DS1822
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MEMORY
The DS1822’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 (T
H
and T
L
) and configuration
register. Note that if the DS1822 alarm function is not used, the T
H
and T
L
registers can serve as general-
purpose memory. All memory commands are described in detail in the DS1822 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 T
H
and T
L
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 are reserved for internal use by the device and
cannot be overwritten.
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 DS1822 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 DS1822 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 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 scratchpad 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 DS1822 will indicate the status of the recall by transmitting 0
while the recall is in progress and 1 when the recall is done.
DS1822 MEMORY MAP Figure 7
SCRATCHPAD (Power-up State)
byte 0 Temperature LSB (50h)
byte 1 Temperature MSB (05h)
EEPROM
byte 2 T
H
Register or User Byte 1* T
H
Register or User Byte 1
byte 3 T
L
Register or User Byte 2* T
L
Register or User Byte 2
byte 4 Configuration Register* 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
(85°C)
