19-6322; Rev 6/12
DS1821
Programmable Digital Thermostat and Thermometer
FEATURES
Requires no external components
Unique 1-Wire
®
interface requires only one
port pin for communication
Operates over a -55°C to +125°C (-67°F to
+257°F) temperature range
Functions as a standalone thermostat with
user-definable trip-points
Provides 8-bit (1°C resolution) centigrade
temperature measurements
Accuracy is
±1°C
over 0°C to +85°C range
Converts temperature to a digital word in
1 second (max)
Available in 3-pin TO92 and 8-pin SO
packages
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system
PIN ASSIGNMENT
DQ
DS1821
1 2 3
GND
NC
NC
1
8
V
DD
NC
NC
NC
DS1821S
2
3
4
7
6
5
8-pin 208-mil SO
(DS1821S)
(BOTTOM VIEW)
TO92
(DS1821C)
PIN DESCRIPTION
GND
DQ
V
DD
NC
1-Wire is a registered trademark of Maxim Integrated
Products, Inc.
The DS1821 can function as a standalone thermostat with user-programmable trip-points or as 8-bit
temperature sensor with a 1-Wire digital interface. The thermostat trip-points are stored in nonvolatile
memory, so DS1821 units can be programmed prior to system insertion for true standalone operation.
The DS1821 has an operating temperature range of –55°C to +125°C and is accurate to
±1°C
over a range
of 0°C to +85°C. Communication with the DS1821 is accomplished through the open-drain DQ pin; this
pin also serves as the thermostat output.
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DESCRIPTION
GND
DQ
V
DD
1 2
3
- Ground
- Data In/Out and Thermostat Output
- Power Supply Voltage
- No Connect
DS1821
ORDERING INFORMATION
DS1821C+
DS1821S+
PART
PACKAGE MARKING
DS1821
DS1821S
DS1821S
DS1821S+T&R
+Denotes a lead(Pb)-free/RoHS-compliant package.
T&R = Tape and reel.
PIN-PACKAGE
3 TO92
8 SO (208 mils)
8 SO (208 mils), 2000 Piece
Tape-and-Reel
DETAILED PIN DESCRIPTIONS
Table 1
TO92
1
2
8-PIN
SO*
2
1
SYMBOL
GND
DQ
DESCRIPTION
Ground pin.
Open drain data input/output pin
– 1-Wire operation;
Open drain
thermostat output pin
–thermostat operation.
3
8
V
DD
Power supply pin.
*All pins not specified in this table are “No Connect” pins.
Figure 1 shows a block diagram of the DS1821 and pin descriptions are given in Table 1. The DS1821
can operate as a standalone thermostat with user-programmable trip-points or as 8-bit temperature sensor
with a 1-Wire digital interface. The open-drain DQ pin functions as the thermostat output for thermostat
operation and as the data I/O pin for 1-Wire communications. The 1-Wire interface provides user access
to the nonvolatile (EEPROM) thermostat trip-point registers (T
H
and T
L
), the status/configuration register,
and the temperature register.
When configured as standalone thermostat, temperature conversions start immediately at power-up. In
this mode, the DQ pin becomes active when the temperature of the DS1821 exceeds the limit
programmed into the T
H
register, and remains active until the temperature drops below the limit
programmed into the T
L
register.
The DS1821 uses Maxim’s exclusive 1-Wire bus protocol that implements bus communication with one
control signal. This system is explained in detail in the 1-Wire BUS SYSTEM section of this datasheet.
OVERVIEW
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DS1821
DS1821 BLOCK DIAGRAM
Figure 1
V
DD
4.7K
DQ
POWER
SUPPLY
SENSE
1-WIRE
INTERFACE
AND
I/O CONTROL
DS1821
CONFIGURATION REGISTER
AND CONTROL LOGIC
TEMPERATURE SENSOR
TH REGISTER
TL REGISTER
V
DD
DIGITAL
COMPARATOR/
LOGIC
GND
TEMPERATURE SENSOR FUNCTIONALITY
The core functionality of the DS1821 is its proprietary direct-to-digital temperature sensor, which
provides 8-bit (1°C increment) centigrade temperature readings over the range of -55°C to +125°C.
A block diagram of the temperature measurement circuitry is shown in Figure 2. This circuit measures
the temperature by counting the number of clock cycles generated by an oscillator with a low temperature
coefficient (temp-co) during a gate period determined by a high temp-co oscillator. The low temp-co
counter is preset with a base count that corresponds to –55°C. If the counter reaches 0 before the gate
period is over, the temperature register, which is preset to –55°C, is incremented by one degree, and the
counter is again preset with a starting value determined by the slope accumulator circuitry. The preset
counter value is unique for every temperature increment and compensates for the parabolic behavior of
the oscillators over temperature.
At this time, the counter is clocked again until it reaches 0. If the gate period is not over when the counter
reaches 0, the temperature register is incremented again. This process of presetting the counter, counting
down to zero, and incrementing the temperature register is repeated until the counter takes less time to
reach zero than the duration of the gate period of the high temp-co oscillator. When this iterative process
is complete, the value in the temperature register will indicate the centigrade temperature of the device.
