DS1925 datasheet(31/46 Pages) MAXIM

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Transaction Sequence

The protocol for accessing the DS1925 through the

1-Wire port is as follows:

● Initialization

● ROM Function Command

● Memory/Control Function Command

● Transaction/Data

Initialization

All transactions on the 1-Wire bus begin with an initializa-

tion 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 pres-

ence pulse lets the bus master know that the DS1925 is

on the bus and is ready to operate. For more details, see

the 1-Wire Signaling section.

1-Wire ROM Function Commands

Once the bus master has detected a presence, it can

issue one of the eight ROM function commands that the

DS1925 supports. All ROM function commands are 8 bits

long. A list of these commands follows (see Figure 11).

Read ROM [33h]

This command allows the bus master to read the

DS1925’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 slave on the bus. If more than one slave is

present on the bus, a data collision occurs when all slaves

try to transmit at the same time (open drain produces a

wired-AND result). The resultant family code and 48-bit

serial number results 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 specific

DS1925 on a multidrop bus. Only the DS1925 that exactly

matches the 64-bit ROM sequence responds to the fol-

lowing memory function command. All other slaves wait

for a reset pulse. This command can be used with a 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

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 participat-

ing in the search outputs the true value of its registration

number bit. On the second slot, each slave device par-

ticipating 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 dis-

cussion, including an example.

Conditional Search ROM [ECh]

The Conditional Search ROM command operates simi-

larly to the Search ROM command except that only those

devices that fulfill certain conditions participate in the

search. This function provides an efficient means for the

bus master to identify devices on a multidrop system that

have to signal an important event. 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 wait-

ing for a reset pulse.

The DS1925 responds to the conditional search if one of

the three alarm flags of the Alarm Status register (address

0214h) reads 1. The temperature alarm only occurs if

enabled (see the Temperature Sensor Alarm section).

The BOR alarm is always enabled. The first alarm that

occurs makes the device respond to the Conditional

Search ROM command.

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

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