DS1825U+ ,Programmable Resolution 1-Wire Digital Thermometer With 4-Bit IDAPPLICATIONS Thermostatic Controls Industrial Systems 1-Wire is a registered trademark of Dallas ..
DS1830 ,Reset Sequence PushbuttonPIN DESCRIPTION- Pushbutton Reset Input1 PBRST2 TD - Time Delay Select Input3 TOL - V Tolerance Sel ..
DS1830AS ,Reset Sequence PushbuttonFEATURES 5V (DS1830) or 3.3V (DS1830A) power-onPIN ASSIGNMENTreset Excellent for systems that nee ..
DS1831AS ,Multisupply MicroMonitor™FEATURESPIN ASSIGNMENT 5V power-on resetDS1831 3.3V power-on reset Two referenced comparators wi ..
DS1831BS ,Multisupply MicroMonitor™DS1831/A/BMultisupply Micromonitor
DS1831C ,3.3V/2.5V Multisupply MicromonitorFEATURESPIN ASSIGNMENT 2.5V power-on resetDS1831C 3.3V power-on reset Two referenced comparators ..
DZ23C10 ,Zener DiodesDZ23C2V7–DZ23C51Vishay Telefunken300 mW Dual Surface Mount Zener Diodes
DZ23C15-7-F , 300mW DUAL SURFACE MOUNT ZENER DIODE
DZ23C16-7-F , 300mW DUAL SURFACE MOUNT ZENER DIODE
DZ23C18 ,Zener DiodesAbsolute Maximum RatingsT = 25
DS1825-DS1825U+
Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
FEATURES Unique 1-Wire Interface Requires Only One
Port Pin for Communication Each Device has a Unique 64-Bit Serial Code
Stored in an On-Board ROM Multidrop Capability Simplifies Distributed
Temperature-Sensing Applications 4 Pin-Programmable Bits to Uniquely Identify
Up to 16 Sensor Locations on a Bus Requires No External Components Can be Powered from Data Line. Power Supply
Range: 3.0V to 3.7V Measures Temperatures from -55°C to +125°C
(-67°F to +257°F) 0.5C Accuracy from -10°C to +85°C Thermometer Resolution is User-Selectable
from 9 to 12 Bits Converts Temperature to 12-Bit Digital Word in
750ms (max) User-Definable (NV) Alarm Settings Alarm Search Command Identifies and
Addresses Devices Whose Temperature is
Outside of Programmed Limits (Temperature
Alarm Condition) Available in 8-Pin SOP Package Software Compatible with the DS1822
PIN ASSIGNMENT
PIN DESCRIPTION GND - Ground
DQ - Data In/Out
N.C. - No Connect
VDD - Power Supply Voltage
AD0 to AD3 - Address Pins
APPLICATIONS Thermostatic Controls
Industrial Systems
Consumer Products
Thermometers
Thermally-Sensitive Systems
DESCRIPTION The DS1825 digital thermometer provides 9 to 12-bit centigrade temperature measurements and has an alarm
function with NV user-programmable upper and lower trigger points. The DS1825 communicates over a 1-Wire bus
that by definition requires only one data line (and ground) for communication with a central microprocessor. It has
an operating temperature range of -55°C to +125°C and is accurate to 0.5C over the range of -10°C to +85°C. In
addition, the DS1825 can derive power directly from the data line (“parasite power”), eliminating the need for an
external power supply.
ORDERING INFORMATION
ORDERING NUMBER PACKAGE MARKING DESCRIPTION DS1825U 1825 8-pin µSOP
DS1825U/T&R 1825 8-pin µSOP Tape-and-Reel
DS1825U+ 1825 (See Note 1) 8-pin SOP, Lead Free
DS1825
Programmable Resolution 1-Wire
Digital Thermometer With 4-Bit ID
AD3
AD2
AD1
AD0 GND
N.C.
VDD
8-pin SOP
(DS1825U)
DS1825 µSOP
(DS1825U)
1-Wire is a registered trademark of Dallas Semiconductor.
