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DS18B20 Programmable Resolution 1-Wire Digital Thermometer
www.maxim-ic.com
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 Requires No External Components Can Be Powered from Data Line; Power Supply Range is 3.0V to 5.5V Measures Temperatures from -55C to +125C (-67F to +257F) 0.5C Accuracy from -10C to +85C Thermometer Resolution is User Selectable from 9 to 12 Bits Converts Temperature to 12-Bit Digital Word in 750ms (Max) User-Definable Nonvolatile (NV) Alarm Settings Alarm Search Command Identifies and Addresses Devices Whose Temperature is Outside Programmed Limits (Temperature Alarm Condition) Available in 8-Pin SO (150 mils), 8-Pin SOP, and 3-Pin TO-92 Packages Software Compatible with the DS1822 Applications Include Thermostatic Controls, Industrial Systems, Consumer Products, Thermometers, or Any Thermally Sensitive System
(R)
PIN CONFIGURATIONS
DALLAS 18B20 123 VDD DQ
3 4
N.C. N.C.
1
8
N.C. N.C. N.C. GND
DALLAS 18B20
2
7 6 5
SO (150 mils) (DS18B20Z)
GND DQ VDD
DQ N.C. N.C. GND
1 2 3 4
8 7 6 5
VDD N.C. N.C. N.C.
12
3
SOP (DS18B20U)
18B20
(BOTTOM VIEW)
TO-92 (DS18B20)
DESCRIPTION
The DS18B20 digital thermometer provides 9-bit to 12-bit Celsius temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20 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 -55C to +125C and is accurate to 0.5C over the range of -10C to +85C. In addition, the DS18B20 can derive power directly from the data line ("parasite power"), eliminating the need for an external power supply. Each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same 1-Wire bus. Thus, it is simple to use one microprocessor to control many DS18B20s distributed over a large area. 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.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
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REV: 042208
DS18B20
ORDERING INFORMATION
PART DS18B20 DS18B20+ DS18B20/T&R DS18B20+T&R DS18B20-SL/T&R DS18B20-SL+T&R DS18B20U DS18B20U+ DS18B20U/T&R DS18B20U+T&R DS18B20Z DS18B20Z+ DS18B20Z/T&R DS18B20Z+T&R TEMP RANGE -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C -55C to +125C PIN-PACKAGE 3 TO-92 3 TO-92 3 TO-92 (2000 Piece) 3 TO-92 (2000 Piece) 3 TO-92 (2000 Piece)* 3 TO-92 (2000 Piece)* 8 SOP 8 SOP 8 SOP (3000 Piece) 8 SOP (3000 Piece) 8 SO 8 SO 8 SO (2500 Piece) 8 SO (2500 Piece) TOP MARK 18B20 18B20 18B20 18B20 18B20 18B20 18B20 18B20 18B20 18B20 DS18B20 DS18B20 DS18B20 DS18B20
+Denotes a lead-free package. A "+" will appear on the top mark of lead-free packages. T&R = Tape and reel. *TO-92 packages in tape and reel can be ordered with straight or formed leads. Choose "SL" for straight leads. Bulk TO-92 orders are straight leads only.
