DS1822
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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 DS1822s 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 DS1822s 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 OPERATION—ALARM SIGNALING
section for an explanation of alarm flag operation.
DS1822 FUNCTION COMMANDS
After the bus master has used a ROM command to address the DS1822 with which it wishes to
communicate, the master can issue one of the DS1822 function commands. These commands allow the
master to write to and read from the DS1822’s scratchpad memory, initiate temperature conversions and
determine the power supply mode. The DS1822 function commands, which are described below, are
summarized in Table 4 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 DS1822 returns to its
low-power idle state. If the device is being used in parasite power mode, within 10μs (max) after this
command is issued the master must enable a strong pullup on the 1-Wire bus for the duration of the
conversion (t
conv
) as described in the POWERING THE DS1822 section. If the DS1822 is powered by an
external supply, the master can issue read time slots after the Convert T command and the DS1822 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 three bytes of data to the DS1822’s scratchpad. The first data
byte is written into the T
H
register (byte 2 of the scratchpad), the second byte is written into the T
L
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 9
th
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.
COPY SCRATCHPAD [48h]
This command copies the contents of the scratchpad T
H
, T
L
and configuration registers (bytes 2, 3, and 4)
to EEPROM. If the device is being used in parasite power mode, within 10μs (max) after this command is