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
DESCRIPTION (cont.) Each DS1825 has a unique 64-bit serial code, which allows multiple DS1825s to function on the same 1-Wire bus;
thus, it is simple to use one microprocessor to control many DS1825s distributed over a large area. In addition, the
4-bit location address can be used to identify specific temperature sensors in the system without requiring a wide
lookup table. Applications that can benefit from this feature include HVAC environmental controls, temperature
monitoring systems inside buildings, equipment or machinery, and process monitoring and control systems.
ABSOLUTE MAXIMUM RATINGS* Voltage on Any Pin Relative to Ground -0.5V to +6.0V
Operating Temperature Range -55C to +125C
Storage Temperature Range -55C to +125C
Solder Dip Temperature (10s) +260C
Reflow Oven Temperature +220C
These are stress ratings 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.
DC ELECTRICAL CHARACTERISTICS (-55°C to +125°C; VDD= 3.0V to 3.7V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES Supply Voltage VDD Local Power +3.0 +3.7 V 1
Parasite Power +3.0 +3.7 Pullup Supply Voltage VPU Local Power +3.0 VDD V 1, 2
-10°C to +85°C ±0.5 °C Thermometer Error tERR -55°C to +125°C ±2 °C 3
Programming
Resistor: AD0-AD3 RPGM 0 10 k 12
DQ Input Logic Low VIL(DQ) -0.3 +0.7 V 1, 4, 5
Local Power +2.2
DQ Input Logic High VIH(DQ)
Parasite Power +3.0
The lower of
3.7
or
VDD + 0.3
V 1, 6
Sink Current IL VI/O = 0.4V 4.0 mA 1
Standby Current IDDS 500 1000 nA 7, 8
Active Current IDD VDD = 3.7V 0.65 1.5 mA 9
DQ Input Current IDQ 5 µA 10
Drift ±0.2 °C 11
NOTES: 1. All voltages are referenced to ground.
2. The Pullup Supply Voltage specification assumes that the pullup device is ideal, and therefore the high level of the pullup is equal to VPU.
In order to meet the VIH spec of the DS1825, the actual supply rail for the strong pullup transistor must include margin for the voltage drop
across the transistor when it is turned on; thus: VPU_ACTUAL = VPU_IDEAL + VTRANSISTOR.
3. See typical performance curve in Figure 18
4. Logic low voltages are specified at a sink current of 4mA.
5. To guarantee a presence pulse under low voltage parasite power conditions, VILMAX may have to be reduced to as low as 0.5V.
6. Logic high voltages are specified at a source current of 1mA.
7. Standby current specified up to 70C. Standby current typically is 3A at 125C.
8. To minimize IDDS, DQ should be within the following ranges: GND DQ GND + 0.3V or VDD - 0.3V DQ VDD.
9. Active current refers to supply current during active temperature conversions or EEPROM writes.
10. DQ line is high (“hi-Z” state).
11. Drift data is based on a 1000 hour stress test at 125°C.
12. Inputs AD0-AD3 must be tied either High or Low. A "Low" is a connection to the GND terminal. A "High" connection varies with usage of
the DS1825. When connected as a parasite powered sensor, a connection to DQ is considered a High. When powered through the VDD
pin, a connection to VDD is a High. If left floating, the input values are indeterminate and may be either logical "0" or logical "1." See
Figures 20 and 21 for details. When optional programming resistors are used, their maximum values are 10,000.
