Datasheet

64-BIT Lasered ROM code
Each DS18B20 contains a unique 64–bit code (see Figure
8) 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.
Memory
The DS18B20’s memory is organized as shown in Figure
9. The memory consists of an SRAM scratchpad with
nonvolatile EEPROM storage for the high and low alarm
trigger registers (T
H
and T
L
) 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 regis-
ter data, which is explained in detail in the Configuration
Register section. Bytes 5, 6, and 7 are reserved for inter-
nal 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 signifi-
cant 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 T
H
, T
L
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 the scratch-
pad at any time using the Recall E
2
[B8h] command. The
master can issue read time slots following the Recall E
2
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 8. 64-Bit Lasered ROM Code
Figure 9. DS18B20 Memory Map
8-BIT CRC 48-BIT SERIAL NUMBER 8-BIT FAMILY CODE (28h)
MSB LSB MSB LSB MSB LSB
BYTE 0
BYTE 1
TEMPERATURE LSB (50h)
TEMPERATURE MSB (05h)
(85°C)
BYTE 2
BYTE 3
T
H
REGISTER OR USER BYTE 1*
T
L
REGISTER OR USER BYTE 2*
BYTE 4
BYTE 5
CONFIGURATION REGISTER*
RESERVED (FFh)
BYTE 6
BYTE 7
RESERVED
RESERVED (10h)
BYTE 8
CRC*
*POWER-UP STATE DEPENDS ON VALUE(S) STORED IN EEPROM.
T
H
REGISTER OR USER BYTE 1*
T
L
REGISTER OR USER BYTE 2*
CONFIGURATION REGISTER*
SCRATCHPAD
(POWER-UP STATE)
EEPROM
DS18B20 Programmable Resolution
1-Wire Digital Thermometer
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