DS1820

1Wire

Digital Thermometer

DS1820

030598 1/27

FEATURES

Unique 1Wire

interface requires only one port pin

for communication

Multidrop capability simplifies distributed temperature
sensing applications

Requires no external components

Can be powered from data line

Zero standby power required

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 9bit digital value.

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

Userdefinable, nonvolatile temperature alarm set-
tings

Alarm search command identifies and addresses
devices whose temperature is outside of pro-
grammed limits (temperature alarm condition)

Applications include thermostatic controls, industrial
systems, consumer products, thermometers, or any
thermally sensitive system

PIN ASSIGNMENT

DALLAS
DS2434

GND

VDD

DALLAS
DS1820

DS1820S

16PIN SSOP

DS1820

PR35 PACKAGE

1
2
3
4
5
6
7
8

16
15
14
13
12
11
10

NC
NC
NC
NC
NC
NC
NC
GND

NC
NC
NC
NC
NC
NC

VDD

BOTTOM VIEW

See Mech. Drawings

Section

See Mech. Drawings

Section

PIN DESCRIPTION

GND

Ground

Data In/Out

Optional V

No Connect

DESCRIPTION

The DS1820 Digital Thermometer provides 9bit tem-
perature readings which indicate the temperature of the
device.

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

Because each DS1820 contains a unique silicon serial
number, multiple DS1820s can exist on the same
1Wire 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.

DS1820

030598 2/27

DETAILED PIN DESCRIPTION

PIN

16PIN SSOP

PIN

PR35

SYMBOL

DESCRIPTION

GND

Ground.

Data Input/Output pin. For 1Wire operation: Open drain. (See
"Parasite Power" section.)

Optional V

pin. See "Parasite Power" section for details of

connection.

DS1820S (16pin 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) 64bit lasered ROM, 2) temperature
sensor, and 3) nonvolatile temperature alarm triggers
TH and TL. The device derives its power from the
1Wire 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 1Wire 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.

Communication to the DS1820 is via a 1Wire port. With
the 1Wire 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 64bit
lasered ROM portion of each device and can single out

a specific device if many are present on the 1Wire 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.

DS1820 BLOCK DIAGRAM Figure 1

64BIT ROM

SCRATCHPAD

MEMORY AND

CONTROL LOGIC

AND

1WIRE PORT

TEMPERATURE SENSOR

8BIT CRC

GENERATOR

POWER

SUPPLY

SENSE

INTERNAL V

HIGH TEMPERATURE

TRIGGER, TH

LOW TEMPERATURE

TRIGGER, TL

+5V

DS1820

I/O

4.7K

GND

DS1820

030598 3/27

PARASITE POWER

The block diagram (Figure 1) shows the parasite pow-
ered circuitry. This circuitry "steals" power whenever the
I/O or V

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 "1Wire Bus Sys-
tem"). The advantages of parasite power are twofold:
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.

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 pullup resistor. This problem is particu-
larly acute if several DS1820's are on the same I/O and
attempting to convert simultaneously.

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 pullup on the I/O line
whenever temperature conversions or copies to the E

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 pullup within 10

s maxi-

mum after issuing any protocol that involves copying to
the E

memory or initiates temperature conversions.

When using the parasite power mode, the V

pin must

be tied to ground.

Another method of supplying current to the DS1820 is
through the use of an external power supply tied to the

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

that the strong pullup 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 1Wire bus during the conversion time. In
addition, any number of DS1820's may be placed on the
1Wire 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.

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

be applied to the DS1820.

For situations where the bus master does not know
whether the DS1820's on the bus are parasite powered
or supplied with external V

, 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 pullup 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 1Wire bus if it is parasite powered; it will send
back a "1" if it is powered from the V

pin. If the master

receives a "0", it knows that it must supply the strong
pullup on the I/O line during temperature conversions.
See "Memory Command Functions" section for more
detail on this command protocol.

STRONG PULLUP FOR SUPPLYING DS1820 DURING TEMPERATURE CONVERSION Figure 2

DS1820

030598 4/27

USING V

TO SUPPLY TEMPERATURE CONVERSION CURRENT Figure 3

+5V

DS1820

I/O

4.7K

TO OTHER 1WIRE
DEVICES

EXTERNAL +5V SUPPLY

OPERATION MEASURING TEMPERATURE

The DS1820 measures temperature through the use of
an onboard proprietary temperature measurement
technique. A block diagram of the temperature mea-
surement circuitry is shown in Figure 4.

