DS1921 datasheet(26/44 Pages) DALLAS

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DS1921H/Z

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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 App Note 187: 1-Wire Search Algorithm for a

detailed discussion, including an 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

bit functions TAS, THS, and TLS in the Control Register, address 20Eh. The Conditional Search ROM

provides an efficient means for the bus master to determine devices on a multidrop system that have to

signal an important event, such as a temperature leaving the tolerance band. After each pass of the

conditional search that successfully determined the 64-bit ROM code for a specific device on the

multidrop bus, that particular device can be individually accessed as if a Match ROM had been issued,

since all other devices will have dropped out of the search process and will be waiting for a reset pulse.

For the conditional search, one can select any combination of the three search conditions by writing the

associated bit to a logical 1. These bits correspond directly to the flags in the Status Register of the

device. If the flag in the status register reads 1 and the corresponding bit in the Control Register is a

logical 1 too, the device will respond to the Conditional Search command. If more than one bit search

condition is selected, the first event occurring will make the device respond to the Conditional Search

command.

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 will occur

on the bus as multiple slaves transmit simultaneously (open drain pull-downs will produce a wired-AND

result).

DS1921 Datasheet(PDF) 26 Page - Dallas Semiconductor