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
AC ELECTRICAL CHARACTERISTICS: NV MEMORY (-55°C to +100°C; VDD = 3.0V to 3.7V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NV Write Cycle Time twr 2 10 ms
EEPROM Writes NEEWR -55°C to +55°C 50k writes
EEPROM Data Retention tEEDR -55°C to +55°C 10 years
AC ELECTRICAL CHARACTERISTICS (-55°C to +125°C; VDD = 3.0V to 3.7V)
PARAMETER SYMBOL CONDITION MIN TYP MAX UNITS NOTES 9-bit resolution 93.75 ms 1
10-bit resolution 187.5 ms 1
11-bit resolution 375 ms 1
Temperature Conversion
Time tCONV
12-bit resolution 750 ms 1
Time to Strong Pullup On tSPON Start Convert T
Command Issued 10 µs
Time Slot tSLOT 60 120 µs 1
Recovery Time tREC 1 µs 1
Write 0 Low Time tLOW0 60 120 µs 1
Write 1 Low Time tLOW1 1 15 µs 1
Read Data Valid tRDV 15 µs 1
Reset Time High tRSTH 480 µs 1
Reset Time Low tRSTL 480 µs 1, 2
Presence Detect High tPDHIGH 15 60 µs 1
Presence Detect Low tPDLOW 60 240 µs 1
Capacitance: DQ CIN/OUT 25 pF
Capacitance: AD0-AD3 CIN_AD 50 pF
NOTES: 1. Refer to timing diagrams in Figure 18.
2. Under parasite power, if tRSTL > 960s, a power on reset may occur.
Table 1. DETAILED PIN DESCRIPTIONS
PIN SYMBOL DESCRIPTION 4 GND
Ground. DQ
Data Input/Output pin. Open-drain 1-Wire interface pin. Also provides power to the device when used in parasite power mode
(see Parasite Power section.) VDD
Optional VDD pin. VDD must be grounded for operation in parasite power mode.
5 AD0
Location Address Input Pin LSB 6 AD1
Location Address Input Pin 7 AD2
Location Address Input Pin 8 AD3
Location Address Input Pin MSB 3 N.C.
No Connection DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
OVERVIEW Figure 1 shows a block diagram of the DS1825, and pin descriptions are given in Table 1. The 64-bit ROM stores
the device’s unique serial code. The scratchpad memory contains the 2-byte temperature register that stores the
digital output from the temperature sensor. In addition, the scratchpad provides access to the 1-byte upper and
lower alarm trigger registers (TH and TL), and the 1-byte configuration register. The configuration register allows the
user to set the resolution of the temperature-to-digital conversion to 9, 10, 11, or 12 bits. It is also used for the hard-
wired address programmed by the AD0-AD3 pins. The TH, TL, and configuration registers are NV (EEPROM), so
they will retain data when the device is powered down.
The DS1825 uses Dallas’ exclusive 1-Wire bus protocol that implements bus communication using one control
signal. The control line requires a weak pullup resistor since all devices are linked to the bus through a 3-state or
open-drain port (the DQ pin in the case of the DS1825). In this bus system, the microprocessor (the master device)
identifies and addresses devices on the bus using each device’s unique 64-bit code. Because each device has a
unique code, the number of devices that can be addressed on one bus is virtually unlimited. The 1-Wire bus
protocol, including detailed explanations of the commands and “time slots,” is covered in the 1-Wire BUS SYSTEM
section of this data sheet.
Another feature of the DS1825 is the ability to operate without an external power supply. Power is instead supplied
through the 1-Wire pullup resistor through the DQ pin when the bus is high. The high bus signal also charges an
internal capacitor (CPP), which then supplies power to the device when the bus is low. This method of deriving
power from the 1-Wire bus is referred to as “parasite power.” As an alternative, the DS1825 can also be powered
by an external supply on VDD.
Figure 1. DS1825 BLOCK DIAGRAM Power
Supply
Sense
64-Bit ROM
And
1-wire Port
16-bit Temp Reg
8-bit TH Register
8-bit TL Register
8-bit Config. Reg
8-bit CRC Gen
Memory
Control Logic
VDD
GND
VPULLUP
AD0-AD3
4.7k
Parasite
Power
Circuit
Cpp
Address Pin
Input Latch
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
OPERATIONMEASURING TEMPERATURE The core functionality of the DS1825 is its direct-to-digital temperature sensor. The resolution of the temperature
sensor is user-configurable to 9, 10, 11, or 12 bits, corresponding to increments of 0.5C, 0.25C, 0.125C, and
0.0625C, respectively. The default resolution at power-up is 12-bit. The DS1825 powers-up in a low-power idle
state; to initiate a temperature measurement and A-to-D conversion, the master must issue a Convert T [44h]
command. Following the conversion, the resulting thermal data is stored in the 12-bit temperature register in the
scratchpad memory and the DS1825 returns to its idle state. If the DS1825 is powered by an external supply, the
master can issue “read time slots” (see the 1-Wire BUS SYSTEM section) after the Convert T command and the
DS1825 will respond by transmitting 0 while the temperature conversion is in progress and 1 when the conversion
is done. If the DS1825 is powered with parasite power, this notification technique cannot be used since the bus
must be pulled high by a strong pullup during the entire temperature conversion. The bus requirements for parasite
power are explained in detail in the POWERING THE DS1825 section of this data sheet.