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

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 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 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.

Internally, this calculation is done inside the DS1820 to
provide 0.5

C resolution. The temperature reading is

provided in a 16bit, signextended 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 1Wire 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.

Note that temperature is represented in the DS1820 in
terms of a

C LSB, yielding the following 9bit format:

MSB

LSB

= 25

The most significant (sign) bit is duplicated into all of the
bits in the upper MSB of the twobyte temperature reg-
ister in memory. This "signextension" yields the 16bit
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

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:

TEMPERATURE = TEMP_READ 0.25

)

(COUNT_PER_C COUNT_REMAIN)

COUNT_PER_C

DS1820

030598 5/27

TEMPERATURE MEASURING CIRCUITRY Figure 4

SLOPE ACCUMULATOR

PRESET

COUNTER

STOP

INC

COMPARE

TEMPERATURE REGISTER

LOW TEMPERATURE

COEFFICIENT OSCILLATOR

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR

SET/CLEAR

LSB

TEMPERATURE/DATA RELATIONSHIPS Table 1

TEMPERATURE

DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125

00000000 11111010

00FA

+25

00000000 00110010

0032h

00000000 00000001

0001h

00000000 00000000

0000h

11111111 11111111

FFFFh

11111111 11001110

FFCEh

11111111 10010010

FF92h

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
8bit only, the 0.5

C bit is ignored for comparison. The

most significant bit of TH or TL directly corresponds to
the sign bit of the 16bit 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 nonalarming devices.

DS1820

030598 6/27

64BIT LASERED ROM

Each DS1820 contains a unique ROM code that is
64bits long. The first eight bits are a 1Wire family
code (DS1820 code is 10h). The next 48 bits are a
unique serial number. The last eight bits are a CRC of
the first 56 bits. (See Figure 5.) The 64bit ROM and
ROM Function Control section allow the DS1820 to
operate as a 1Wire device and follow the 1Wire proto-
col detailed in the section "1Wire Bus System". The
functions required to control sections of the DS1820 are
not accessible until the ROM function protocol has been
satisfied. This protocol is described in the ROM function
protocol flowchart (Figure 6). The 1Wire bus 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. After a ROM functions
sequence has been successfully executed, the func-
tions specific to the DS1820 are accessible and the bus
master may then provide and one of the six memory and
control function commands.

CRC GENERATION

The DS1820 has an 8bit CRC stored in the most signif-
icant byte of the 64bit ROM. The bus master can com-
pute a CRC value from the first 56bits of the 64bit
ROM and compare it to the value stored within the
DS1820 to determine if the ROM data has been
received errorfree by the bus master. The equivalent
polynomial function of this CRC is:

CRC = X

+ X

+ 1

The DS1820 also generates an 8bit CRC value using
the same polynomial function shown above and pro-

vides this value to the bus master to validate the transfer
of data bytes. In each case where a CRC is used for data
transfer validation, the bus master must calculate a
CRC value using the polynomial function given above
and compare the calculated value to either the 8bit
CRC value stored in the 64bit ROM portion of the
DS1820 (for ROM reads) or the 8bit CRC value com-
puted within the DS1820 (which is read as a ninth byte
when the scratchpad is read). The comparison of CRC
values and decision to continue with an operation are
determined entirely by the bus master. There is no cir-
cuitry inside the DS1820 that prevents a command
sequence from proceeding if the CRC stored in or calcu-
lated by the DS1820 does not match the value gener-
ated by the bus master.

The 1Wire CRC can be generated using a polynomial
generator consisting of a shift register and XOR gates
as shown in Figure 7. Additional information about the
Dallas 1Wire Cyclic Redundancy Check is available in
Application Note 27 entitled "Understanding and Using
Cyclic Redundancy Checks with Dallas Semiconductor
Touch Memory Products".

