DS1821S-C05+T&R - Maxim Integrated Products, Inc.

1 of 18 071107

FEATURES

 Requires no external components

 Unique 1-Wire® interface requires only one

port pin for communication

 Operates over a -55°C to +125°C (-67°F to

+257°F) temperature range

 Functions as a standalone thermostat with

user-definable trip-points

 Provides 8-bit (1°C resolution) centigrade

temperature measurements

 Accuracy is ±1°C over 0°C to +85°C range

 Converts temperature to a digital word in 1

second (max)

 Available in 3-pin PR35 and 8-pin SO

packages

 Applications include thermostatic controls,

industrial systems, consumer products,

thermometers, or any thermally sensitive

system

PIN ASSIGNMENT

PIN DESCRIPTION

GND - Ground

DQ - Data In/Out and Thermostat Output

VDD - Power Supply Voltage

NC - No Connect

DESCRIPTION

The DS1821 can function as a standalone thermostat with user-programmable trip-points or as 8-bit

temperature sensor with a 1-Wire digital interface. The thermostat trip-points are stored in nonvolatile

memory, so DS1821 units can be programmed prior to system insertion for true standalone operation.

The DS1821 has an operating temperature range of –55°C to +125°C and is accurate to ±1°C over a range

of 0°C to +85°C. Communication with the DS1821 is accomplished through the open-drain DQ pin; this

pin also serves as the thermostat output.

DS1821

Programmable Digital Thermostat and

Thermomete

8-pin 208-mil SO

(

DS1821S

)

(

BOTTOM VIEW

)

2 3

DALLAS

DS1821

GND

VDD

2 3

PR35

(DS1821)

VDD

DS1821S

1-Wire is a registered trademark of Dallas Semico nd uct o r C orp .,

a wholly owned subsidiary of Maxim Integrated Products, Inc.

www.maxim-ic.com

DS1821

2 of 18

ORDER INFORMATION

ORDERING

NUMBER PACKAGE

MARKING DESCRIPTION

DS1821 DS1821 DS1821 in 3-pin PR35

DS1821+ DS1821 (See Note) DS1821 in 3-pin PR35

DS1821S DS1821S DS1821 in 208 mil 8-pin SO

DS1821S/T&R DS1821S DS1821 in 208 mil 8-pin SO, 2000 Piece Tape-and-Reel

DS1821S+ DS1821S (See Note) DS1821 in 208 mil 8-pin SO

DS1821S+T&R DS1821S (See Note) DS1821 in 208 mil 8-pin SO, 2000 Piece Tape-and-Reel

Note: A “+” symbol will also be marked on the package.

+ Denotes lead-free package.

DETAILED PIN DESCRIPTIONS Table 1

PR35 8-PIN

SO* SYMBOL DESCRIPTION

1 2 GND

Ground pin.

2 1 DQ Open drain data input/output pin – 1-Wire operation; Open drain

thermostat output pin –thermostat operation.

3 8 VDD Power supply pin.

*All pins not specified in this table are “No Connect” pins.

OVERVIEW

Figure 1 shows a block diagram of the DS1821 and pin descriptions are given in Table 1. The DS1821

can operate as a standalone thermostat with user-programmable trip-points or as 8-bit temperature sensor

with a 1-Wire digital interface. The open-drain DQ pin functions as the thermostat output for thermostat

operation and as the data I/O pin for 1-Wire communications. The 1-Wire interface provides user access

to the nonvolatile (EEPROM) thermostat trip-point registers (TH and TL), the status/configuration register,

and the temperature register.

When configured as standalone thermostat, temperature conversions start immediately at power-up. In

this mode, the DQ pin becomes active when the temperature of the DS1821 exceeds the limit

programmed into the TH register, and remains active until the temperature drops below the limit

programmed into the TL register.

The DS1821 uses Dallas’ exclusive 1-Wire bus protocol that implements bus communication with one

control signal. This system is explained in detail in the 1-Wire BUS SYSTEM section of this datasheet.