The DS1825 output temperature data is calibrated in degrees centigrade; for Fahrenheit applications, a lookup
table or conversion routine must be used. The temperature data is stored as a 16-bit sign-extended two’s
complement number in the temperature register (see Figure 2). The sign bits (S) indicate if the temperature is
positive or negative: for positive numbers S = 0 and for negative numbers S = 1. If the DS1825 is configured for 12-
bit resolution, all bits in the temperature register will contain valid data. For 11-bit resolution, bit 0 is undefined. For
10-bit resolution, bits 1 and 0 are undefined, and for 9-bit resolution bits 2, 1 and 0 are undefined. Table 3 gives
examples of digital output data and the corresponding temperature reading for 12-bit resolution conversions.
Figure 2. TEMPERATURE REGISTER FORMAT bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
LS Byte 23 22 21 20 2-1 2-2 2-3 2-4
bit 15 bit 14 bit 13 bit 12 bit 11 bit 10 bit 9 bit 8
MS Byte S S S S S 26 25 24
Table 3. TEMPERATURE/DATA RELATIONSHIP
TEMPERATURE DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT
(Hex) +125°C 0000 0111 1101 0000 07D0h
+85°C* 0000 0101 0101 0000 0550h
+25.0625°C 0000 0001 1001 0001 0191h
+10.125°C 0000 0000 1010 0010 00A2h
+0.5°C 0000 0000 0000 1000 0008h
0°C 0000 0000 0000 0000 0000h
-0.5°C 1111 1111 1111 1000 FFF8h
-10.125°C 1111 1111 0101 1110 FF5Eh
-25.0625°C 1111 1110 0110 1111 FE6Fh
-55°C 1111 1100 1001 0000 FC90h
*The power-on reset value of the temperature register is +85°C
OPERATIONALARM SIGNALING After the DS1825 performs a temperature conversion, the temperature value is compared to the user-defined two’s
complement alarm trigger values stored in the 1-byte TH and TL registers (see Figure 3). The sign bit (S) indicates if
the value is positive or negative: for positive numbers S = 0 and for negative numbers S = 1. The TH and TL
registers are NV (EEPROM) so they will retain data when the device is powered down. TH and TL can be accessed
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
Figure 3. TH AND TL REGISTER FORMAT bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
S 26 25 25 25 22 21 20
Only bits 11 through 4 of the temperature register are used in the TH and TL comparison since TH and TL are 8-bit
registers. If the measured temperature is lower than or equal to TL or higher than or equal to TH, an alarm condition
exists and an alarm flag is set inside the DS1825. This flag is updated after every temperature measurement;
therefore, if the alarm condition goes away, the flag will be turned off after the next temperature conversion.
The master device can check the alarm flag status of all DS1825s on the bus by issuing an Alarm Search [ECh]
command. Any DS1825s with a set alarm flag will respond to the command, so the master can determine exactly
which DS1825s have experienced an alarm condition. If an alarm condition exists and the TH or TL settings have
changed, another temperature conversion should be done to validate the alarm condition.
POWERING THE DS1825 The DS1825 can be powered by an external supply on the VDD pin, or it can operate in “parasite power” mode,
which allows the DS1825 to function without a local external supply. Parasite power is very useful for applications
that require remote temperature sensing or that are very space constrained. Figure 1 shows the DS1825’s
parasite-power control circuitry, which “steals” power from the 1-Wire bus through the DQ pin when the bus is high.
