Rainbow Electronics DS1720 User Manual

DS1720

PRELIMINARY

DS1720

Econo – 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

1

DQ

CLK/CONV

RST
GND

DS1720S 8–PIN SOIC (208 MIL)

See Mech Drawings Section

8

V

2
3
4

DD

7

T

HIGH

6

T

LOW

5

T

COM

PIN DESCRIPTION

DQ – 3–Wire Input/Output
CLK/CONV

RST – 3–Wire Reset Input
GND – Ground
T

HIGH

T

LOW

T

COM

V

DD

– 3–Wire Clock Input and

Stand–alone
Convert Input

– High Temperature Trigger
– Low Temperature Trigger
– High/Low Combination Trigger
– 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
driven high if the DS1720’s temperature is greater than
or equal to a user–defined temperature TH. T
driven high if the DS1720’s temperature is less than or
equal to a user–defined temperature TL. T

COM

HIGH

is

LOW

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–

is

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.

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DS1720

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.

DS1720 FUNCTIONAL BLOCK DIAGRAM Figure 1

STATUS REGISTER AND

CONTROL LOGIC

CLK

DQ

ADDRESS

AND

RESET

TEMPERATURE SENSOR

HIGH TEMP TRIGGER, TH

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.

RST

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LOW TEMP TRIGGER, TL

DIGITAL COMPARATOR/LOGIC

T

T

T

HIGH

LOW

COM

TEMPERATURE MEASURING CIRCUITRY Figure 2

SLOPE ACCUMULATOR

DS1720

PRESET

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER

COUNTER

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.

COMPARE

PRESET

INC

=0

STOP

=0

TEMPERATURE REGISTER

SET/CLEAR
LSB

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

MSB LSB

XXXXXXX1 11 0 01 11 0

1

/2°C LSB, yielding the following 9–bit format:

TEMPERATURE/DATA RELATIONSHIPS

T able 1

DIGITAL
OUTPUT

TEMP

(Binary)

+85°C 0 10101010 00AA
+25°C 0 00110010 0032h

1

+

°C 0 00000001 0001h

/2

+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

DIGITAL

OUTPUT

(Hex)

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

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DS1720

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

3 RST Reset input pin for 3–wire communication port.
4 GND Ground pin.
5 T

6 T
7 T
8 V

COM

LOW
HIGH

DD

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

.

High/Low Combination Trigger. Goes high when temperature exceeds TH; will
reset to low when temperature falls below TL.

Low Temperature Trigger. Goes high when temperature falls below TL.
High Temperature Trigger. Goes high when temperature exceeds TH.
Supply Voltage. 2.7V – 5.5V input power pin.

OPERATION–THERMOSTAT CONTROLS

Three thermally triggered outputs, T
T

, are provided to allow the DS1720 to be used as a

COM

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
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

output can be used to indicate that a high

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

output functions similarly to the T

LOW

When the DS1720’s measured temperature equals or

HIGH

, T

HIGH

LOW

output.

, and

HIGH

THERMOSTAT OUTPUT OPERATION Figure 3

T

HIGH

T

LOW

falls below the value stored in the low temperature regis­ter, the T

output becomes active. T

LOW

LOW

remains
active until the DS1720’s temperature becomes greater
than the value stored in the low temperature register,
TL. The T

output can be used to indicate that a low

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

output goes high when the measured tem-

COM

perature 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.

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T

COM

TL TH

T (°C)