The shift register bits are initialized to zero. Then start-
ing with the least significant bit of the family code, one bit
at a time is shifted in. After the 8th bit of the family code
has been entered, then the serial number is entered.
After the 48th bit of the serial number has been entered,
the shift register contains the CRC value. Shifting in the
eight bits of CRC should return the shift register to all
zeros.

64BIT LASERED ROM Figure 5

8BIT CRC CODE

48BIT SERIAL NUMBER

8BIT FAMILY CODE (10h)

MSB

LSB

MSB

LSB

MSB

LSB

DS1820 T

PRESENCE

PULSE

33h

READ ROM

COMMAND

55h

MATCH ROM

COMMAND

F0h

SEARCH ROM

COMMAND

CCh

SKIP ROM

COMMAND

DS1820 T

FAMILY

CODE

1 BYTE

BIT 0

MATCH?

BIT 0

MATCH?

BIT 1

MATCH?

BIT 1

MATCH?

BIT 63

MATCH?

BIT 63

MATCH?

DS1820 T

SERIAL NUMBER

6 BYTES

DS1820 T

CRC BYTE

DS1820 T

BIT 0

DS1820 T

BIT 0

DS1820 T

BIT 1

DS1820 T

BIT 1

DS1820 T

BIT 63

DS1820 T

BIT 63

MASTER T

BIT 1

MASTER T

BIT 0

MASTER T

BIT 0

MASTER T

BIT 1

MASTER T

BIT 63

MASTER T

BIT 63

MASTER T

RESET PULSE

MASTER T

ROM

FUNCTION COMMAND

MASTER T

MEMORY OR CONTROL

FUNCTION COMMAND

ECh

ALARM SEARCH

COMMAND

ALARM

CONDITION

DS1820

030598 7/27

ROM FUNCTIONS FLOW CHART Figure 6

DS1820

030598 8/27

1WIRE CRC CODE Figure 7

(MSB)

(LSB)

XOR

INPUT

MEMORY

The DS1820's memory is organized as shown in
Figure 8. The memory consists of a scratchpad RAM
and a nonvolatile, electrically erasable (E

) RAM, which

stores the high and low temperature triggers TH and TL.
The scratchpad helps insure data integrity when com-
municating over the 1Wire bus. Data is first written to
the scratchpad where it can be read back. After the data
has been verified, a copy scratchpad command will
transfer the data to the nonvolatile (E

) RAM. This pro-

cess insures data integrity when modifying the memory.

The scratchpad is organized as eight bytes of memory.
The first two bytes contain the measured temperature

information. The third and fourth bytes are volatile
copies of TH and TL and are refreshed with every pow-
eron reset. The next two bytes are not used; upon
reading back, however, they will appear as all logic 1's.
The seventh and eighth bytes are count registers, which
may be used in obtaining higher temperature resolution
(see "Operationmeasuring Temperature" section).

There is a ninth byte which may be read with a Read
Scratchpad command. This byte contains a cyclic
redundancy check (CRC) byte which is the CRC over all
of the eight previous bytes. This CRC is implemented in
the fashion described in the section titled "CRC Genera-
tion".

DS1820 MEMORY MAP Figure 8

TEMPERATURE LSB

TEMPERATURE MSB

TH/USER BYTE 1

TL/USER BYTE 2

RESERVED

COUNT REMAIN

CRC

BYTE

TH/USER BYTE 1

TL/USER BYTE 2

SCRATCHPAD

RAM

COUNT PER

DS1820

030598 9/27

1WIRE BUS SYSTEM

The 1Wire bus is a system which has a single bus mas-
ter and one or more slaves. The DS1820 behaves as a
slave. The discussion of this bus system is broken down
into three topics: hardware configuration, transaction
sequence, and 1Wire signaling (signal types and tim-
ing).

HARDWARE CONFIGURATION

The 1Wire bus has only a single line by definition; it is
important that each device on the bus be able to drive it

at the appropriate time. To facilitate this, each device
attached to the 1Wire bus must have open drain or
3state outputs. The 1Wire port of the DS1820 (I/O
pin) is open drain with an internal circuit equivalent to
that shown in Figure 9. A multidrop bus consists of a
1Wire bus with multiple slaves attached. The 1Wire
bus requires a pullup resistor of approximately 5K

HARDWARE CONFIGURATION Figure 9

+5V

4.7K

1OO OHM
MOSFET

DS1820 1WIRE PORT

BUS MASTER

Typ.