The stolen charge powers the DS1825 while the bus is high, and some of the charge is stored on the parasite
power capacitor (CPP) to provide power when the bus is low. When the DS1825 is used in parasite power mode,
the VDD pin must be connected to ground.
In parasite power mode, the 1-Wire bus and CPP can provide sufficient current to the DS1825 for most operations
as long as the specified timing and voltage requirements are met (refer to the DC ELECTRICAL
CHARACTERISTICS and the AC ELECTRICAL CHARACTERISTICS sections of this data sheet). However, when
the DS1825 is performing temperature conversions or copying data from the scratchpad memory to EEPROM, the
operating current can be as high as 1.5mA. This current can cause an unacceptable voltage drop across the weak
1-Wire pullup resistor and is more current than can be supplied by CPP. To assure that the DS1825 has sufficient
supply current, it is necessary to provide a strong pullup on the 1-Wire bus whenever temperature conversions are
taking place or data is being copied from the scratchpad to EEPROM. This can be accomplished by using a
MOSFET to pull the bus directly to the rail as shown in Figure 4. The 1-Wire bus must be switched to the strong
pullup within 10s (max) after a Convert T [44h] or Copy Scratchpad [48h] command is issued, and the bus must
be held high by the pullup for the duration of the conversion (tconv) or data transfer (twr = 10ms). No other activity can
take place on the 1-Wire bus while the pullup is enabled.
The DS1825 can also be powered by the conventional method of connecting an external power supply to the VDD
pin, as shown in Figure 5. The advantage of this method is that the MOSFET pullup is not required, and the 1-Wire
bus is free to carry other traffic during the temperature conversion time.
The use of parasite power is not recommended for temperatures above 100C since the DS1825 may not be able
to sustain communications due to the higher leakage currents that can exist at these temperatures. For
applications in which such temperatures are likely, it is strongly recommended that the DS1825 be powered by an
external power supply.
In some situations the bus master may not know whether the DS1825s on the bus are parasite powered or
powered by external supplies. The master needs this information to determine if the strong bus pullup should be
used during temperature conversions. To get this information, the master can issue a Skip ROM [CCh] command
followed by a Read Power Supply [B4h] command followed by a “read time slot”. During the read time slot, parasite
powered DS1825s will pull the bus low, and externally powered DS1825s will let the bus remain high. If the bus is
pulled low, the master knows that it must supply the strong pullup on the 1-Wire bus during temperature
conversions.
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
Figure 4. SUPPLYING THE PARASITE-POWERED DS1825 DURING
TEMPERATURE CONVERSIONS
Figure 5. POWERING THE DS1825 WITH AN EXTERNAL SUPPLY
64-BIT LASERED ROM CODE Each DS1825 contains a unique 64-bit code (see Figure 6) stored in ROM. The least significant 8 bits of the ROM
code contain the DS1825’s 1-Wire family code: 3Bh. The next 48 bits contain a unique serial number. The most
significant 8 bits contain a cyclic redundancy check (CRC) byte that is calculated from the first 56 bits of the ROM
code. A detailed explanation of the CRC bits is provided in the CRC GENERATION section. The 64-bit ROM code
and associated ROM function control logic allow the DS1825 to operate as a 1-Wire device using the protocol
detailed in the 1-Wire BUS SYSTEM section of this data sheet.
Figure 6. 64-BIT LASERED ROM CODE 8-BIT CRC 48-BIT SERIAL NUMBER 8-BIT FAMILY CODE (3Bh)
MEMORY The DS1825’s memory is organized as shown in Figure 7. The memory consists of an SRAM scratchpad with NV
EEPROM storage for the high and low alarm trigger registers (TH and TL) and configuration register. Note that if the
DS1825 alarm function is not used, the TH and TL registers can serve as general-purpose memory. All memory
commands are described in detail in the DS1825 FUNCTION COMMANDS section.