DS1821

3 of 18

DS1821 BLOCK DIAGRAM Figure 1

TEMPERATURE SENSOR FUNCTIONALITY

The core functionality of the DS1821 is its proprietary direct-to-digital temperature sensor, which

provides 8-bit (1°C increment) centigrade temperature readings over the range of -55°C to +125°C.

A block diagram of the temperature measurement circuitry is shown in Figure 2. This circuit measures

the temperature by counting the number of clock cycles generated by an oscillator with a low temperature

coefficient (temp-co) during a gate period determined by a high temp-co oscillator. The low temp-co

counter is preset with a base count that corresponds to –55°C. If the counter reaches 0 before the gate

period is over, the temperature register, which is preset to –55°C, is incremented by one degree, and the

counter is again preset with a starting value determined by the slope accumulator circuitry. The preset

counter value is unique for every temperature increment and compensates for the parabolic behavior of

the oscillators over temperature.

At this time, the counter is clocked again until it reaches 0. If the gate period is not over when the counter

reaches 0, the temperature register is incremented again. This process of presetting the counter, counting

down to zero, and incrementing the temperature register is repeated until the counter takes less time to

reach zero than the duration of the gate period of the high temp-co oscillator. When this iterative process

is complete, the value in the temperature register will indicate the centigrade temperature of the device.

VDD

4.7K

GND

TH RE

TER

TL RE

TER

CONFIGURATION REGISTER

ND CONTROL LOGIC

TEMPERAT

1-WIRE

INTERFACE

AND

I/O CONTROL

DD POWER

SUPPLY

SENSE

DIGITAL

COMPARATOR/

LOGIC

DS1821

4 of 18

TEMPERATURE MEASURING CIRCUITRY Figure 2

OPERATING MODES

The DS1821 has two operating modes: 1-Wire mode and thermostat mode. The power-up operating

mode is determined by the user-programmable T/R¯ bit in the status/configuration register: if T/R¯ = 0 the

device powers-up in 1-Wire mode, and if T/R¯ = 1 the device powers-up in thermostat mode. The T/R¯ bit

is stored in nonvolatile memory (EEPROM), so it will retain its value when the device is powered down.

1-Wire MODE

The DS1821 arrives from the factory in 1-Wire mode (T/R¯ = 0). In this mode, the DQ pin of the DS1821

is configured as a 1-Wire port for communication with a microprocessor using the protocols described in

the 1-Wire BUS SYSTEM section of this datasheet. These communications can include reading and

writing the high and low thermostat trip-point registers (TH and TL) and the configuration register, and

reading the temperature, counter, and slope accumulator registers. Also in this mode, the microprocessor

can initiate and stop temperature measurements as described in the OPERATION – MEASURING

TEMPERATURE section of this datasheet.

The TH and TL registers and certain bits (THF, TLF, T/R¯, POL and 1SHOT) in the status/configuration

down. This allows these registers to be pre-programmed when the DS1821 is to be used as a standalone

thermostat. Writes to these nonvolatile registers can take up to 10ms. To avoid data corruption, no

writes to nonvolatile memory should be initiated while a write to nonvolatile memory is in progress.

SLOPE ACCUMULATOR

PRESET COMPARE

LOW TEMPERATURE

COEFFICIENT OSCILLATOR COUNTER PRESET

=0 TEMPERATURE REGISTER

HIGH TEMPERATURE

COEFFICIENT OSCILLATOR COUNTER

INC

STOP

SET/CLEAR

LSB

DS1821

5 of 18

Nonvolatile write status can be monitored by reading the NVB bit in the status/configuration register:

NVB = 1 – a write to EEPROM memory is in progress, NVB = 0 – nonvolatile memory is idle.

