Dallas Semiconductor DS1720S Datasheet
DS1720
Econo – Digital Thermometer and
Thermostat
DS1720
PRELIMINARY
030598 1/12
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
GND
V
DD
RST
CLK/CONV
DQ
T
LOW
T
HIGH
T
COM
1
2
3
4
8
7
6
5
DS1720S 8–PIN SOIC (208 MIL)
See Mech Drawings Section
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
T
HIGH
– High Temperature Trigger
T
LOW
– Low Temperature Trigger
T
COM
– High/Low Combination Trigger
V
DD
– 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 outputs, the DS1720 can also act as a thermostat. T
HIGH
is
driven high if the DS1720’s temperature is greater than
or equal to a user–defined temperature TH. T
LOW
is
driven high if the DS1720’s temperature is less than or
equal to a user–defined temperature TL. T
COM
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
030598 2/12
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. T o 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 FUNCTIONAL BLOCK DIAGRAM Figure 1
RST
ADDRESS
AND
RESET
CLK
DQ
STATUS REGISTER AND
CONTROL LOGIC
TEMPERATURE SENSOR
HIGH TEMP TRIGGER, TH
LOW TEMP TRIGGER, TL
DIGITAL COMPARATOR/LOGIC
T
HIGH
T
LOW
T
COM
DS1720
030598 3/12
TEMPERATURE MEASURING CIRCUITRY Figure 2
SLOPE ACCUMULATOR
PRESET
PRESET
COUNTER
COUNTER
=0
=0
STOP
INC
COMPARE
TEMPERATURE REGISTER
LOW TEMPERATURE
COEFFICIENT OSCILLATOR
HIGH TEMPERATURE
COEFFICIENT OSCILLATOR
SET/CLEAR
LSB
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
T able 1
TEMP
DIGITAL
OUTPUT
(Binary)
DIGITAL
OUTPUT
(Hex)
+85°C 0 10101010 00AA
+25°C 0 00110010 0032h
+
1
/2
°C 0 00000001 0001h
+0°C 0 00000000 0000h
–1/2°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 T able 1. After the MSB, the DS1720 will
output 0s.
Note that temperature is represented in the DS1720 in
terms of a
1
/2°C LSB, yielding the following 9–bit format:
XXXXXXX1 110 01 110
MSB LSB
T = –25°C
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. By loading the value of the slope accumulator
into the count register (using the READ SLOPE command), this value may then be read, 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
(COUNT_PER_C – COUNT_REMAIN)
COUNT_PER_C
T
HIGH
T
LOW
T
COM
TL TH
T (°C)
DS1720
030598 4/12
DETAILED PIN DESCRIPTION Table 2
PIN SYMBOL DESCRIPTION
1 DQ Data Input/Output pin for 3–wire communication port.
2 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
.
3 RST Reset input pin for 3–wire communication port.
4 GND Ground pin.
5 T
COM
High/Low Combination Trigger. Goes high when temperature exceeds TH; will
reset to low when temperature falls below TL.
6 T
LOW
Low Temperature Trigger . Goes high when temperature falls below TL.
7 T
HIGH
High Temperature Trigger. Goes high when temperature exceeds TH.
8 V
DD
Supply Voltage. 2.7V – 5.5V input power pin.
OPERATION–THERMOSTAT CONTROLS
Three thermally triggered outputs, T
HIGH
, T
LOW
, and
T
COM
, 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 T
HIGH
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 T
HIGH
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 T
LOW
output functions similarly to the T
HIGH
output.
When the DS1720’s measured temperature equals or
falls below the value stored in the low temperature register, the T
LOW
output becomes active. T
LOW
remains
active until the DS1720’s temperature becomes greater
than the value stored in the low temperature register,
TL. The T
LOW
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 T
COM
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.
THERMOSTAT OUTPUT OPERATION Figure 3
Loading...