PIN DESCRIPTION
PIN SO 1, 2, 6, 7, 8 3 4 5 SOP 2, 3, 5, 6, 7 8 1 4 TO-92 -- 3 2 1 NAME N.C. VDD DQ GND No Connection Optional VDD. VDD must be grounded for operation in parasite power mode. Data Input/Output. Open-drain 1-Wire interface pin. Also provides power to the device when used in parasite power mode (see the Powering the DS18B20 section.) Ground FUNCTION
OVERVIEW
Figure 1 shows a block diagram of the DS18B20, and pin descriptions are given in the Pin Description table. 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 temperatureto-digital conversion to 9, 10, 11, or 12 bits. The TH, TL, and configuration registers are nonvolatile (EEPROM), so they will retain data when the device is powered down. The DS18B20 uses Maxim's 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 via a 3-state or open-drain port (the DQ pin in the case of the DS18B20). 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
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DS18B20
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. Another feature of the DS18B20 is the ability to operate without an external power supply. Power is instead supplied through the 1-Wire pullup resistor via 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 DS18B20 may also be powered by an external supply on VDD. Figure 1. DS18B20 Block Diagram
VPU
4.7k
PARASITE POWER CIRCUIT
MEMORY CONTROL LOGIC
DS18B20
TEMPERATURE SENSOR
DQ
INTERNAL VDD 64-BIT ROM AND 1-Wire PORT
GND
CPP
SCRATCHPAD
ALARM HIGH TRIGGER (TH) REGISTER (EEPROM) ALARM LOW TRIGGER (TL) REGISTER (EEPROM)
VDD
POWERSUPPLY SENSE
CONFIGURATION REGISTER (EEPROM) 8-BIT CRC GENERATOR
OPERATION--MEASURING TEMPERATURE
The core functionality of the DS18B20 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 DS18B20 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 2-byte temperature register in the scratchpad memory and the DS18B20 returns to its idle state. If the DS18B20 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 DS18B20 will respond by transmitting 0 while the temperature conversion is in progress and 1 when the conversion is done. If the DS18B20 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 DS18B20 section. The DS18B20 output temperature data is calibrated in degrees Celsius; 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 DS18B20 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 1 gives examples of digital output data and the corresponding temperature reading for 12-bit resolution conversions.
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DS18B20
Figure 2. Temperature Register Format
BIT 7 23 BIT 15 MS BYTE
S = SIGN
LS BYTE
BIT 6 22 BIT 14 S
BIT 5 21 BIT 13 S
BIT 4 20 BIT 12 S
BIT 3 2-1 BIT 11 S
BIT 2 2-2 BIT 10 2
6
BIT 1 2-3 BIT 9 2
5
BIT 0 2-4 BIT 8 24
S
Table 1. Temperature/Data Relationship TEMPERATURE (C) +125 +85* +25.0625 +10.125 +0.5 0 -0.5 -10.125 -25.0625 -55 DIGITAL OUTPUT (BINARY) 0000 0111 1101 0000 0000 0101 0101 0000 0000 0001 1001 0001 0000 0000 1010 0010 0000 0000 0000 1000 0000 0000 0000 0000 1111 1111 1111 1000 1111 1111 0101 1110 1111 1110 0110 1111 1111 1100 1001 0000 DIGITAL OUTPUT (HEX) 07D0h 0550h 0191h 00A2h 0008h 0000h FFF8h FF5Eh FE6Fh FC90h
*The power-on reset value of the temperature register is +85C.
OPERATION--ALARM SIGNALING
After the DS18B20 performs a temperature conversion, the temperature value is compared to the userdefined 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 nonvolatile (EEPROM) so they will retain data when the device is powered down. TH and TL can be accessed through bytes 2 and 3 of the scratchpad as explained in the Memory section. Figure 3. TH and TL Register Format
BIT 7 S BIT 6 26 BIT 5 25 BIT 4 25 BIT 3 25 BIT 2 22 BIT 1 21 BIT 0 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 DS18B20. 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.
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DS18B20
The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search [ECh] command. Any DS18B20s with a set alarm flag will respond to the command, so the master can determine exactly which DS18B20s 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 DS18B20
The DS18B20 can be powered by an external supply on the VDD pin, or it can operate in "parasite power" mode, which allows the DS18B20 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 DS18B20's parasite-power control circuitry, which "steals" power from the 1-Wire bus via the DQ pin when the bus is high. The stolen charge powers the DS18B20 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 DS18B20 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 DS18B20 for most operations as long as the specified timing and voltage requirements are met (see the DC Electrical Characteristics and AC Electrical Characteristics). However, when the DS18B20 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 DS18B20 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 DS18B20 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 DS18B20 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 DS18B20 be powered by an external power supply. In some situations the bus master may not know whether the DS18B20s 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 DS18B20s will pull the bus low, and externally powered DS18B20s 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.
