFF8h
-10.125°C
1111 1111 0101 1110
FF5Eh
-25.0625°C
1111 1110 0110 1111
FE6Fh
-55°C
1111 1100 1001 0000
FC90h
*The power on reset value of the temperature register is +85°C
DS1822
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OPERATION--ALARM SIGNALING
After the DS1822 performs a temperature conversion, the temperature value is compared to the user-
defined two's complement alarm trigger values stored in the 1-byte T
H
and T
L
registers (see Figure 3).
The sign bit (S)
indicates if the value is positive or negative: for positive numbers S = 0 and for negative
numbers S = 1. The T
H
and T
L
registers are NV (EEPROM) so they will retain data when the device is
powered down. T
H
and T
L
can be accessed through bytes 2 and 3 of the scratchpad as explained in the
MEMORY section of this data sheet.
T
H
AND T
L
REGISTER FORMAT Figure 3
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
S
2
6
2
5
2
5
2
5
2
2
2
1
2
0
Only bits 11 through 4 of the temperature register are used in the T
H
and T
L
comparison since T
H
and T
L
are 8-bit registers. If the measured temperature is lower than or equal to T
L
or higher than T
H
, an alarm
condition exists and an alarm flag is set inside the DS1822. This flag is updated after every temperature
measurement; 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 DS1822s on the bus by issuing an Alarm Search
[ECh] command. Any DS1822s with a set alarm flag will respond to the command, so the master can
determine exactly which DS1822s 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 DS1822
The DS1822 can be powered by an external supply on the V
DD
pin, or it can operate in "parasite power"
mode, which allows the DS1822 to function without a local external supply. Parasite power is very useful
for applications that require remote temperature sensing or that are very space constrained. Figure 1
shows the DS1822'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 DS1822 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
DS1822 is used in parasite power mode, the V
DD
pin must be connected to ground.
In parasite power mode, the 1-Wire bus and C
PP
can provide sufficient current to the DS1822 for most
operations as long as the specified timing and voltage requirements are met (refer to the DC
ELECTRICAL CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this
data sheet). However, when the DS1822 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 DS1822 has sufficient supply current, it is necessary to provide a
strong pullup on the 1-Wire bus whenever temperature conversions are taking 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 4. The 1-Wire bus must be switched to the strong pullup within
10
ms (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 DS1822 can also be powered by the conventional method of connecting an external power supply to
the V
DD
pin, as shown in Figure 5. 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 temperature conversion time.
DS1822
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The use of parasite power is not recommended for temperatures above 100
°C since the DS1822 may not
be able to sustain communications due to the higher leakage currents that can exist at these temperatures.
For applications in which such temperatures are likely, it is strongly recommended that the DS1822 be
powered by an external power supply.
In some situations the bus master may not know whether the DS1822s on the bus are parasite powered or
powered by external supplies. The master needs this information 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 followed by a Read Power Supply [B4h] command followed by a "read-time slot".
During the read time slot, parasite powered DS1822s will pull the bus low, and externally powered
DS1822s 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.
SUPPLYING THE PARASITE-POWERED DS1822 DURING TEMPERATURE
CONVERSIONS Figure 4
POWERING THE DS1822 WITH AN EXTERNAL SUPPLY Figure 5
64-BIT LASERED ROM CODE
Each DS1822 contains a unique 64bit code (see Figure 6) stored in ROM. The least significant 8 bits of
the ROM code contain the DS1822's 1-Wire family code: 22h. The next 48 bits contain a unique serial
number. The most significant 8 bits contain a cyclic redundancy check (CRC) byte that is calculated from
the first 56 bits of the ROM code. A detailed explanation of the CRC bits is provided in the CRC
GENERATION section. The 64-bit ROM code and associated ROM function control logic allow the
DS1822 to operate as a 1-Wire device using the protocol detailed in the 1-WIRE BUS SYSTEM section
of this data sheet.
64-BIT LASERED ROM CODE Figure 6
8-BIT CRC
48-BIT SERIAL NUMBER
8-BIT FAMILY CODE (22h)
MSB
MSB
LSB
LSB
LSB
MSB
V
DD
(External Supply)
DS1822
GND
V
DD
DQ
V
PU
4.7k
To Other
1-Wire Devices
1-Wire Bus
Micro-
processor
V
PU
V
PU
4.7k
1-Wire Bus
Micro-
processor
DS1822
GND
V
DD
DQ
To Other
1-Wire Devices
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