DS1820.PDF

DS1820

1–Wire TM Digital Thermometer

FEATURES

•

PIN ASSIGNMENT

Unique 1–Wire TM interface requires only one port pin

for communication

DALLAS

DS1820

DALLAS

DS2434

•

Multidrop capability simplifies distributed temperature

sensing applications

•

BOTTOM VIEW

Requires no external components

•

Can be powered from data line

•

Zero standby power required

VDD

GND

•

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 digital value.

•

Converts temperature to digital word in 200 ms (typ.)

DS1820

PR35 PACKAGE

DS1820S

16–PIN SSOP

See Mech. Drawings

Section

See Mech. Drawings

Section

•

User–definable, nonvolatile temperature alarm set-

tings

•

Alarm search command identifies and addresses

devices whose temperature is outside of pro-

grammed limits (temperature alarm condition)

PIN DESCRIPTION

GND

– Ground

– Data In/Out

•

Applications include thermostatic controls, industrial

systems, consumer products, thermometers, or any

thermally sensitive system

V DD

– Optional V DD

– No Connect

DESCRIPTION

The DS1820 Digital Thermometer provides 9–bit tem-

perature readings which indicate the temperature of the

device.

Because each DS1820 contains a unique silicon serial

number, multiple DS1820s can exist on the same

1–Wire bus. This allows for placing temperature sen-

sors in many different places. Applications where this

feature is useful include HVAC environmental controls,

sensing temperatures inside buildings, equipment or

machinery, and in process monitoring and control.

Information is sent to/from the DS1820 over a 1–Wire

interface, so that only one wire (and ground) needs to be

connected from a central microprocessor to a DS1820.

Power for reading, writing, and performing temperature

conversions can be derived from the data line itself with

no need for an external power source.

030598 1/27

DS1820

DETAILED PIN DESCRIPTION

16–PIN SSOP

PR35

SYMBOL

DESCRIPTION

GND

Ground.

8 2 DQ Data Input/Output pin. For 1–Wire operation: Open drain. (See

“Parasite Power” section.)

7 3 V DD Optional V DD pin . See “Parasite Power” section for details of

connection.

DS1820S (16–pin SSOP): All pins not specified in this table are not to be connected.

OVERVIEW

The block diagram of Figure 1 shows the major compo-

nents of the DS1820. The DS1820 has three main data

components: 1) 64–bit lasered ROM, 2) temperature

sensor, and 3) nonvolatile temperature alarm triggers

TH and TL. The device derives its power from the

1–Wire communication line by storing energy on an

internal capacitor during periods of time when the signal

line is high and continues to operate off this power

source during the low times of the 1–Wire line until it

returns high to replenish the parasite (capacitor) supply.

As an alternative, the DS1820 may also be powered

from an external 5 volts supply.

a specific device if many are present on the 1–Wire line

as well as indicate to the Bus Master how many and

what types of devices are present. After a ROM function

sequence has been successfully executed, the memory

and control functions are accessible and the master

may then provide any one of the six memory and control

function commands.

One control function command instructs the DS1820 to

perform a temperature measurement. The result of this

measurement will be placed in the DS1820’s scratch-

pad memory, and may be read by issuing a memory

function command which reads the contents of the

scratchpad memory. The temperature alarm triggers

TH and TL consist of one byte EEPROM each. If the

alarm search command is not applied to the DS1820,

these registers may be used as general purpose user

memory. Writing TH and TL is done using a memory

function command. Read access to these registers is

through the scratchpad. All data is read and written

least significant bit first.

Communication to the DS1820 is via a 1–Wire port. With

the 1–Wire port, the memory and control functions will

not be available before the ROM function protocol has

been established. The master must first provide one of

five ROM function commands: 1) Read ROM, 2) Match

ROM, 3) Search ROM, 4) Skip ROM, or 5) Alarm

Search. These commands operate on the 64–bit

lasered ROM portion of each device and can single out

DS1820 BLOCK DIAGRAM Figure 1

MEMORY AND

CONTROL LOGIC

64–BIT ROM

AND

1–WIRE PORT

TEMPERATURE SENSOR

INTERNAL V DD

SCRATCHPAD

HIGH TEMPERATURE

TRIGGER, TH

LOW TEMPERATURE

TRIGGER, TL

V DD

POWER

SUPPLY

SENSE

8–BIT CRC

GENERATOR

030598 2/27

DS1820

PARASITE POWER

The block diagram (Figure 1) shows the parasite pow-

ered circuitry. This circuitry “steals” power whenever the

I/O or V DD pins are high. I/O will provide sufficient power

as long as the specified timing and voltage require-

ments are met (see the section titled “1–Wire Bus Sys-

tem”). The advantages of parasite power are two–fold:

1) by parasiting off this pin, no local power source is

needed for remote sensing of temperature, and 2) the

ROM may be read in absence of normal power.

V DD pin, as shown in Figure 3. The advantage to this is

that the strong pull–up is not required on the I/O line, and

the bus master need not be tied up holding that line high

during temperature conversions. This allows other data

traffic on the 1–Wire bus during the conversion time. In

addition, any number of DS1820’s may be placed on the

1–Wire bus, and if they all use external power, they may

all simultaneously perform temperature conversions by

issuing the Skip ROM command and then issuing the

Convert T command. Note that as long as the external

power supply is active, the GND pin may not be floating.

In order for the DS1820 to be able to perform accurate

temperature conversions, sufficient power must be pro-

vided over the I/O line when a temperature conversion is

taking place. Since the operating current of the DS1820

is up to 1 mA, the I/O line will not have sufficient drive

due to the 5K pull–up resistor. This problem is particu-

larly acute if several DS1820’s are on the same I/O and

attempting to convert simultaneously.