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DS18B20
Figure 4. Supplying the Parasite-Powered DS18B20 During Temperature Conversions
VPU
DS18B20 P
VPU 4.7k 1-Wire BUS GND DQ VDD
TO OTHER 1-WIRE DEVICES
Figure 5. Powering the DS18B20 with an External Supply
P
VPU 4.7k 1-Wire BUS
DS18B20
GND DQ VDD
VDD (EXTERNAL SUPPLY)
TO OTHER 1-WIRE DEVICES
64-BIT LASERED ROM CODE
Each DS18B20 contains a unique 64-bit code (see Figure 6) stored in ROM. The least significant 8 bits of the ROM code contain the DS18B20's 1-Wire family code: 28h. 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 DS18B20 to operate as a 1-Wire device using the protocol detailed in the 1-Wire Bus System section. Figure 6. 64-Bit Lasered ROM Code
8-BIT CRC MSB LSB 48-BIT SERIAL NUMBER MSB LSB 8-BIT FAMILY CODE (28h) MSB LSB
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DS18B20
MEMORY
The DS18B20's memory is organized as shown in Figure 7. The memory consists of an SRAM scratchpad with nonvolatile EEPROM storage for the high and low alarm trigger registers (TH and TL) and configuration register. Note that if the DS18B20 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 DS18B20 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. Bytes 5, 6, and 7 are reserved for internal use by the device and cannot be overwritten. Byte 8 of the scratchpad is read-only and contains the CRC code for bytes 0 through 7 of the scratchpad. The DS18B20 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 DS18B20 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 is reloaded into the corresponding scratchpad locations. Data can also be reloaded from EEPROM to 2 the scratchpad at any time using the Recall E [B8h] command. The master can issue read time slots following the Recall E2 command and the DS18B20 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. DS18B20 Memory Map SCRATCHPAD (POWER-UP STATE) Byte 0 Temperature LSB (50h) (85C) Byte 1 Temperature MSB (05h) Byte 2 TH Register or User Byte 1* Byte 3 TL Register or User Byte 2* Byte 4 Configuration Register* Byte 5 Reserved (FFh) Byte 6 Reserved Byte 7 Reserved (10h) Byte 8 CRC* *Power-up state depends on value(s) stored in EEPROM.
EEPROM TH Register or User Byte 1 TL Register or User Byte 2 Configuration Register
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DS18B20
CONFIGURATION REGISTER
Byte 4 of the scratchpad memory contains the configuration register, which is organized as illustrated in Figure 8. The user can set the conversion resolution of the DS18B20 using the R0 and R1 bits in this register as shown in Table 2. The power-up default of these bits is R0 = 1 and R1 = 1 (12-bit resolution). Note that there is a direct tradeoff between resolution and conversion time. Bit 7 and bits 0 to 4 in the configuration register are reserved for internal use by the device and cannot be overwritten. Figure 8. Configuration Register
BIT 7 0
BIT 6 R1
BIT 5 R0
BIT 4 1
BIT 3 1
BIT 2 1
BIT 1 1
BIT 0 1
Table 2. Thermometer Resolution Configuration R1 0 0 1 1 R0 0 1 0 1 RESOLUTION (BITS) 9 10 11 12 MAX CONVERSION TIME 93.75ms (tCONV/8) 187.5ms (tCONV/4) 375ms (tCONV/2) 750ms (tCONV)
CRC GENERATION
CRC bytes are provided as part of the DS18B20'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 DS18B20. 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 DS18B20 that prevents a command sequence from proceeding if the DS18B20 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 DS18B20 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 DS18B20 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 Maxim 1-Wire cyclic redundancy check
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DS18B20
is available in Application Note 27: Understanding and Using Cyclic Redundancy Checks with Maxim iButton Products. Figure 9. CRC Generator
INPUT
XOR (MSB)
XOR (LSB)
XOR
1-WIRE BUS SYSTEM
The 1-Wire bus system uses a single bus master to control one or more slave devices. The DS18B20 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).
