Datasheet

DS2408
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TRANSACTION SEQUENCE
The protocol for accessing the DS2408 through the 1-Wire port is as follows:
 Initialization
 ROM Function Command
 Control Function Command
 Transaction/Data
Illustrations of the transaction sequence for the various control function commands are found later in this
document.
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 the DS2408 is on the bus and is ready to
operate. For more details, see the 1-Wire Signaling section.
ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the seven ROM function commands. All
ROM function commands are eight bits long. A list of these commands follows (see the flowchart in
Figure 12).
Read ROM [33h]
This command allows the bus master to read the DS2408's 8-bit family code, unique 48-bit serial number,
and 8-bit CRC. This command can only be used if there is a single device on the bus. If more than one
slave is present on the bus, a data collision will occur when all slaves try to transmit at the same time
(open drain will produce a wired-AND result). The resultant family code and 48-bit serial number will
result in a mismatch of the CRC.
Match ROM [55h]
The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a spe-
cific DS2408 on a multidrop bus. Only the DS2408 that exactly matches the 64-bit ROM sequence will
respond to the following control function command. All slaves that do not match the 64-bit ROM se-
quence will wait for a reset pulse. This command can be used with either single or multiple devices on the
bus.
Search ROM [F0h]
When a system is initially brought up, the bus master might not know the number of devices on the
1-Wire bus or their 64-bit ROM codes. The Search ROM command allows the bus master to use a
process of elimination to identify the 64-bit ROM codes of all slave devices on the bus. The search ROM
process is the repetition of a simple three-step routine: read a bit, read the complement of the bit, then
write the desired value of that bit. The bus master performs this simple, three-step routine on each bit of
the ROM. After one complete pass, the bus master knows the contents of the ROM in one device. The
remaining number of devices and their ROM codes may be identified by additional passes. See
Application Note 187 for a detailed discussion on the Search ROM command process including a
software example.
Conditional Search [ECh]
The Conditional Search ROM command operates similarly to the Search ROM command except that only
devices fulfilling the specified condition will participate in the search. The condition is specified by the
Conditional Search channel and polarity selection (addresses 008Bh, 008Ch), the bit functions CT and