Rainbow Electronics DS2431 - Manual (Page 15)

DS2431: 1024-Bit, 1-Wire EEPROM
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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 Application Note 187: 1-Wire Search Algorithm
for a detailed discussion, including an example.
SKIP ROM [CCh]
This command can save time in a single-drop bus system by allowing the bus master to access the memory
functions without providing the 64-bit ROM code. If more than one slave is present on the bus and, for example, a
Read command is issued following the Skip ROM command, data collision occurs on the bus as multiple slaves
transmit simultaneously (open-drain pulldowns produce a wired-AND result).
RESUME [A5h]
To maximize the data throughput in a multidrop environment, the Resume function is available. This function
checks the status of the RC bit and, if it is set, directly transfers control to the Memory functions, similar to a Skip
ROM command. The only way to set the RC bit is through successfully executing the Match ROM, Search ROM, or
Overdrive Match ROM command. Once the RC bit is set, the device can repeatedly be accessed through the
Resume Command function. Accessing another device on the bus clears the RC bit, preventing two or more
devices from simultaneously responding to the Resume Command function.
OVERDRIVE SKIP ROM [3Ch]
On a single-drop bus this command can save time by allowing the bus master to access the memory functions
without providing the 64-bit ROM code. Unlike the normal Skip ROM command, the Overdrive Skip ROM sets the
DS2431 in the Overdrive mode (OD = 1). All communication following this command has to occur at Overdrive
speed until a reset pulse of minimum 480µs duration resets all devices on the bus to standard speed (OD = 0).
When issued on a multidrop bus, this command sets all Overdrive-supporting devices into Overdrive mode. To
subsequently address a specific Overdrive-supporting device, a reset pulse at Overdrive speed has to be issued
followed by a Match ROM or Search ROM command sequence. This speeds up the time for the search process. If
more than one slave supporting Overdrive is present on the bus and the Overdrive Skip ROM command is followed
by a Read command, data collision occurs on the bus as multiple slaves transmit simultaneously (open-drain
pulldowns produce a wired-AND result).
OVERDRIVE MATCH ROM [69h]
The Overdrive Match ROM command followed by a 64-bit ROM sequence transmitted at Overdrive Speed allows
the bus master to address a specific DS2431 on a multidrop bus and to simultaneously set it in Overdrive mode.
Only the DS2431 that exactly matches the 64-bit ROM sequence responds to the subsequent memory function
command. Slaves already in Overdrive mode from a previous Overdrive Skip or successful Overdrive Match
command remain in Overdrive mode. All overdrive-capable slaves return to standard speed at the next Reset Pulse
of minimum 480µs duration. The Overdrive Match ROM command can be used with a single or multiple devices on
the bus.