= RECEIVE

= TRANSMIT

The idle state for the 1Wire bus is high. If for any reason
a transaction needs to be suspended, the bus MUST be
left in the idle state if the transaction is to resume. Infinite
recovery time can occur between bits so long as the
1Wire bus is in the inactive (high) state during the re-
covery period. If this does not occur and the bus is left
low for more than 480

s, all components on the bus will

be reset.

TRANSACTION SEQUENCE

The protocol for accessing the DS1820 via the 1Wire
port is as follows:

Initialization

ROM Function Command

Memory Function Command

Transaction/Data

INITIALIZATION

All transactions on the 1Wire bus begin with an initial-
ization sequence. The initialization sequence consists
of a reset pulse transmitted by the bus master followed
by presence pulse(s) transmitted by the slave(s).

The presence pulse lets the bus master know that the
DS1820 is on the bus and is ready to operate. For more
details, see the "1Wire Signaling" section.

ROM FUNCTION COMMANDS

Once the bus master has detected a presence, it can
issue one of the five ROM function commands. All ROM
function commands are 8bits long. A list of these com-
mands follows (refer to flowchart in Figure 6):

DS1820

030598 10/27

Read ROM [33h]

This command allows the bus master to read the
DS1820's 8bit family code, unique 48bit serial num-
ber, and 8bit CRC. This command can only be used if
there is a single DS1820 on the bus. If more than one
slave is present on the bus, a data collision will occur
when all slaves try to transmit at the same time (open
drain will produce a wired AND result).

Match ROM [55h]

The match ROM command, followed by a 64bit ROM
sequence, allows the bus master to address a specific
DS1820 on a multidrop bus. Only the DS1820 that
exactly matches the 64bit ROM sequence will respond
to the following memory function command. All slaves
that do not match the 64bit ROM sequence will wait for
a reset pulse. This command can be used with a single
or multiple devices on the bus.

Skip ROM [CCh]

This command can save time in a single drop bus sys-
tem by allowing the bus master to access the memory
functions without providing the 64bit ROM code. If
more than one slave is present on the bus and a read
command is issued following the Skip ROM command,
data collision will occur on the bus as multiple slaves
transmit simultaneously (open drain pulldowns will pro-
duce a wired AND result).

Search ROM [F0h]

When a system is initially brought up, the bus master
might not know the number of devices on the 1Wire
bus or their 64bit ROM codes. The search ROM com-
mand allows the bus master to use a process of elimina-
tion to identify the 64bit ROM codes of all slave devices
on the bus.

Alarm Search [ECh]

The flowchart of this command is identical to the Search
ROM command. However, the DS1820 will respond to
this command only if an alarm condition has been
encountered at the last temperature measurement. An
alarm condition is defined as a temperature higher than
TH or lower than TL. The alarm condition remains set as
long as the DS1820 is powered up, or until another tem-
perature measurement reveals a nonalarming value.
For alarming, the trigger values stored in EEPROM are
taken into account. If an alarm condition exists and the
TH or TL settings are changed, another temperature

conversion should be done to validate any alarm condi-
tions.

Example of a ROM Search

The ROM search process is the repetition of a simple
3step routine: read a bit, read the complement of the
bit, then write the desired value of that bit. The bus mas-
ter performs this simple, 3step routine on each bit of
the ROM. After one complete pass, the bus master
knows the contents of the ROM in one device. The
remaining number of devices and their ROM codes may
be identified by additional passes.

The following example of the ROM search process
assumes four different devices are connected to the
same 1Wire bus. The ROM data of the four devices is
as shown:

ROM1

00110101...

ROM2

10101010...

ROM3

11110101...

ROM4

00010001...

The search process is as follows:

1. The bus master begins the initialization sequence by

issuing a reset pulse. The slave devices respond by
issuing simultaneous presence pulses.

2. The bus master will then issue the Search ROM

command on the 1Wire bus.