The use of parasite power is not recommended above

100 ° C, since it may not be able to sustain communica-

tions given the higher leakage currents the DS1820

exhibits at these temperatures. For applications in

which such temperatures are likely, it is strongly recom-

mended that V DD be applied to the DS1820.

There are two ways to assure that the DS1820 has suffi-

cient supply current during its active conversion cycle.

The first is to provide a strong pull–up on the I/O line

whenever temperature conversions or copies to the E 2

memory are taking place. This may be accomplished by

using a MOSFET to pull the I/O line directly to the power

supply as shown in Figure 2. The I/O line must be

switched over to the strong pull–up within 10 m s maxi-

mum after issuing any protocol that involves copying to

the E 2 memory or initiates temperature conversions.

When using the parasite power mode, the V DD pin must

be tied to ground.

For situations where the bus master does not know

whether the DS1820’s on the bus are parasite powered

or supplied with external V DD , a provision is made in the

DS1820 to signal the power supply scheme used. The

bus master can determine if any DS1820’s are on the

bus which require the strong pull–up by sending a Skip

ROM protocol, then issuing the read power supply com-

mand. After this command is issued, the master then

issues read time slots. The DS1820 will send back “0”

on the 1–Wire bus if it is parasite powered; it will send

back a “1” if it is powered from the V DD pin. If the master

receives a “0”, it knows that it must supply the strong

pull–up on the I/O line during temperature conversions.

See “Memory Command Functions” section for more

detail on this command protocol.

Another method of supplying current to the DS1820 is

through the use of an external power supply tied to the

STRONG PULL–UP FOR SUPPLYING DS1820 DURING TEMPERATURE CONVERSION Figure 2

+5V

DS1820

+5V

4.7K

GND

V DD

m P

I/O

030598 3/27

DS1820

USING V DD TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

TO OTHER 1–WIRE

DEVICES

DS1820

+5V

4.7K

V DD

EXTERNAL +5V SUPPLY

m P

I/O

OPERATION – MEASURING TEMPERATURE

The DS1820 measures temperature through the use of

an on–board proprietary temperature measurement

technique. A block diagram of the temperature mea-

surement circuitry is shown in Figure 4.

provided in a 16–bit, sign–extended two’s complement

reading. Table 1 describes the exact relationship of out-

put data to measured temperature. The data is trans-

mitted serially over the 1–Wire interface. The DS1820

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.

The DS1820 measures temperature by counting the

number of clock cycles that an oscillator with a low tem-

perature coefficient goes through during a gate period

determined by a high temperature coefficient oscillator.

The counter is preset with a base count that corre-

sponds 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, indicat-

ing that the temperature is higher than –55

Note that temperature is represented in the DS1820 in

terms of a 1 / 2 ° C LSB, yielding the following 9–bit format:

MSB

LSB

= –25 ° C

At the same time, the counter is then preset with a value

determined by the slope accumulator circuitry. This cir-

cuitry is needed to compensate for the parabolic behav-

ior 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 most significant (sign) bit is duplicated into all of the

bits in the upper MSB of the two–byte temperature reg-

ister in memory. This “sign–extension” yields the 16–bit

temperature readings as shown in Table 1.

Higher resolutions may be obtained by the following

procedure. First, read the temperature, and truncate

the 0.5

The slope accumulator is used to compensate for the

non–linear behavior of the oscillators over temperature,

yielding a high resolution temperature measurement.

This is done by changing the number of counts neces-

sary 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 accumu-

lator) at a given temperature must be known.

C bit (the LSB) from the read value. This value is

TEMP_READ. The value left in the counter may then be

read. This value is the count remaining

(COUNT_REMAIN) after the gate period has ceased.

The last value needed is 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:

(COUNT_PER_C – COUNT_REMAIN )

COUNT_PER_C

TEMPERATURE = TEMP_READ – 0.25

Internally, this calculation is done inside the DS1820 to

provide 0.5

C resolution. The temperature reading is

030598 4/27

DS1820

TEMPERATURE MEASURING CIRCUITRY Figure 4

SLOPE ACCUMULATOR

PRESET

COMPARE

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER

PRESET

SET/CLEAR

LSB

INC

TEMPERATURE REGISTER

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

COUNTER

STOP

TEMPERATURE/DATA RELATIONSHIPS Table 1

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125 ° C

00000000 11111010

00FA

+25 ° C

00000000 00110010

0032h

+ 1 /2 °

00000000 00000001

0001h

+0 ° C

00000000 00000000

0000h

– 1 / 2 °

11111111 11111111

FFFFh

–25

11111111 11001110

FFCEh

–55 ° C

11111111 10010010

FF92h

C bit is ignored for comparison. The

most significant bit of TH or TL directly corresponds to

the sign bit of the 16–bit temperature register. If the

result of a temperature measurement is higher than TH

or lower than TL, an alarm flag inside the device is set.

This flag is updated with every temperature measure-

ment. As long as the alarm flag is set, the DS1820 will

respond to the alarm search command. This allows

many DS1820s to be connected in parallel doing simul-

taneous temperature measurements. If somewhere the

temperature exceeds the limits, the alarming device(s)

can be identified and read immediately without having to

read non–alarming devices.

030598 5/27

OPERATION – ALARM SIGNALING

After the DS1820 has performed a temperature conver-

sion, the temperature value is compared to the trigger

values stored in TH and TL. Since these registers are

8–bit only, the 0.5

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