DS2411P Dallas Semiconducotr, DS2411P Datasheet - Page 6

• Manufacturer Part Number: DS2411P
• Description: Silicon Serial Number with VCC Input
• Manufacturer: Dallas Semiconducotr

DS2411
TRANSACTION SEQUENCE
The communication sequence for accessing the DS2411 through the 1-Wire bus is as follows:
§ Initialization
§ ROM Function Command
§ Read Data
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 a presence pulse(s) transmitted by the
slave(s). The presence pulse lets the bus master know that the DS2411 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 three ROM function commands. All
ROM function command codes are 1 byte long. A list of these commands follows (see the flowchart in
Figure 5).
Read ROM [33h]
This command allows the bus master to read the DS2411’s 8-bit family code, unique 48-bit serial
number, and 8-bit CRC. This command should only be used if there is a single slave device on the bus. If
more than one slave is present on the bus, a data collision results when all slaves try to transmit at the
same time (open drain produces a wired-AND result), and the resulting registration number read by the
master will be invalid.
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 registration numbers. By taking advantage of the wired-AND property of the bus, the
master can use a process of elimination to identify the registration numbers of all slave devices. For each
bit of the registration number, starting with the least significant bit, the bus master issues a triplet of time
slots. On the first slot, each slave device participating in the search outputs the true value of its
registration number bit. On the second slot, each slave device participating in the search outputs the
complemented value of its registration number bit. On the third slot, the master writes the true value of
the bit to be selected. All slave devices that do not match the bit written by the master stop participating
in the search. If both of the read bits are zero, the master knows that slave devices exist with both states of
the bit. By choosing which state to write, the bus master branches in the romcode tree. After one complete
pass, the bus master knows the registration number of a single device. Additional passes identify the
registration numbers of the remaining devices. Refer to App Note 187: 1-Wire Search Algorithm for a
detailed discussion, including an example.
Overdrive Skip ROM [3Ch]
This command causes all overdrive-capable slave devices on the 1-Wire network to enter overdrive speed
(OD = 1). All communication following this command has to occur at overdrive speed until a reset pulse
of minimum 480ms duration resets all devices on the bus to regular speed (OD = 0).
To subsequently address a specific overdrive-supporting device, a reset pulse at overdrive speed has to be
issued followed by a read ROM or search ROM command sequence. Overdrive speeds up the time for the
search process.
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