3. The bus master reads a bit from the 1Wire bus.

Each device will respond by placing the value of the
first bit of their respective ROM data onto the 1Wire
bus. ROM1 and ROM4 will place a 0 onto the
1Wire bus, i.e., pull it low. ROM2 and ROM3 will
place a 1 onto the 1Wire bus by allowing the line to
stay high. The result is the logical AND of all devices
on the line, therefore the bus master sees a 0. The
bus master reads another bit. Since the Search
ROM data command is being executed, all of the
devices on the 1Wire bus respond to this second
read by placing the complement of the first bit of their
respective ROM data onto the 1Wire bus. ROM1
and ROM4 will place a 1 onto the 1Wire, allowing
the line to stay high. ROM2 and ROM3 will place a
0 onto the 1Wire, thus it will be pulled low. The bus
master again observes a 0 for the complement of the
first ROM data bit. The bus master has determined
that there are some devices on the 1Wire bus that
have a 0 in the first position and others that have a 1.

DS1820

030598 11/27

The data obtained from the two reads of the 3step
routine have the following interpretations:

There are still devices attached which have
conflicting bits in this position.

All devices still coupled have a 0bit in this
bit position.

All devices still coupled have a 1bit in this
bit position.

There are no devices attached to the 1Wire
bus.

4. The bus master writes a 0. This deselects ROM2

and ROM3 for the remainder of this search pass,
leaving only ROM1 and ROM4 connected to the
1Wire bus.

5. The bus master performs two more reads and

receives a 0bit followed by a 1bit. This indicates
that all devices still coupled to the bus have 0's as
their second ROM data bit.

6. The bus master then writes a 0 to keep both ROM1

and ROM4 coupled.

7. The bus master executes two reads and receives

two 0bits. This indicates that both 1bits and 0bits
exist as the third bit of the ROM data of the attached
devices.

8. The bus master writes a 0bit. This deselects ROM1

leaving ROM4 as the only device still connected.

9. The bus master reads the remainder of the ROM bits

for ROM4 and continues to access the part if
desired. This completes the first pass and uniquely
identifies one part on the 1Wire bus.

10. The bus master starts a new ROM search sequence

by repeating steps 1 through 7.

11. The bus master writes a 1bit. This decouples

ROM4, leaving only ROM1 still coupled.

12. The bus master reads the remainder of the ROM bits

for ROM1 and communicates to the underlying logic
if desired. This completes the second ROM search
pass, in which another of the ROMs was found.

13. The bus master starts a new ROM search by repeat-

ing steps 1 through 3.

14. The bus master writes a 1bit. This deselects

ROM1 and ROM4 for the remainder of this search
pass, leaving only ROM2 and ROM3 coupled to the
system.

15. The bus master executes two read time slots and

receives two zeros.

16. The bus master writes a 0bit. This decouples

ROM3, and leaving only ROM2.

17. The bus master reads the remainder of the ROM bits

for ROM2 and communicates to the underlying logic
if desired. This completes the third ROM search
pass, in which another of the ROMs was found.

18. The bus master starts a new ROM search by repeat-

ing steps 13 through 15.

19. The bus master writes a 1bit. This decouples

ROM2, leaving only ROM3.

20. The bus master reads the remainder of the ROM bits

for ROM3 and communicates to the underlying logic
if desired. This completes the fourth ROM search
pass, in which another of the ROMs was found.

Note the following:

The bus master learns the unique ID number (ROM data
pattern) of one 1Wire device on each ROM Search
operation. The time required to derive the part's unique
ROM code is:

960

s + (8 + 3 x 64) 61

s = 13.16 ms

The bus master is therefore capable of identifying 75 dif-
ferent 1Wire devices per second.

I/O SIGNALING

The DS1820 requires strict protocols to insure data
integrity. The protocol consists of several types of
signaling on one line: reset pulse, presence pulse, write
0, write 1, read 0, and read 1. All of these signals, with
the exception of the presence pulse, are initiated by the
bus master.

The initialization sequence required to begin any com-
munication with the DS1820 is shown in Figure 11. A
reset pulse followed by a presence pulse indicates the
DS1820 is ready to send or receive data given the cor-
rect ROM command and memory function command.