HARDWARE CONFIGURATION
The 1-Wire bus has by definition only a single data line. Each device (master or slave) 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 the bus is available for use by another device. The 1-Wire port of the DS18B20 (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
VPU
DS18B20 1-Wire PORT
4.7k Rx 1-Wire BUS DQ PIN 5A TYP Tx Rx = RECEIVE Tx = TRANSMIT Rx
TX 100 MOSFET
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DS18B20
TRANSACTION SEQUENCE
The transaction sequence for accessing the DS18B20 is as follows: Step 1. Initialization Step 2. ROM Command (followed by any required data exchange) Step 3. DS18B20 Function Command (followed by any required data exchange) It is very important to follow this sequence every time the DS18B20 is accessed, as the DS18B20 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 DS18B20) 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 commands, and each command is 8 bits long. The master device must issue an appropriate ROM command before issuing a DS18B20 function command. A flowchart for operation of the ROM commands is shown in Figure 11. 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(R) Book of Standards at www.maxim-ic.com/ibuttonbook. 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.
iButton is a registered trademark of Maxim Integrated Products, Inc.
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DS18B20
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 DS18B20s 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 DS18B20s 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. See the Operation--Alarm Signaling section for an explanation of alarm flag operation.
DS18B20 FUNCTION COMMANDS
After the bus master has used a ROM command to address the DS18B20 with which it wishes to communicate, the master can issue one of the DS18B20 function commands. These commands allow the master to write to and read from the DS18B20's scratchpad memory, initiate temperature conversions and determine the power supply mode. The DS18B20 function commands, which are described below, are summarized in Table 3 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 DS18B20 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 DS18B20 section. If the DS18B20 is powered by an external supply, the master can issue read time slots after the Convert T command and the DS18B20 will respond by transmitting a 0 while the temperature conversion is in progress and a 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 DS18B20'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 byte is written into the configuration register (byte 4). Data must be transmitted least significant bit first. All three bytes MUST be written before the master issues a reset, or the data may be corrupted. READ SCRATCHPAD [BEh] This command allows the master to read the contents of the scratchpad. The data transfer starts with the least significant bit of byte 0 and continues through the scratchpad until the 9th byte (byte 8 - CRC) is read. The master may issue a reset to terminate reading at any time if only part of the scratchpad data is needed.
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DS18B20
COPY SCRATCHPAD [48h] This command copies the contents of the scratchpad TH, TL and configuration registers (bytes 2, 3 and 4) to EEPROM. 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 at least 10ms as described in the Powering the DS18B20 section. RECALL E2 [B8h] This command recalls the alarm trigger values (TH and TL) and configuration data from EEPROM and places the data in bytes 2, 3, and 4, respectively, in the scratchpad memory. The master device can issue read time slots following the Recall E2 command and the DS18B20 will indicate the status of the recall by transmitting 0 while the recall is in progress and 1 when the recall is done. The recall operation happens automatically at power-up, so valid data is available in the scratchpad as soon as power is applied to the device. READ POWER SUPPLY [B4h] The master device issues this command followed by a read time slot to determine if any DS18B20s on the bus are using parasite power. During the read time slot, parasite powered DS18B20s will pull the bus low, and externally powered DS18B20s will let the bus remain high. See the Powering the DS18B20 section for usage information for this command. Table 3. DS18B20 Function Command Set 1-Wire BUS COMMAND DESCRIPTION PROTOCOL ACTIVITYAFTER COMMAND IS ISSUED TEMPERATURE CONVERSION COMMANDS Convert T Initiates temperature DS18B20 transmits conversion. conversion status to master 44h (not applicable for parasitepowered DS18B20s). MEMORY COMMANDS Read Reads the entire scratchpad DS18B20 transmits up to 9 BEh Scratchpad including the CRC byte. data bytes to master. Write Writes data into scratchpad Master transmits 3 data bytes Scratchpad bytes 2, 3, and 4 (TH, TL, to DS18B20. 4Eh and configuration registers). Copy Copies TH, TL, and None Scratchpad configuration register data 48h from the scratchpad to EEPROM. 2 Recalls TH, TL, and DS18B20 transmits recall Recall E configuration register data status to master. B8h from EEPROM to the scratchpad. Read Power Signals DS18B20 power DS18B20 transmits supply B4h Supply supply mode to the master. status to master.
