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DS1720
Econ-Digital Thermometer and Thermostat
FEATURES Requires no external components Supply voltage range covers from 2.7V to
5.5V 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 Temperature is read as a 9–bit value Converts temperature to digital word in 1
second (max) Thermostatic settings are user–definable and
non–volatile Data is read from/written via a 3–wire serial
interface (CLK, DQ, RST) Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system 8–pin SOIC (208 mil) package
PIN ASSIGNMENT
PIN DESCRIPTION DQ – 3–Wire Input/Output
CLK/CONV – 3–Wire Clock Input and
Stand–alone
Convert Input
RST – 3–Wire Reset Input
GND – Ground
THIGH – High Temperature Trigger
TLOW – Low Temperature Trigger
TCOM – High/Low Combination Trigger
VDD – Power Supply Voltage (3V-5V)
DESCRIPTION The DS1720 Digital Thermometer and Thermostat provides 9–bit temperature readings which indicate
the temperature of the device. With three thermal alarm out-puts, the DS1720 can also act as a thermostat.
THIGH is driven high if the DS1720’s temperature is greater than or equal to a user–defined temperature
TH. TLOW is driven high if the DS1720’s temperature is less than or equal to a user–defined temperature
TL. TCOM is driven high when the temperature exceeds TH and stays high until the temperature falls below
that of TL.
User–defined temperature settings are stored in non–volatile memory, so parts can be programmed prior
to insertion in a system, as well as used in stand–alone applications without a CPU. Temperature settings
and temperature readings are all communicated to/from the DS1720 over a simple 3–wire interface.
DS1720
ECON-Digital Thermometer and
Thermostat
DS1720S 8-Pin SOIC (208-mil)
DQ
CLK/CONV
RST
GND
VDD
THIGH
TLOW
TCOM
DS1720
ORDER INFORMATION
ORDERING
NUMBER
PACKAGE
MARKING
DESCRIPTION DS1720S+ DS1720 (see note) DS1720 in Lead-Free 208mil 8-pin SO
DS1720S+T&R DS1720 (see note) DS1720 in Lead-Free 208mil 8-pin SO, 2500 Piece
Tape-and-Reel
DS1720S DS1720 DS1720 in 208mil 8-pin SO
DS1720S/T&R DS1720 DS1720 in 208mil 8-pin SO, 2500 Piece Tape-and-Reel
Note: A “+” symbol will also be marked on the package near the Pin 1 indicator.
OPERATION–MEASURING TEMPERATURE A block diagram of the DS1720 is shown in Figure 1. The DS1720 measures temperatures through the
use of an on–board proprietary temperature measurement technique. A block diagram of the temperature
measurement circuitry is shown in Figure 2.
The DS1720 measures temperature by counting the number of clock cycles that an oscillator with a low
temperature coefficient goes through during a gate period determined by a high temperature coefficient
oscillator. The counter is preset with a base count that corresponds 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, indicating 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 circuitry is needed to compensate
for the parabolic behavior 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 nonlinear behavior of the oscillators over
temperature, yielding a high resolution temperature measurement. This is done by changing the number
of counts necessary 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 accumulator) at a given temperature must be known.
DS1720
DS1720 FUNCTIONAL BLOCK DIAGRAM Figure 1 TEMPERATURE MEASURING CIRCUITRY Figure 2 This calculation is done inside the DS1720 to provide 0.5°C resolution. The temperature reading is
provided in a 9–bit, two’s complement reading by issuing a READ TEMPERATURE command. Table 1
describes the exact relationship of output data to measured temperature. The data is transmitted serially
through the 3–wire serial interface, LSB first. The DS1720 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.
TEMPERATURE/DATA RELATIONSHIPS Table 1
TEMP DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex) +85°C 0 10101010 00AA
+25°C 0 00110010 0032h
+½°C 0 00000001 0001h
+0°C 0 00000000 0000h
-½°C 1 11111111 01FFh
-25°C 1 11001110 01CEh
Since data is transmitted over the 3–wire bus LSB first, temperature data can be written to/read from the
DS1720 as either a 9–bit word (taking RST low after the 9th (MSB) bit), or as two transfers of 8–bit
words, with the most significant 7 bits being ignored or set to zero, as illustrated in Table 1. After the
MSB, the DS1720 will output 0s.
Note that temperature is represented in the DS1720 in terms of a ½°C LSB, yielding the following 9–bit
format:
MSB LSB DS1720
Higher resolutions may be obtained by reading the temperature, and truncating 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 by
issuing a READ COUNTER command. This value is the count remaining (COUNT_REMAIN) after the
gate period has ceased. The value of the slope accumulator may be read (using the READ SLOPE
command), yielding 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 +
DETAILED PIN DESCRIPTION Table 2
PIN SYMBOL DESCRIPTION 1 DQ
Data Input/Output pin for 3–wire communication port. CLK/CONV
Clock input pin for 3–wire communication port. When the DS1720 is used in a stand–alone application with no 3–wire port, this pin can be used as a
convert pin. Temperature conversion will begin on the falling edge of CONV. RST
Reset input pin for 3–wire communication port. 4 GND
Ground pin. 5 TCOM
High/Low Combination Trigger. Goes high when temperature exceeds TH; will reset to low when temperature falls below TL.
6 TLOW
Low Temperature Trigger. Goes high when temperature falls below TL. 7 THIGH
High Temperature Trigger. Goes high when temperature exceeds TH. 8 VDD
Supply Voltage. 2.7V – 5.5V input power pin.
OPERATION–THERMOSTAT CONTROLS Three thermally triggered outputs, THIGH, TLOW, and TCOM, are provided to allow the DS1720 to be used
as a thermostat, as shown in Figure 3. When the DS1720’s temperature meets or exceeds the value stored
in the high temperature trip register, the output THIGH becomes active (high) and remains active until the
DS1720’s measured temperature becomes less than the stored value in the high temperature register, TH.