THERMOSTAT MODE

In thermostat mode (T/R¯ = 1), the DS1821 can operate as a standalone thermostat that triggers according

to the TH and TL trip-points programmed while the device was in 1-Wire mode. In thermostat mode the

DS1821 powers-up performing continuous temperature conversions, and the DQ pin acts as the

thermostat output. Detailed operation of the thermostat output is provided in the OPERATION –

STANDALONE THERMOSTAT section of this datasheet.

Communications can be re-established with the DS1821 while it is in thermostat mode by pulling VDD to

0V while the DQ line is held high, and then toggling the DQ line low 16 times as shown in Figure 12.

This temporarily places the DS1821 in 1-Wire mode, allowing microprocessor communication with the

DS1821 via the DQ pin. At this time any I/O function can be performed, such as reading/writing the TH,

TL or configuration registers or reading the temperature register. To return to thermostat mode, the same

procedure can be performed (pulling VDD to 0V while the DQ line is held high, and then clocking the DQ

line 16 times) or the power can be cycled. Note that temporarily putting the DS1821 into 1-Wire mode

does not change the power-up mode of the device; this can only be changed by rewriting the T/R¯ bit in

the status/configuration register. Also note that holding both VDD and DQ low for more than

approximately 10 seconds will cause the DS1821 to be powered down.

OPERATION – MEASURING TEMPERATURE

DS1821 output temperature data is calibrated in degrees centigrade and is stored in two’s complement

format in the 1-byte (8-bit) temperature register (see Figure 3), which the user can access when the

DS1821 is in 1-Wire mode (T/R¯ = 0 in the status/configuration register). The sign bit (S) indicates if the

temperature is positive or negative; for positive numbers S = 0 and for negative numbers S = 1. Table 2

gives examples of digital output data and the corresponding temperature reading. For Fahrenheit

measurements, a lookup table or conversion routine must be used.

The DS1821 can be configured by the user to take continuous temperature measurements (continuous

conversion mode) or single measurements (one-shot mode). The desired configuration can be achieved

by setting the nonvolatile1SHOT bit in the status/configuration register: 1SHOT = 0 – continuous

conversion mode, 1SHOT = 1 – one-shot mode. Note that the 1SHOT setting only controls the operation

of the device in 1-Wire mode; in thermostat mode, continuous temperature conversions are started

automatically at power-up.

In continuous conversion mode, the Start Convert T [EEh] command initiates continuous temperature

conversions, which can be stopped using the Stop Convert T [22h] command. In one-shot mode the Start

Convert T [EEh] command initiates a single temperature conversion after which the DS1821 returns to a

low-power standby state. In this mode, the microprocessor can monitor the DONE bit in the

configuration register to determine when the conversion is complete: DONE = 0 ― conversion in

progress, DONE = 1 ― conversion complete. The DONE bit does not provide conversion status in

continuous conversion mode since measurements are constantly in progress (i.e., DONE will always be

0).

TEMPERATURE, TH and TL REGISTER FORMAT Figure 3

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

S 26 2

5 2

4 2

3 2

2 2

1 2

DS1821

6 of 18

TEMPERATURE/DATA RELATIONSHIP Table 2

TEMPERATURE DIGITAL OUTPUT

(Binary)

DIGITAL OUTPUT

(Hex)

+125°C* 0111 1101 7Dh

+85°C 0101 0101 55h

+25°C 0001 1001 19h

0°C 0000 0000 00h

-1°C 1111 1111 FFh

-25°C 1110 0111 E7h

-55°C 1100 1001 C9h

HIGH-RESOLUTION TEMPERATURE READINGS

The user can calculate temperature values with higher than 8-bit resolution using the data remaining in

the counter and slope accumulator when the temperature conversion is complete. To do this the user must

first read the temperature from the 8-bit temperature register. This value is called TEMP_READ in the

high-resolution equation (see Eq. 1). The 9-bit counter value must then be obtained by issuing the Read

Counter [A0h] command. This value is the count remaining in the counter at the end of the gate period

and is called COUNT_REMAIN in Eq. 1. Next the Load Counter [41h] command must be issued, which

loads the 9-bit slope accumulator value into the counter register. The slope accumulator value (called