Note 1: Note 2: Note 3:
NOTES
1
2
3
1
For parasite-powered DS18B20s, the master must enable a strong pullup on the 1-Wire bus during temperature conversions and copies from the scratchpad to EEPROM. No other bus activity may take place during this time. The master can interrupt the transmission of data at any time by issuing a reset. All three bytes must be written before a reset is issued. 12 of 22
DS18B20
Figure 11. ROM Commands Flowchart
Initialization Sequence
MASTER TX RESET PULSE
DS18B20 TX PRESENCE PULSE
MASTER TX ROM COMMAND
33h READ ROM COMMAND
N
55h MATCH ROM COMMAND
N
F0h SEARCH ROM COMMAND
N
ECh ALARM SEARCH COMMAND
N
CCh SKIP ROM COMMAND
N
Y
Y
Y
Y
Y
MASTER TX BIT 0
DS18B20 TX BIT 0 DS18B20 TX BIT 0 MASTER TX BIT 0 DS18B20 TX FAMILY CODE 1 BYTE N BIT 0 MATCH? DS18B20 TX SERIAL NUMBER 6 BYTES N BIT 0 MATCH? Y
DS18B20 TX BIT 0 DS18B20 TX BIT 0 MASTER TX BIT 0
DEVICE(S) WITH ALARM FLAG SET? Y
N
Y
DS18B20 TX BIT 1 DS18B20 TX CRC BYTE MASTER TX BIT 1 DS18B20 TX BIT 1 MASTER TX BIT 1
N BIT 1 MATCH?
N BIT 1 MATCH? Y
Y
DS18B20 TX BIT 63 MASTER TX BIT 63 DS18B20 TX BIT 63 MASTER TX BIT 63
N BIT 63 MATCH?
N BIT 63 MATCH? Y
Y
MASTER TX FUNCTION COMMAND (FIGURE 12)
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DS18B20
Figure 12. DS18B20 Function Commands Flowchart
MASTER TX FUNCTION COMMAND 44h CONVERT TEMPERATURE ? Y N 48h COPY SCRATCHPAD ? Y N
N
PARASITE POWER ?
Y
N
PARASITE POWER ?
Y
DS18B20 BEGINS CONVERSION
MASTER ENABLES STRONG PULLUP ON DQ
MASTER ENABLES STRONG PULL-UP ON DQ
DS18B20 CONVERTS TEMPERATURE DEVICE CONVERTING TEMPERATURE ? Y N
DATA COPIED FROM SCRATCHPAD TO EEPROM COPY IN PROGRESS ? Y N
MASTER DISABLES STRONG PULLUP
MASTER DISABLES STRONG PULLUP
MASTER RX "0s"
MASTER RX "1s"
MASTER RX "0s"
MASTER RX "1s"
N
B4h READ POWER SUPPLY ? Y
N
B8h RECALL E2 ? Y
N
BEh READ SCRATCHPAD ? Y
N
4Eh WRITE SCRATCHPAD ? Y MASTER TX TH BYTE TO SCRATCHPAD
N
PARASITE POWERED ?
Y MASTER BEGINS DATA RECALL FROM E2 PROM
MASTER RX DATA BYTE FROM SCRATCHPAD
MASTER TX TL BYTE TO SCRATCHPAD MASTER RX "1s" MASTER RX "0s" DEVICE BUSY RECALLING DATA ? Y N MASTER TX RESET ? N Y
MASTER TX CONFIG. BYTE TO SCRATCHPAD
N
HAVE 8 BYTES BEEN READ ? Y
MASTER RX "0s"
MASTER RX "1s"
MASTER RX SCRATCHPAD CRC BYTE
RETURN TO INITIALIZATION SEQUENCE (FIGURE 11) FOR NEXT TRANSACTION
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DS18B20
1-WIRE SIGNALING
The DS18B20 uses a strict 1-Wire communication protocol to ensure data integrity. Several signal types are defined by this protocol: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. The bus master initiates all these signals, with the exception of the presence pulse.
