Note: For certain operations that consist only of writing from the OWM to the slave, it is technically possible for the master to
communicate with more than one slave at a time on the same 1-Wire bus. For this to work, the exact same data must be transmitted to all
slave devices, and any values read back from the slaves must either be identical as well or must be disregarded by the master device
(because different slaves can attempt to transmit different values). The descriptions below assume, however, that the master is
communicating with only one slave device at a time because this is the method that is normally used.
As explained above, the ROM ID contents play an important role in addressing and selecting devices on the 1-Wire bus. All
devices except one are in an idle/inactive state after the Match ROM command or the Search ROM command is executed.
They return to the active state only after receiving a 1-Wire reset pulse.
Devices with overdrive capability are distinguished from others by their family code and two additional ROM commands:
Overdrive Skip ROM and Overdrive Match ROM. The first transmission of the ROM command itself takes place at the
normal speed that is understood by all 1-Wire devices. After a device with overdrive capability is addressed and set into
overdrive mode (that is, after the appropriate ROM command is received), further communication to that device must
occur at overdrive speed. Because all deselected devices remain in the idle state as long as no reset pulse of regular
duration is detected, even multiple overdrive components can reside on the same 1-Wire bus. A reset pulse of regular
duration resets all 1-Wire devices on the bus and simultaneously sets all overdrive-capable devices back to the default
standard speed.
20.3.2 Read ROM Command
The Read ROM command allows the OWM to obtain the 8-byte ROM ID of any slave device connected to the 1-Wire bus.
Each slave device on the bus responds to this command by transmitting all eight bytes of its ROM ID value starting with the
least significant byte (Family Code) and ending with the most significant byte (CRC).
Because this command is addressed to all 1-Wire devices on the bus, if more than one slave is present on the bus there is a
data collision as multiple slaves attempt to transmit their ROM IDs at once. This condition is detectable by the OWM
because the CRC value does not match the ROM ID value received. In this case, the OWM should reset the 1-Wire bus and
select a single slave device on the bus to continue either by using the Match ROM command (if the ROM ID values are
already known) or the Search ROM command (if the master has not yet identified some or all devices on the bus).
After the Read ROM command is complete, all slave devices on the 1-Wire bus are selected or active, and communication
proceed to the Transport layer.
20.3.3 Skip ROM and Overdrive Skip ROM Commands
The Skip ROM command is used to activate all slave devices present on the 1-Wire bus regardless of their ROM ID.
Normally, this command is used when only a single 1-Wire slave device is connected to the bus. After the Skip ROM
command is complete, all slave devices on the 1-Wire bus are selected or active and communication proceeds to the
Transport layer.
The Overdrive Skip ROM command operates in an identical manner except that running it also causes the receiving slave
devices to shift communication speed from standard speed to overdrive speed. The Overdrive Skip ROM command byte
itself (0x3C) is transmitted at standard speed. All subsequent communication is sent at overdrive speed.
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