COUNT_PER_C in Eq. 1) can then be read from the counter by again issuing the Read Counter [A0h]

command. The slope accumulator value is called “COUNT_PER_C” because it represents the number of

counts needed for an accurate measurement at a given temperature (i.e., the counts per degree C). The

high-resolution temperature can then be calculated using Eq. 1:

Eq. 1) TEMPERATURE = TEMP_READ

−

0.5 + CPERCOUNT REMAINCOUNTCPERCOUNT

)___(

−

High-resolution temperature readings cannot be used while in continuous conversion mode. Also, the

Read Counter [A0h] and Load Counter [41h] commands must not be used while in continuous conversion

mode.

DS1821

7 of 18

OPERATION – THERMOSTAT

When the DS1821 is in thermostat mode (T/R¯ = 1 in the status/configuration register), temperature

conversions are performed continuously beginning at power-up (regardless of the value of the 1SHOT

bit), and the DQ pin serves as the thermostat output. The DQ output will become active when the

temperature of the DS1821 exceeds the user-defined limit in the TH register, and will remain active until

the temperature drops below the user-defined limit in the TL register as illustrated in Figure 4. Thus, the

user can select TH and TL to provide the desired amount of thermostat output hysteresis.

The user-defined 8-bit centigrade trip-point values (TH and TL) must be stored in two’s complement

format as shown in Figure 3. The sign bit (S) indicates if the temperature is positive or negative; for

positive numbers S = 0 and for negative numbers S = 1. The non-volatile TH and TL registers must be

programmed when the DS1821 is in 1-Wire mode as explained in the OPERATING MODES section of

this datasheet. The DS1821 can be temporarily switched from thermostat mode to 1-Wire mode to

change the TH and TL values as also explained in the OPERATING MODES section.

The polarity (i.e., the active state) of the DQ output is user-selectable with the nonvolatile POL bit in the

status/configuration register. DQ is active-high when POL = 1, and DQ is active-low when POL = 0.

Two bits in the status/configuration register, THF and TLF, provide additional thermostatic information.

The value of these bits is normally 0. The THF (temperature high flag) bit will be set to 1 if the measured

temperature is ever greater than the value in the TH register and will remain a 1 until the user rewrites the

bit with a 0. The TLF (temperature low flag) bit will be set to 1 if the temperature is ever lower than the

value in the TL register and will remain a 1 until the user rewrites the bit with a 0. These bits provide a

record of the device temperature relative to the thermostat trip-points over a period of time. They are

stored in nonvolatile memory, so the data stored in THF and TLF can be analyzed after any number of

power cycles. The THF and TLF bits function in both 1-Wire and thermostat mode.

DQ OPERATION IN THERMOSTATE MODE Figure 4

STATUS/CONFIGURATION REGISTER

The status/configuration register provides information to the user about conversion status, EEPROM

activity and thermostat activity. It also allows the user to program various DS1821 options such as

power-up operating mode, thermostat output polarity and conversion mode. The status/configuration

Note that the THF, TLF T/R¯, POL and 1SHOT bits are stored in nonvolatile memory (EEPROM).

CONFIGURATION REGISTER Figure 5

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0

DONE 1 NVB THF* TLF* T/R

* POL* 1SHOT*

*Stored in EEPROM

TL THTemp (°C)

Operating Mode = Thermostat

POL=1

(

is active hi

)

DS1821

8 of 18

CONFIGURATION REGISTER BIT DESCRIPTIONS Table 3

Bit Name

(User Access) Functional Description

DONE — Temperature Conversion Done

(Read Only)

DONE = 0 — Temperature conversion is in progress.

DONE = 1 — Temperature conversion is complete.

NOTE: DONE = 0 at POR.

NVB — Non-volatile Memory Busy

(Read Only)

NVB = 0 — Nonvolatile memory is not busy.