Byte 0 and byte 1 of the scratchpad contain the LSB and the MSB of the temperature register, respectively. These
bytes are read-only. Bytes 2 and 3 provide access to TH and TL registers. Byte 4 contains the configuration register
data, which is explained in detail in the CONFIGURATION REGISTER section of this data sheet. Bytes 5, 6, and 7
MSB MSB LSB LSBLSBMSB
VPU
VPU
4.7K
1-Wire Bus
Micro-
processor
DS1825
GND VDD DQ
To Other
1-Wire Devices
VDD (External Supply) DS1825GND VDD DQ
VPU
4.7K
To Other
1-Wire Devices1-Wire Bus
Micro-
processor
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
Byte 8 of the scratchpad is read-only and contains the cyclic redundancy check (CRC) code for bytes 0 through 7
of the scratchpad. The DS1825 generates this CRC using the method described in the CRC GENERATION
section.
Data is written to bytes 2, 3, and 4 of the scratchpad using the Write Scratchpad [4Eh] command; the data must be
transmitted to the DS1825 starting with the least significant bit of byte 2. To verify data integrity, the scratchpad can
be read (using the Read Scratchpad [BEh] command) after the data is written. When reading the scratchpad, data
is transferred over the 1-Wire bus starting with the least significant bit of byte 0. To transfer the TH, TL, and
configuration data from the scratchpad to EEPROM, the master must issue the Copy Scratchpad [48h] command.
Data in the EEPROM registers is retained when the device is powered down; at power-up the EEPROM data
(including the hard-wired address inputs AD0-AD3)is reloaded into the corresponding scratchpad locations. Data
can also be reloaded from EEPROM to the scratchpad at any time using the Recall E2 [B8h] command. The master
can issue read time slots following the Recall E2 command and the DS1825 will indicate the status of the recall by
transmitting 0 while the recall is in progress and 1 when the recall is done.
Figure 7. DS1825 MEMORY MAP SCRATCHPAD (Power-up State) byte 0 Temperature LSB (50h)
byte 1 Temperature MSB (05h)
EEPROM byte 2 TH Register or User Byte 1 TH Register or User Byte 1
byte 3 TL Register or User Byte 2 TL Register or User Byte 2
byte 4 Configuration Register* Configuration Register*
byte 5 Reserved
byte 6 Reserved
byte 7 Reserved
byte 8 CRC
* Lower four bits of Configuration Register
are hardwired through AD0-AD3
CONFIGURATION REGISTER Byte 4 of the scratchpad memory is the configuration register, as shown in Figure 8. The configuration register
allows the user to set the conversion resolution using the R0 and R1 bits and read the programmed value of the
address pins. The conversion resolution power-up default is R0 = 1 and R1 = 1 (12-bit resolution). Table 4 shows
the resolution configuration settings and maximum conversion time. Note that there is a direct tradeoff between
resolution and conversion time. AD0-AD3 bits report the pin programmed location information and are sampled at
power-up. In Parasite Power mode, the address pins must be connected to DQ or GND and in VDD powered mode,
the address pins must be connected to VDD or GND. Pins tied to DQ/VDD are reported with a logical 1 and pins tied
to GND are reported as a logical 0. Pins connected to DQ/ VDD or GND through a resistor are valid logical 1s or
logical 0s if the resistor is less than 10k. Floating or high impedance (>10k) connections are indeterminate. Bit 7
and Bit 4 of the configuration register are reserved for internal use and cannot be overwritten.
Figure 8. CONFIGURATION REGISTER FORMAT Note: Bit 0 through Bit 3 are programmed through the four Location Programming Address pins AD0-AD3.
Reading the configuration register provides location information on up to 16 individual DS1825s.
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0 R1 R0 1 AD3 AD2 AD1 AD0
(85°C)
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
Table 4. THERMOMETER RESOLUTION CONFIGURATION
R1 R0 Resolution Max Conversion Time 0 0 9-bit 93.75ms (tCONV/8)
0 1 10-bit 187.5ms (tCONV/4)
1 0 11-bit 375ms (tCONV/2)
1 1 12-bit 750ms (tCONV)
CRC GENERATION CRC bytes are provided as part of the DS1825’s 64-bit ROM code and in the 9th byte of the scratchpad memory.