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DS1821
TEMPERATURE MEASURING CIRCUITRY
Figure 2
SLOPE ACCUMULATOR
PRESET
COMPARE
LOW TEMPERATURE
COEFFICIENT OSCILLATOR
COUNTER
PRESET
SET/CLEAR
LSB
=0
INC
TEMPERATURE REGISTER
HIGH TEMPERATURE
COEFFICIENT OSCILLATOR
COUNTER
=0
STOP
The DS1821 has two operating modes: 1-Wire mode and thermostat mode. The power-up operating
mode is determined by the user-programmable T/R bit in the status/configuration register: if T/R = 0 the
¯
¯
device powers-up in 1-Wire mode, and if T/R = 1 the device powers-up in thermostat mode. The T/R bit
¯
¯
is stored in nonvolatile memory (EEPROM), so it will retain its value when the device is powered down.
OPERATING MODES
1-Wire MODE
The DS1821 arrives from the factory in 1-Wire mode (T/R = 0). In this mode, the DQ pin of the DS1821
¯
is configured as a 1-Wire port for communication with a microprocessor using the protocols described in
the 1-Wire BUS SYSTEM section of this datasheet. These communications can include reading and
writing the high and low thermostat trip-point registers (T
H
and T
L
) and the configuration register, and
reading the temperature, counter, and slope accumulator registers. Also in this mode, the microprocessor
can initiate and stop temperature measurements as described in the OPERATION – MEASURING
TEMPERATURE section of this datasheet.
The T
H
and T
L
registers and certain bits (THF, TLF, T/R, POL and 1SHOT) in the status/configuration
¯
register are stored in nonvolatile EEPROM memory, so they will retain data when the device is powered
down. This allows these registers to be pre-programmed when the DS1821 is to be used as a standalone
thermostat. Writes to these nonvolatile registers can take up to 10ms. To avoid data corruption, no
writes to nonvolatile memory should be initiated while a write to nonvolatile memory is in progress.
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DS1821
Nonvolatile write status can be monitored by reading the NVB bit in the status/configuration register:
NVB = 1 – a write to EEPROM memory is in progress, NVB = 0 – nonvolatile memory is idle.
THERMOSTAT MODE
In thermostat mode (T/R = 1), the DS1821 can operate as a standalone thermostat that triggers according
¯
to the T
H
and T
L
trip-points programmed while the device was in 1-Wire mode. In thermostat mode the
DS1821 powers-up performing continuous temperature conversions, and the DQ pin acts as the
thermostat output. Detailed operation of the thermostat output is provided in the OPERATION –
STANDALONE THERMOSTAT section of this datasheet.
Communications can be re-established with the DS1821 while it is in thermostat mode by pulling V
DD
to
0V while the DQ line is held high, and then toggling the DQ line low 16 times as shown in Figure 12.
This temporarily places the DS1821 in 1-Wire mode, allowing microprocessor communication with the
DS1821 via the DQ pin. At this time any I/O function can be performed, such as reading/writing the T
H
,
T
L
or configuration registers or reading the temperature register. To return to thermostat mode, the same
procedure can be performed (pulling V
DD
to 0V while the DQ line is held high, and then clocking the DQ
line 16 times) or the power can be cycled. Note that temporarily putting the DS1821 into 1-Wire mode
does not change the power-up mode of the device; this can only be changed by rewriting the T/R bit in
¯
the status/configuration register. Also note that holding both V
DD
and DQ low for more than
approximately 10 seconds will cause the DS1821 to be powered down.
DS1821 output temperature data is calibrated in degrees centigrade and is stored in two’s complement
format in the 1-byte (8-bit) temperature register (see Figure 3), which the user can access when the
DS1821 is in 1-Wire mode (T/R = 0 in the status/configuration register). The sign bit (S) indicates if the
¯
temperature is positive or negative; for positive numbers S = 0 and for negative numbers S = 1. Table 2
gives examples of digital output data and the corresponding temperature reading. For Fahrenheit
measurements, a lookup table or conversion routine must be used.
The DS1821 can be configured by the user to take continuous temperature measurements (continuous
conversion mode) or single measurements (one-shot mode). The desired configuration can be achieved
by setting the nonvolatile1SHOT bit in the status/configuration register: 1SHOT = 0 – continuous
conversion mode, 1SHOT = 1 – one-shot mode. Note that the 1SHOT setting only controls the operation
of the device in 1-Wire mode; in thermostat mode, continuous temperature conversions are started
automatically at power-up.
In continuous conversion mode, the Start Convert T [EEh] command initiates continuous temperature
conversions, which can be stopped using the Stop Convert T [22h] command. In one-shot mode the Start
Convert T [EEh] command initiates a single temperature conversion after which the DS1821 returns to a
low-power standby state. In this mode, the microprocessor can monitor the DONE bit in the
configuration register to determine when the conversion is complete: DONE = 0 ― conversion in
progress, DONE = 1 ― conversion complete. The DONE bit does not provide conversion status in
continuous conversion mode since measurements are constantly in progress (i.e., DONE will always be
0).
OPERATION – MEASURING TEMPERATURE
TEMPERATURE, 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
4
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2
3
2
2
2
1
2
0
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