NVB = 1 — A write to EEPROM memory is in progress

THF* — Temperature High Flag

(Read/Write)

THF = 0 — The measured temperature has not exceeded the

value stored in the TH register.

THF = 1 — At some point in time the measured temperature has

been higher than the value stored in the TH register. THF will

remain a 1 until it is over-written with a 0 by the user.

TLF* — Temperature Low Flag

(Read/Write)

TLF = 0 — The measured temperature has not been lower than

the value stored in the TL register.

TLF = 1 — At some point in time the measured temperature has

been lower than the value stored in the TL register. TLF will

remain a 1 until it is over-written with a 0 by the user.

T/R¯* — Power-up Operating Mode

(Read/Write)

T/R¯ = 0 — DS1821 powers up in 1- wire mode.

T/R¯ = 1 — DS1821 powers up in thermostat mode.

POL* — Thermostat Output (DQ) Polarity

(Read/Write)

POL = 0 — Thermostat output (DQ) is active low.

POL = 1 — Thermostat output (DQ) is active high.

1SHOT* — Conversion Mode

(Read/Write)

1SHOT = 0 — Continuous conversion mode. The Start Convert

T [EEh] command initiates continuous temperature conversions,

which can be stopped with the Stop Convert T [22h] command.

1SHOT = 1 — One-shot mode. The Start Convert T [EEh]

command initiates a single temperature conversion after which

the DS1821 returns to a low-power standby state.

*Stored in EEPROM

DS1821

9 of 18

1-Wire BUS SYSTEM

The 1-Wire bus system uses a single bus master (i.e., a microprocessor) to control slave devices. The

DS1821 functions as a slave device when it is used in 1-Wire mode; however, since the DS1821 is not

addressable or multi-droppable, a single 1-Wire-mode DS1821 must be the only slave device on the bus.

All data and commands are transmitted least significant bit first over the 1-Wire bus.

The following discussion of the 1-Wire bus system is broken down into three topics: hardware

configuration, transaction sequence, and 1-Wire signaling (signal types and timing).

HARDWARE CONFIGURATION

The 1-Wire bus has by definition only a single data line. Each device (in this case, the master and one

DS1821) interfaces to the data line via an open drain or 3–state port. This allows each device to “release”

the data line when the device is not transmitting data so that the bus is available for use by the other

device. The 1-Wire port of the DS1821 (the DQ pin) is open drain with an internal circuit equivalent to

that shown in Figure 6.

The 1-Wire bus requires an external pullup resistor of approximately 5 kΩ; thus, the idle state for the 1-

Wire 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 1-Wire

bus is in the inactive (high) state during the recovery period. If the bus is held low for more than 480 μs,

the DS1821 will be reset.

HARDWARE CONFIGURATION Figure 6

TRANSACTION SEQUENCE

The transaction sequence for accessing the DS1821 via the 1-Wire port is as follows:

• Initialization

• DS1821 Function Command

• Data Transmitted/Received

5 μA

Typ.

DS1821 1-WIRE PORT

100 Ω

FET

RX = RECEIVE

TX = TRANSMIT

VDD

4.7K

1-wire bus

Micro-

processor

DS1821

10 of 18

INITIALIZATION

All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence

consists of a reset pulse transmitted by the bus master followed by a presence pulse transmitted by the

DS1821. The presence pulse lets the bus master know that the DS1821 is on the bus and ready to

operate. Timing for the reset and presence pulses is detailed in the 1-Wire SIGNALING section.

DS1821 FUNCTION COMMANDS

The DS1821 function commands in this section allow the master to communicate with and configure the

DS1821. The DS1821 function commands are summarized in Table 4.

READ TEMPERATURE [AAh]

Provides read access to the 1-byte temperature register.

START CONVERT T [EEh]

Initiates temperature conversions. If the part is in one-shot mode (1SHOT = 1), only one conversion will

be performed. If it is in continuous mode (1SHOT = 0), continuous conversions will be performed until a

Stop Convert T command is received.