INITIALIZATION PROCEDURE--RESET AND PRESENCE PULSES
All communication with the DS18B20 begins with an initialization sequence that consists of a reset pulse from the master followed by a presence pulse from the DS18B20. This is illustrated in Figure 13. When the DS18B20 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. During the initialization sequence the bus master transmits (TX) the reset pulse by pulling the 1-Wire bus low for a minimum of 480s. 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 DS18B20 detects this rising edge, it waits 15s to 60s and then transmits a presence pulse by pulling the 1-Wire bus low for 60s to 240s. Figure 13. Initialization Timing
MASTER TX RESET PULSE 480s minimum DS18B20 waits 15-60s MASTER RX 480s minimum DS18B20 TX presence pulse 60-240s
VPU 1-WIRE BUS GND
LINE TYPE LEGEND Bus master pulling low DS18B20 pulling low Resistor pullup
READ/WRITE TIME SLOTS
The bus master writes data to the DS18B20 during write time slots and reads data from the DS18B20 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 DS18B20 and a Write 0 time slot to write a logic 0 to the DS18B20. All write time slots must be a minimum of 60s in duration with a minimum of a 1s 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 14). To generate a Write 1 time slot, after pulling the 1-Wire bus low, the bus master must release the 1-Wire bus within 15s. 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 60s).
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DS18B20
The DS18B20 samples the 1-Wire bus during a window that lasts from 15s to 60s after the master initiates the write time slot. If the bus is high during the sampling window, a 1 is written to the DS18B20. If the line is low, a 0 is written to the DS18B20. Figure 14. Read/Write Time Slot Timing Diagram
START OF SLOT START OF SLOT
MASTER WRITE "0" SLOT 60s < TX "0" < 120s VPU 1-WIRE BUS GND DS18B20 Samples
MIN TYP MAX
1s < TREC < > 1s
MASTER WRITE "1" SLOT
DS18B20 Samples
MIN TYP MAX
15s
15s
30s
15s
15s
30s
MASTER READ "0" SLOT VPU 1-WIRE BUS GND Master samples > 1s 15s 45s 15s > 1 s
MASTER READ "1" SLOT 1s < TREC <
Master samples
LINE TYPE LEGEND Bus master pulling low Resistor pullup DS18B20 pulling low
READ TIME SLOTS
The DS18B20 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 Scratchpad [BEh] or Read Power Supply [B4h] command, so that the DS18B20 can provide the requested data. In addition, the master can generate read time slots after issuing Convert T [44h] or Recall E2 [B8h] commands to find out the status of the operation as explained in the DS18B20 Function Commands section. All read time slots must be a minimum of 60s in duration with a minimum of a 1s recovery time between slots. A read time slot is initiated by the master device pulling the 1-Wire bus low for a minimum of 1s and then releasing the bus (see Figure 14). After the master initiates the read time slot, the DS18B20 will begin transmitting a 1 or 0 on bus. The DS18B20 transmits a 1 by leaving the bus high and transmits a 0 by pulling the bus low. When transmitting a 0, the DS18B20 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
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DS18B20
data from the DS18B20 is valid for 15s after the falling edge that initiated the read time slot. Therefore, the master must release the bus and then sample the bus state within 15s from the start of the slot. Figure 15 illustrates that the sum of TINIT, TRC, and TSAMPLE must be less than 15s for a read time slot. Figure 16 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 15s period. Figure 15. Detailed Master Read 1 Timing
VPU 1-WIRE BUS GND TINT > 1s TRC 15s Master samples VIH of Master
Figure 16. Recommended Master Read 1 Timing
VPU 1-WIRE BUS GND TINT = TRC = small small 15s Master samples VIH of Master
LINE TYPE LEGEND Bus master pulling low Resistor pullup
RELATED APPLICATION NOTES
The following application notes can be applied to the DS18B20 and are available on our website at www.maxim-ic.com. Application Note 27: Understanding and Using Cyclic Redundancy Checks with Maxim iButton Products Application Note 122: Using Dallas' 1-Wire ICs in 1-Cell Li-Ion Battery Packs with Low-Side N-Channel Safety FETs Master Application Note 126: 1-Wire Communication Through Software Application Note 162: Interfacing the DS18x20/DS1822 1-Wire Temperature Sensor in a Microcontroller Environment Application Note 208: Curve Fitting the Error of a Bandgap-Based Digital Temperature Sensor Application Note 2420: 1-Wire Communication with a Microchip PICmicro Microcontroller Application Note 3754: Single-Wire Serial Bus Carries Isolated Power and Data Sample 1-Wire subroutines that can be used in conjunction with Application Note 74: Reading and Writing iButtons via Serial Interfaces can be downloaded from the Maxim website.