The bus master transmits (TX) a reset pulse (a low sig-
nal for a minimum of 480

s). The bus master then

releases the line and goes into a receive mode (RX).
The 1Wire bus is pulled to a high state via the 5K
pullup resistor . After detecting the rising edge on the

DS1820

030598 16/27

INITIALIZATION PROCEDURE "RESET AND PRESENCE PULSES" Figure 11

Master T

"reset pulse"

480

s minimum

960

s maximum

Master R

480

s minimum

GND

DS1820

waits

15 - 60

DS1820 T

"presence pulse"

60 - 240

LINE TYPE LEGEND:

Bus master active low

Both bus master and
DS1820 active low

DS1820 active low

Resistor pullup

1WIRE

BUS

DS1820 COMMAND SET Table 2

INSTRUCTION

DESCRIPTION

PROTOCOL

1WIRE BUS

AFTER ISSUING

PROTOCOL

NOTES

TEMPERATURE CONVERSION COMMANDS

Convert T

Initiates temperature conversion.

44h

<read temperature

busy status>

MEMORY COMMANDS

Read Scratchpad

Reads bytes from scratchpad and
reads CRC byte.

BEh

<read data up to 9

bytes>

Write Scratchpad

Writes bytes into scratchpad at
addresses 2 and 3 (TH and TL
temperature triggers).

4Eh

<write data into 2

bytes at addr. 2 and

addr. 3>

Copy Scratchpad

Copies scratchpad into nonvolatile
memory (addresses 2 and 3 only).

48h

Recall E

Recalls values stored in nonvolatile
memory into scratchpad (tempera-
ture triggers).

B8h

<read temperature

busy status>

Read Power Supply

Signals the mode of DS1820
power supply to the master.

B4h

NOTES:

1. Temperature conversion takes up to 500 ms. After receiving the Convert T protocol, if the part does not

receive power from the V

pin, the I/O line for the DS1820 must be held high for at least 500 ms to provide

power during the conversion process. As such, no other activity may take place on the 1Wire bus for at least
this period after a Convert T command has been issued.

2. After receiving the Copy Scratchpad protocol, if the part does not receive power from the V

pin, the I/O line

for the DS1820 must be held high for at least 10 ms to provide power during the copy process. As such, no
other activity may take place on the 1Wire bus for at least this period after a Copy Scratchpad command has
been issued.

DS1820

030598 17/27

Read Scratchpad [BEh]

This command reads the contents of the scratchpad.
Reading will commence at byte 0, and will continue
through the scratchpad until the 9th (byte8, CRC) byte
is read. If not all locations are to be read, the master may
issue a reset to terminate reading at any time.

Copy Scratchpad [48h]

This command copies the scratchpad into the E

memory of the DS1820, storing the temperature trigger
bytes in nonvolatile memory. If the bus master issues
read time slots following this command, the DS1820 will
output "0" on the bus as long as it is busy copying the
scratchpad to E

; it will return a "1" when the copy pro-

cess is complete. If parasite powered, the bus master
has to enable a strong pullup for at least 10 ms immedi-
ately after issuing this command.

Convert T [44h]

This command begins a temperature conversion. No
further data is required. The temperature conversion will
be performed and then the DS1820 will remain idle. If
the bus master issues read time slots following this com-
mand, the DS1820 will output "0" on the bus as long as it
is busy making a temperature conversion; it will return a
"1" when the temperature conversion is complete. If par-
asite powered, the bus master has to enable a strong
pullup for 500 ms immediately after issuing this com-
mand.

Recall E2 [B8h]

This command recalls the temperature trigger values
stored in E

to the scratchpad. This recall operation hap-

pens automatically upon powerup to the DS1820 as
well, so valid data is available in the scratchpad as soon
as the device has power applied. With every read data
time slot issued after this command has been sent, the
device will output its temperature converter busy flag
"0"=busy, "1"=ready.

Read Power Supply [B4h]

With every read data time slot issued after this com-
mand has been sent to the DS1820, the device will sig-
nal its power mode: "0"=parasite power, "1"=external
power supply provided.

READ/WRITE TIME SLOTS

DS1820 data is read and written through the use of time
slots to manipulate bits and a command word to specify
the transaction.

Write Time Slots

A write time slot is initiated when the host pulls the data
line from a high logic level to a low logic level. There are
two types of write time slots: Write One time slots and
Write Zero time slots. All write time slots must be a mini-
mum of 60

s in duration with a minimum of a one

recovery time between individual write cycles.