STOP CONVERT T [22h]

Stops temperature conversions when the device is in continuous conversion mode (1SHOT = 0). This

opcode has no function if the device is in one-shot mode (1SHOT = 1).

WRITE TH [01h]

WRITE TL [02h]

Provides write access to the 8-bit TH and TL registers, respectively.

READ TH [A1h]

READ TL [A2h]

Provides read access to the 8-bit TH and TL registers, respectively.

WRITE STATUS [0Ch]

Provides write access to the 8-bit status/configuration register.

READ STATUS [ACh]

Provides read access to the 8-bit status/configuration register.

READ COUNTER [A0h]

Provides read access to data in the 9-bit counter register for use in high-resolution temperature

calculations. This is explained in detail in the HIGH-RESOLUTION TEMPERATURE READINGS

section.

LOAD COUNTER [41h]

Loads the 9-bit data from the slope accumulator register into the counter register so that it can be accessed

using the Read Counter [A0h] command. Use of the Load Counter command is explained in detail in the

HIGH-RESOLUTION TEMPERATURE READINGS section.

DS1821

11 of 18

DS1821 FUNCTION COMMAND SET Table 4

Command Description Protocol 1-Wire Bus Activity

After Command is Issued

TEMPERATURE CONVERSION COMMANDS

Read

Temperature

Reads last converted temperature

value from temperature register.

AAh Master receives 8-bit temperature

value from DS1821.

Start Convert T Initiates temperature conversions. EEh None

Stop Convert T Halts temperature conversions. 22h None

THERMOSTAT and STATUS/CONFIGURATION COMMANDS

Write TH Writes data to the TH register. 01h Master transmits 8-bit TH value to

DS1821.

Write TL Writes data to the TL register. 02h Master transmits 8-bit TL value to

DS1821.

Read TH Reads data from the TH register. A1h Master receives 8-bit TH value from

DS1821.

Read TL Reads data from the TL register. A2h Master receives 8-bit TL value from

DS1821.

Write Status Writes data to the

status/configuration register.

0Ch Master transmits 8-bit

status/configuration value to DS1821.

Read Status Reads data from the

status/configuration register.

ACh Master receives 8-bit

status/configuration value from

DS1821.

HIGH-RESOLUTION COMMANDS

Read Counter Reads data from the counter

A0h Master receives 9-bit counter value

from DS1821.

Load Counter Loads slope accumulator data into

the counter register

41h None

1-Wire SIGNALING

The DS1821 uses a strict 1-Wire communication protocol to insure data integrity. Several signal types are

defined by this protocol: 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.

INITIALIZATION PROCEDURE: RESET AND PRESENCE PULSES

All communication with the DS1821 begins with an initialization sequence that consists of a reset pulse

from the master followed by a presence pulse from the DS1821. This is illustrated in Figure 7. When the

DS1821 sends the presence pulse in response to the reset, it is indicating to the master that it is on the bus

and ready to operate given an appropriate function command.

During the initialization sequence the bus master transmits (TX) the reset pulse by pulling the 1-Wire bus

low for a minimum of 480 μs. The bus master then releases the bus and goes into receive mode (RX).

When the bus is released, the 5k pullup resistor pulls the 1-Wire bus high. When the DS1821 detects this

rising edge, it waits 15–60 μs and then transmits a presence pulse by pulling the 1-Wire bus low for 60–

240 μs.

DS1821

12 of 18

INITIALIZATION TIMING Figure 7

READ/WRITE TIME SLOTS

The bus master writes data to the DS1821 during write time slots and reads data from the DS1821 during

read time slots. One bit of data is transmitted over the 1-Wire bus per time slot.

WRITE TIME SLOTS

There are two types of write time slots: “Write 1” time slots and “Write 0” time slots. The bus master

uses a Write 1 time slot to write a logic 1 to the DS1821 and a Write 0 time slot to write a logic 0 to the

DS1821. All write time slots must be a minimum of 60 μs in duration with a minimum of a 1 μs recovery

time between individual write slots. Both types of write time slots are initiated by the master pulling the

1-Wire bus low (see Figure 8).