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DS18B20
DS18B20 OPERATION EXAMPLE 1
In this example there are multiple DS18B20s on the bus and they are using parasite power. The bus master initiates a temperature conversion in a specific DS18B20 and then reads its scratchpad and recalculates the CRC to verify the data.
MASTER MODE Tx Rx Tx Tx Tx Tx Tx Rx Tx Tx Tx DATA (LSB FIRST) Reset Presence 55h 64-bit ROM code 44h DQ line held high by strong pullup Reset Presence 55h 64-bit ROM code BEh COMMENTS Master issues reset pulse. DS18B20s respond with presence pulse. Master issues Match ROM command. Master sends DS18B20 ROM code. Master issues Convert T command. Master applies strong pullup to DQ for the duration of the conversion (tCONV). Master issues reset pulse. DS18B20s respond with presence pulse. Master issues Match ROM command. Master sends DS18B20 ROM code. Master issues Read Scratchpad command. Master reads entire scratchpad including CRC. The master then recalculates the CRC of the first eight data bytes from the scratchpad and compares the calculated CRC with the read CRC (byte 9). If they match, the master continues; if not, the read operation is repeated.
Rx
9 data bytes
DS18B20 OPERATION EXAMPLE 2
In this example there is only one DS18B20 on the bus and it is using parasite power. The master writes to the TH, TL, and configuration registers in the DS18B20 scratchpad and then reads the scratchpad and recalculates the CRC to verify the data. The master then copies the scratchpad contents to EEPROM.
MASTER MODE Tx Rx Tx Tx Tx Tx Rx Tx Tx DATA (LSB FIRST) Reset Presence CCh 4Eh 3 data bytes Reset Presence CCh BEh COMMENTS Master issues reset pulse. DS18B20 responds with presence pulse. Master issues Skip ROM command. Master issues Write Scratchpad command. Master sends three data bytes to scratchpad (TH, TL, and config). Master issues reset pulse. DS18B20 responds with presence pulse. Master issues Skip ROM command. Master issues Read Scratchpad command. Master reads entire scratchpad including CRC. The master then recalculates the CRC of the first eight data bytes from the scratchpad and compares the calculated CRC with the read CRC (byte 9). If they match, the master continues; if not, the read operation is repeated. Master issues reset pulse. DS18B20 responds with presence pulse. Master issues Skip ROM command. Master issues Copy Scratchpad command. Master applies strong pullup to DQ for at least 10ms while copy operation is in progress.