The THIGH output can be used to indicate that a high temperature tolerance boundary has been met or
exceeded, or as part of a closed loop system can be used to activate a cooling system and to deactivate it
when the system temperature returns to tolerance.
The TLOW output functions similarly to the THIGH output. When the DS1720’s measured temperature
equals or falls below the value stored in the low temperature register, the TLOW output becomes active.
TLOW remains active until the DS1720’s temperature becomes greater than the value stored in the low
temperature register, TL. The TLOW output can be used to indicate that a low temperature tolerance
boundary has been met or exceeded, or as part of a closed loop system, can be used to activate a heating
system and to deactivate it when the system temperature returns to tolerance.
The TCOM output goes high when the measured temperature meets or exceeds TH, and will stay high until
the temperature equals or falls below TL. In this way, any amount of hysteresis can be obtained.
(COUNT_PER_C– COUNT_REMAIN)
COUNT_PER_C
DS1720
THERMOSTAT OUTPUT OPERATION Figure 3
OPERATION AND CONTROL The DS1720 must have temperature settings resident in the TH and TL registers for thermostatic operation.
A configuration/status register is also used to determine the method of operation that the DS1720 will use
in a particular application, as well as indicating the status of the temperature conversion operation. The
configuration register is defined as follows:
CONFIGURATION/STATUS REGISTER DONE THF TLF NVB 1 0 CPU 1SHOT
where
DONE = Conversion Done bit. 1=conversion complete, 0=conversion in progress.
THF = Temperature High Flag. This bit will be set to 1 when the temperature is greater than or equal to the value of TH. It will remain 1 until reset by writing 0 into this location or by removing power from the
device. This feature provides a method of determining if the DS1720 has ever been subjected to
temperatures above TH while power has been applied.
TLF = Temperature Low Flag. This bit will be set to 1 when the temperature is less than or equal to the value of TL. It will remain 1 until reset by writing 0 into this location or by removing power from the
device. This feature provides a method of determining if the DS1720 has ever been subjected to
temperatures below TL while power has been applied.
NVB = Nonvolatile Memory Busy Flag. 1=write to an E2 memory cell in progress. 0=nonvolatile
memory is not busy. A copy to E2 may take up to 10 ms.
CPU = CPU use bit. If CPU=0, the CLK/CONV pin acts as a conversion start control, when RST is low.
If CPU is 1, the DS1720 will be used with a CPU communicating to it over the 3–wire port, and the
operation of the CLK/CONV pin is as a normal clock in concert with DQ and RST. This bit is stored in
nonvolatile E2 memory, capable of at least 50,000 writes. The DS1720 is shipped with CPU=0.
1SHOT = One–Shot Mode. If 1SHOT is 1, the DS1720 will perform one temperature conversion upon reception of the Start Convert T protocol. If 1SHOT is 0, the DS1720 will continuously perform
DS1720
For typical thermostat operation, the DS1720 will operate in continuous mode. However, for applications
where only one reading is needed at certain times, and to conserve power, the one–shot mode may be
used. Note that the thermostat outputs (THIGH, TLOW, TCOM) will remain in the state they were in after the
last valid temperature conversion cycle when operating in one–shot mode.
OPERATION IN STAND–ALONE MODE In applications where the DS1720 is used as a simple thermostat, no CPU is required. Since the
temperature limits are nonvolatile, the DS1720 can be programmed prior to insertion in the system. In
order to facilitate operation without a CPU, the CLK/CONV pin (pin 2) can be used to initiate
conversions. Note that the CPU bit must be set to 0 in the configuration register to use this mode of
operation. Whether CPU=0 or 1, the 3–wire port is active. Setting CPU=1 disables the stand–alone mode.
To use the CLK/CONV pin to initiate conversions, RST must be low and CLK/CONV must be high. If
CLK/CONV is driven low and then brought high in less than 10 ms, one temperature conversion will be
performed and then the DS1720 will return to an idle state. If CLK/CONV is driven low and remains low,
continuous con-versions will take place until CLK/CONV is brought high again. With the CPU bit set to
0, the CLK/CONV will override the 1–shot bit if it is equal to 1. This means that even if the part is set for
one–shot mode, driving CLK/CONV low will initiate conversions.
3–WIRE COMMUNICATIONS The 3–wire bus is comprised of three signals. These are the RST (reset) signal, the CLK (clock) signal,
and the DQ (data) signal. All data transfers are initiated by driving the RST input high. Driving the RST
input low terminates communication. (See Figures 4 and 5). A clock cycle is a sequence of a falling edge
followed by a rising edge. For data inputs, the data must be valid during the rising edge of a clock cycle.
Data bits are output on the falling edge of the clock, and remain valid through the rising edge.
When reading data from the DS1720, the DQ pin goes to a high impedance state while the clock is high.
Taking RST low will terminate any communication and cause the DQ pin to go to a high impedance
state.
Data over the 3–wire interface is communicated LSB first. The command set for the 3–wire interface as
shown in Table 3 is as follows; only these protocols should be written to the DS1720, as writing other
protocols to the device may result in permanent damage to the part.
Read Temperature [AAh] This command reads the contents of the register which contains the last temperature conversion result.
The next nine clock cycles will output the contents of this register.
Write TH [01h] This command writes to the TH (HIGH TEMPERATURE) register. After issuing this command, the next
nine clock cycles clock in the 9–bit temperature limit which will set the threshold for operation of the
THIGH output.
Write TL [02h] This command writes to the TL (LOW TEMPERATURE) register. After issuing this command, the next
nine clock cycles clock in the 9–bit temperature limit which will set the threshold for operation of the