To generate a Write 1 time slot, after pulling the 1-Wire bus low, the bus master must release the 1-Wire

bus within 15 μs. When the bus is released, the 5k pullup resistor will pull the bus high. To generate a

Write 0 time slot, after pulling the 1-Wire bus low, the bus master must continue to hold the bus low for

the duration of the time slot (at least 60 μs).

The DS1821 samples the 1-Wire bus during a window that lasts from 15 μs to 60 μs after the master

initiates the write time slot. If the bus is high during the sampling window, a 1 is written to the DS1821.

If the line is low, a 0 is written to the DS1821.

READ TIME SLOTS

The DS1821 can only transmit data to the master when the master issues read time slots. Therefore, the

master must generate read time slots immediately after issuing a read command (e.g., Read Temperature

[AAh]), so that the DS1821 can provide the requested data. All read time slots must be a minimum of 60

μs in duration with a minimum of a 1 μs recovery time between slots. A read time slot is initiated by the

master device pulling the 1-Wire bus low for a minimum of 1 μs and then releasing the bus (see Figure

8). After the master initiates the read time slot, the DS1821 will begin transmitting a 1 or 0 on the bus.

The DS1821 transmits a 1 by leaving the bus high and transmits a 0 by pulling the bus low. When

transmitting a 0, the DS1821 will release the bus by the end of the time slot, and the bus will be pulled

back to its high idle state by the pullup resister. Output data from the DS1821 is valid for 15 μs after the

falling edge that initiated the read time slot. Therefore, the master must release the bus and then sample

the bus state within 15 μs from the start of the slot.

LINE TYPE LEGEND

Bus master pulling low

DS1821 pulling low

Resistor

ull-u

VDD

GND

1-WIRE BUS

480 μs minimum 480

s minimum

DS1821 TX

presence pulse

60-240 μs

MASTER TX RESET PULSE MASTER RX

DS1821

waits 15-60 μs

DS1821

13 of 18

Figure 9 illustrates that the sum of TINIT, TRC, and TSAMPLE must be less than 15 μs for a read time slot.

Figure 10 shows that system timing margin is maximized by keeping TINIT and TRC as short as possible

and by locating the master sample time during read time slots towards the end of the 15 μs period.

READ/WRITE TIME SLOT TIMING DIAGRAM Figure 8

DETAILED MASTER READ 1 TIMING Figure 9

RECOMMENDED MASTER READ 1 TIMING Figure 10

VDD

GND

1-WIRE BUS

VIH of Master

TRC

TINT > 1 μs Master samples

VDD

GND

1-WIRE BUS

VIH of Master

TRC =

small

TINT =

small

Master samples

LINE TYPE LEGEND (Figure 8, Figure 9 and Figure 10)

Bus master pulling low DS1821 pulling low

Resistor pullup

45 μs

15 μs

VDD

GND

1-WIRE BUS

60 μs < TX “0” < 120

1 μs < TREC <

∞

DS1821 samples

MIN TYP MAX

15 μs 30

> 1

MASTER WRITE “0” SLOT MASTER WRITE “1” SLOT

DS1821 samples

MIN TYP MAX

VDD

GND

1-WIRE BUS

MASTER READ “0” SLOT MASTER READ “1” SLOT

Master samples Master samples

START

OF SLOT START

OF SLOT

> 1

1 μs < TREC <

∞

15 μs 15

s 30 μs

> 1 μs

DS1821

14 of 18

DS1821 OPERATION EXAMPLE

In this example, the master device programs the DS1821 with TL = +10°C and TH = +40°C and verifies

that the data has been saved correctly. The master then programs the status/configuration register so that

the device will power-up in thermostat mode (T/R¯ = 1) and the thermostat output will have active high

polarity (POL = 1).

MASTER

MODE DATA (LSB

FIRST) COMMENTS

TX Reset Master issues reset pulse.