Rx
9 data bytes
Tx Rx Tx Tx Tx
Reset Presence CCh 48h DQ line held high by strong pullup
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DS18B20
ABSOLUTE MAXIMUM RATINGS
Voltage Range on Any Pin Relative to Ground.....................................................................-0.5V to +6.0V Operating Temperature Range...........................................................................................-55C to +125C Storage Temperature Range ..............................................................................................-55C to +125C Solder Temperature .......................................................Refer to the IPC/JEDEC J-STD-020 Specification. 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
PARAMETER Supply Voltage Pullup Supply Voltage Thermometer Error Input Logic-Low Input Logic-High Sink Current Standby Current Active Current DQ Input Current Drift SYMBOL VDD VPU tERR VIL VIH Parasite Power IL IDDS IDD IDQ VI/O = 0.4V VDD = 5V
(-55C to +125C; VDD=3.0V to 5.5V)
TYP MAX +5.5 +5.5 VDD 0.5 2 +0.8 The lower of 5.5 or VDD + 0.3 1000 1.5 UNITS NOTES V 1 V C V V mA nA mA A C 1,2 3 1,4,5 1, 6 1 7,8 9 10 11
CONDITIONS MIN Local Power +3.0 Parasite Power +3.0 Local Power +3.0 -10C to +85C -55C to +125C -0.3 Local Power +2.2 +3.0 4.0
750 1 5 0.2
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 DS18B20, 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 17. 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 ("high-Z" state). 11) Drift data is based on a 1000-hour stress test at +125C with VDD = 5.5V.
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DS18B20
AC ELECTRICAL CHARACTERISTICS--NV MEMORY (-55C to +100C; VDD = 3.0V to 5.5V)
PARAMETER NV Write Cycle Time EEPROM Writes EEPROM Data Retention SYMBOL tWR NEEWR tEEDR CONDITIONS -55C to +55C -55C to +55C MIN 50k 10 TYP 2 MAX 10 UNITS ms writes years
AC ELECTRICAL CHARACTERISTICS
PARAMETER Temperature Conversion Time Time to Strong Pullup On Time Slot Recovery Time Write 0 Low Time Write 1 Low Time Read Data Valid Reset Time High Reset Time Low Presence-Detect High Presence-Detect Low Capacitance SYMBOL tCONV
(-55C to +125C; VDD = 3.0V to 5.5V)
UNITS ms s s s s s s s s s s pF 1 1 1 1 1 1 1,2 1 1 NOTES 1
tSPON tSLOT tREC tLOW0 tLOW1 tRDV tRSTH tRSTL tPDHIGH tPDLOW CIN/OUT
CONDITIONS MIN TYP MAX 9-bit resolution 93.75 10-bit resolution 187.5 11-bit resolution 375 12-bit resolution 750 Start Convert T 10 Command Issued 60 120 1 60 120 1 15 15 480 480 15 60 60 240 25
NOTES:
1) See the timing diagrams in Figure 18. 2) Under parasite power, if tRSTL > 960s, a power-on reset may occur. Figure 17. Typical Performance Curve
DS18B20 Typical Error Curve 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5
Thermometer Error (C)
+3s Error
0
10
20
30
40
50
60
70
Mean Error -3s Error Temperature (C)
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DS18B20
Figure 18. Timing Diagrams
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DS18B20
REVISION HISTORY
REVISION DATE 030107 DESCRIPTION In the Absolute Maximum Ratings section, removed the reflow oven temperature value of +220C. Reference to JEDEC specification for reflow remains. In the Operation--Alarm Signaling section, added "or equal to" in the desciption for a TH alarm condition In the Memory section, removed incorrect text describing memory. In the Configuration Register section, removed incorrect text describing configuration register. In the Ordering Information table, added TO-92 straight-lead packages and included a note that the TO-92 package in tape and reel can be ordered with either formed or straight leads. PAGES CHANGED 19 5 7 8 2
101207
042208
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Maxim/Dallas Semiconductor cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim/Dallas Semiconductor product. No circuit patent licenses are implied. Maxim/Dallas Semiconductor reserves the right to change the circuitry and specifications without notice at any time.
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(c) 2008 Maxim Integrated Products
The Maxim logo is a registered trademark of Maxim Integrated Products, Inc. The Dallas logo is a registered trademark of Dallas Semiconductor Corporation.

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