The ROM code CRC is calculated from the first 56 bits of the ROM code and is contained in the most significant
byte of the ROM. The scratchpad CRC is calculated from the data stored in the scratchpad, and therefore it
changes when the data in the scratchpad changes. The CRCs provide the bus master with a method of data
validation when data is read from the DS1825. To verify that data has been read correctly, the bus master must re-
calculate the CRC from the received data and then compare this value to either the ROM code CRC (for ROM
reads) or to the scratchpad CRC (for scratchpad reads). If the calculated CRC matches the read CRC, the data has
been received error free. The comparison of CRC values and the decision to continue with an operation are
determined entirely by the bus master. There is no circuitry inside the DS1825 that prevents a command sequence
from proceeding if the DS1825 CRC (ROM or scratchpad) does not match the value generated by the bus master.
The equivalent polynomial function of the CRC (ROM or scratchpad) is:
CRC = X8 + X5 + X4 + 1
The bus master can re-calculate the CRC and compare it to the CRC values from the DS1825 using the polynomial
generator shown in Figure 9. This circuit consists of a shift register and XOR gates, and the shift register bits are
initialized to 0. Starting with the least significant bit of the ROM code or the least significant bit of byte 0 in the
scratchpad, one bit at a time should shifted into the shift register. After shifting in the 56th bit from the ROM or the
most significant bit of byte 7 from the scratchpad, the polynomial generator will contain the re-calculated CRC.
Next, the 8-bit ROM code or scratchpad CRC from the DS1825 must be shifted into the circuit. At this point, if the
re-calculated CRC was correct, the shift register will contain all 0s. Additional information about the Dallas 1-Wire
cyclic redundancy check is available in Application Note 27 entitled “Understanding and Using Cyclic Redundancy
Checks with Dallas Semiconductor Touch Memory Products.”
Figure 9. CRC GENERATOR
1-Wire BUS SYSTEM The 1-Wire bus system uses a single bus master to control one or more slave devices. The DS1825 is always a
slave. When there is only one slave on the bus, the system is referred to as a “single-drop” system; the system is
“multidrop” if there are multiple slaves 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).
(MSB) (LSB)
XOR XOR XOR
INPUT
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
HARDWARE CONFIGURATION The 1-Wire bus has by definition only a single data line. Each device (master or slave) interfaces to the data line
through an open-drain or 3-state port. This allows each device to “release” the data line when the device is not
transmitting data so the bus is available for use by another device. The 1-Wire port of the DS1825 (the DQ pin) is
open drain with an internal circuit equivalent to that shown in Figure 10.
The 1-Wire bus requires an external pullup resistor of approximately 5k; 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 480s, all components on the bus will be reset.
Figure 10. HARDWARE CONFIGURATION
TRANSACTION SEQUENCE The transaction sequence for accessing the DS1825 is as follows:
Step 1. Initialization
Step 2. ROM Command (followed by any required data exchange)
Step 3. DS1825 Function Command (followed by any required data exchange)
It is very important to follow this sequence every time the DS1825 is accessed, as the DS1825 will not respond if
any steps in the sequence are missing or out of order. Exceptions to this rule are the Search ROM [F0h] and Alarm
Search [ECh] commands. After issuing either of these ROM commands, the master must return to Step 1 in the
sequence.
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 presence pulse(s) transmitted by the slave(s). The presence
pulse lets the bus master know that slave devices (such as the DS1825) are on the bus and are ready to operate.
Timing for the reset and presence pulses is detailed in the 1-Wire SIGNALING section.
