Rainbow Electronics DS1822 - Manual (Page 10)

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, however, that the Skip ROM command can only be followed by the Read Scratchpad
[BEh] command when there is one slave on the bus. This sequence saves time by allowing the master to
read from the device without sending its 64–bit ROM code. This sequence 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 10
µ
s (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 3 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 10
µ
s (max) after this command