The DS1820 samples the I/O line in a window of 15

s to

s after the I/O line falls. If the line is high, a Write One

occurs. If the line is low, a Write Zero occurs (see
Figure 12).

For the host to generate a Write One time slot, the data
line must be pulled to a logic low level and then released,
allowing the data line to pull up to a high level within
15

s after the start of the write time slot.

For the host to generate a Write Zero time slot, the data
line must be pulled to a logic low level and remain low for
60

Read Time Slots

The host generates read time slots when data is to be
read from the DS1820. A read time slot is initiated when
the host pulls the data line from a logic high level to logic
low level. The data line must remain at a low logic level
for a minimum of one

s; output data from the DS1820 is

valid for 15

s after the falling edge of the read time slot.

The host therefore must stop driving the I/O pin low in
order to read its state 15

s from the start of the read slot

(see Figure 12). By the end of the read time slot, the I/O
pin will pull back high via the external pullup resistor. All
read time slots must be a minimum of 60

s in duration

with a minimum of a one

s recovery time between indi-

vidual read slots.

Figure 13 shows that the sum of T

INIT

, T

, and

SAMPLE

must be less than 15

s. Figure 14 shows that

system timing margin is maximized by keeping T

INIT

and T

as small as possible and by locating the master

sample time towards the end of the 15

s period.

DS1820

030598 20/27

Related Application Notes

The following Application Notes can be applied to the
DS1820. These notes can be obtained from the Dallas

Semiconductor "Application Note Book", via our web-
site at http://www.dalsemi.com/, or through our faxback
service at (214) 4500441.

Application Note 27: "Understanding and Using Cyclic Redundancy Checks with Dallas Semiconductor Touch
Memory Product"

Application Note 55: "Extending the Contact Range of Touch Memories"

Application Note 74: "Reading and Writing Touch Memories via Serial Interfaces"

Application Note 104: "Minimalist Temperature Control Demo"

Application Note 105: "High Resolution Temperature Measurement with Dallas DirecttoDirect Temperature Sen-
sors"

Application Note 106: "Complex MicroLANs"

Application Note 108: "MicroLAN In the Long Run"

Sample 1Wire subroutines that can be used in conjunction with AN74 can be downloaded from the website or our
Anonymous FTP Site.

DS1820

030598 21/27

MEMORY FUNCTION EXAMPLE Table 3

Example: Bus Master initiates temperature conversion, then reads temperature (parasite power assumed).

MASTER MODE

DATA (LSB FIRST)

COMMENTS

Reset

Reset pulse (480960

s).

Presence

Presence pulse.

55h

Issue "Match ROM" command.

<64bit ROM code>

Issue address for DS1820.

44h

Issue "Convert T" command.

I/O line is held high for at least 500 ms by bus master to
allow conversion to complete.

Reset

Reset pulse.

Presence

Presence pulse.

55h

Issue "Match ROM" command.

<64bit ROM code>

Issue address for DS1820.

BEh

Issue "Read Scratchpad" command.

<9 data bytes>

Read entire scratchpad plus CRC; the master now recal-
culates the CRC of the eight data bytes received from the
scratchpad, compares the CRC calculated and the CRC
read. If they match, the master continues; if not, this read
operation is repeated.

Reset

Reset Pulse.

Presence

Presence pulse, done.

DS1820

030598 22/27

MEMORY FUNCTION EXAMPLE Table 4

Example: Bus Master writes memory (parasite power and only one DS1820 assumed).

MASTER MODE

DATA (LSB FIRST)

COMMENTS

Reset

Reset pulse.

Presence

Presence pulse.

CCh

Skip ROM command.

4Eh

Write Scratchpad command.

<2 data bytes>

Writes two bytes to scratchpad (TH and TL).

Reset

Reset pulse.

Presence

Presence pulse.

CCh

Skip ROM command.

BEh

Read Scratchpad command.

<9 data bytes>

Read entire scratchpad plus CRC. The master now recal-
culates the CRC of the eight data bytes received from the
scratchpad, compares the CRC and the two other bytes
read back from the scratchpad. If data match, the master
continues; if not, repeat the sequence.

Reset

Reset pulse.

Presence

Presence pulse.