ROM COMMANDS After the bus master has detected a presence pulse, it can issue a ROM command. These commands operate on
the unique 64-bit ROM codes of each slave device and allow the master to single out a specific device if many are
present on the 1-Wire bus. These commands also allow the master to determine how many and what types of
devices are present on the bus or if any device has experienced an alarm condition. There are five ROM
VPU
4.7K
DS1825 1-WIRE PORT
100
MOSFET
RX =
RECEIVE
TX =
TRANSMIT
1-Wire Bus
Pin
DS1825 Programmable Resolution 1-Wire Digital Thermometer With 4-Bit ID
SEARCH ROM [F0h] When a system is initially powered up, the master must identify the ROM codes of all slave devices on the bus,
which allows the master to determine the number of slaves and their device types. The master learns the ROM
codes through a process of elimination that requires the master to perform a Search ROM cycle (i.e., Search ROM
command followed by data exchange) as many times as necessary to identify all of the slave devices. If there is
only one slave on the bus, the simpler Read ROM command (see below) can be used in place of the Search ROM
process. For a detailed explanation of the Search ROM procedure, refer to the iButton Book of Standards at
www.ibutton.com/ibuttons/standard.pdf. After every Search ROM cycle, the bus master must return to Step 1
(Initialization) in the transaction sequence.
READ ROM [33h] This command can only be used when there is one slave on the bus. It allows the bus master to read the slave’s
64-bit ROM code without using the Search ROM procedure. If this command is used when there is more than one
slave present on the bus, a data collision will occur when all the slaves attempt to respond at the same time.
MATCH ROM [55h] The match ROM command followed by a 64-bit ROM code sequence allows the bus master to address a specific
slave device on a multidrop or single-drop bus. Only the slave that exactly matches the 64-bit ROM code sequence
will respond to the function command issued by the master; all other slaves on the bus will wait for a reset pulse.
SKIP ROM [CCh] The master can use this command to address all devices on the bus simultaneously without sending out any ROM
code information. For example, the master can make all DS1825s on the bus perform simultaneous temperature
conversions by issuing a Skip ROM command followed by a Convert T [44h] command.
Note that the Read Scratchpad [BEh] command can follow the Skip ROM command only if there is a single slave
device on the bus. In this case time is saved by allowing the master to read from the slave without sending the
device’s 64-bit ROM code. A Skip ROM command followed by a Read Scratchpad command will cause a data
collision on the bus if there is more than one slave since multiple devices will attempt to transmit data
simultaneously.
ALARM SEARCH [ECh] The operation of this command is identical to the operation of the Search ROM command except that only slaves
with a set alarm flag will respond. This command allows the master device to determine if any DS1825s
experienced an alarm condition during the most recent temperature conversion. After every Alarm Search cycle
(i.e., Alarm Search command followed by data exchange), the bus master must return to Step 1 (Initialization) in
the transaction sequence. Refer to the OPERATIONALARM SIGNALING section for an explanation of alarm flag
operation.
DS1825 FUNCTION COMMANDS After the bus master has used a ROM command to address the DS1825 with which it wishes to communicate, the
master can issue one of the DS1825 function commands. These commands allow the master to write to and read
from the DS1825’s scratchpad memory, initiate temperature conversions and determine the power supply mode.
The DS1825 function commands, which are described below, are summarized in Table 5 and illustrated by the
flowchart in Figure 12.
CONVERT T [44h] This command initiates a single temperature conversion. Following the conversion, the resulting thermal data is
stored in the 2-byte temperature register in the scratchpad memory and the DS1825 returns to its low-power idle
state. If the device is being used in parasite power mode, within 10s (max) after this command is issued the
master must enable a strong pullup on the 1-Wire bus for the duration of the conversion (tconv) as described in the
POWERING THE DS1825 section. If the DS1825 is powered by an external supply, the master can issue read time
slots after the Convert T command and the DS1825 will respond by transmitting 0 while the temperature
conversion is in progress and 1 when the conversion is done. In parasite power mode this notification technique
cannot be used since the bus is pulled high by the strong pullup during the conversion.
WRITE SCRATCHPAD [4Eh] This command allows the master to write 3 bytes of data to the DS1825’s scratchpad. The first data byte is written
into the TH register (byte 2 of the scratchpad), the second byte is written into the TL register (byte 3), and the third