CCh

Skip ROM command.

48h

Copy Scratchpad command; after issuing this command,
the master must wait 6 ms for copy operation to complete.

Reset

Reset pulse.

Presence

Presence pulse, done.

DS1820

030598 23/27

MEMORY FUNCTION EXAMPLE Table 5

Example: Temperature conversion and interpolation (external power supply and only one DS1820 assumed).

MASTER MODE

DATA (LSB FIRST)

COMMENTS

Reset

Reset pulse.

Presence

Presence pulse.

CCh

Skip ROM command.

44h

Convert T command.

<1 data byte>

Read busy flag eight times. The master continues reading
one byte (or bit) after another until the data is FFh (all
bits 1).

Reset

Reset pulse.

Presence

Presence pulse.

CCh

Skip ROM command.

BEh

Read Scratchpad command.

<9 data bytes>

Read entire scratchpad plus CRC. The master now recal-
culates the CRC of the eight data bytes received from the
scratchpad and compares both CRCs. If the CRCs match,
the data is valid. The master saves the temperature value
and stores the contents of the count register and count
per

C register as COUNT_REMAIN and COUNT_PER_C,

respectively.

Reset

Reset pulse.

Presence

Presence pulse, done.

CPU calculates temperature as described in the data sheet
for higher resolution.

DS1820

030598 24/27

ABSOLUTE MAXIMUM RATINGS*

Voltage on Any Pin Relative to Ground

0.5V to +7.0V

Operating Temperature

C to +125

Storage Temperature

C to +125

Soldering Temperature

260

C for 10 seconds

* This is a stress rating only and functional operation of the device at these or any other conditions above those

indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods of time may affect reliability.

RECOMMENDED DC OPERATING CONDITIONS

PARAMETER

SYMBOL

CONDITION

MIN

TYP

MAX

UNITS

NOTES

Supply Voltage

I/O Functions

C Accurate

Temperature
Conversions

2.8

4.3

5.0

5.5

1, 2

Data Pin

I/O

0.5

+5.5

Logic 1

2.0

+0.3

2, 3

Logic 0

0.3

+0.8

2, 4

DC ELECTRICAL CHARACTERISTICS

(55

C to +125

C; V

=3.6V to 5.5V)

PARAMETER

SYMBOL

CONDITION

MIN

TYP

MAX

UNITS

NOTES

Thermometer Error

ERR

C to +70

C to 0

and +70

C to

+125

See Typical Curve

1, 9, 10

Input Logic High

2.2

5.5

2, 3

Input Logic Low

0.3

+0.8

2, 4

Sink Current

I/O

=0.4V

4.0

Standby Current

200

350

Active Current

1.5

5, 6

Input Load Current

DS1820

030598 25/27

AC ELECTRICAL CHARACTERISTICS:

(55

C to +125

C; V

=3.6V to 5.5V)

PARAMETER

SYMBOL

MIN

TYP

MAX

UNITS

NOTES

Temperature Conversion Time

CONV

200

500

Time Slot

SLOT

120

Recovery Time

REC

Write 0 Low Time

LOW0

120

Write 1 Low Time

LOW1

Read Data Valid

RDV

Reset Time High

RSTH

480

Reset Time Low

RSTL

480

4800

Presence Detect High

PDHIGH

Presence Detect Low

PDLOW

240

Capacitance

IN/OUT

NOTES:

1. Temperature conversion will work with

C accuracy down to V

= 3.4 volts.

2. All voltages are referenced to ground.

3. Logic one voltages are specified at a source current of 1 mA.

4. Logic zero voltages are specified at a sink current of 4 mA.

5. I

specified with V

at 5.0 volts.

6. Active current refers to either temperature conversion or writing to the E

memory. Writing to E

memory con-

sumes approximately 200

A for up to 10 ms.

7. Input load is to ground.

8. Standby current specified up to 70

C. Standby current typically is 5

A at 125

9. See Typical Curve for specification limits outside the 0

C to 70

C range. Thermometer error reflects sensor accu-

racy as tested during calibration.

10. Typical accuracy curve valid for 4.3V

5.5V.

1WIRE WRITE ONE TIME SLOT

START OF NEXT CYCLE

REC

LOW1

SLOT

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