DS2407 datasheet(19/31 Pages) DALLAS

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DS2407

012099 19/31

INITIALIZATION

All transactions on the 1–Wire bus begin with an initial-

ization 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

DS2407 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 five ROM function commands. All ROM

function commands are eight bits long. A list of these

commands follows (refer to flowchart in Figure 12):

Read ROM [33h]

This command allows the bus master to read the

DS2407’s 8–bit family code, unique 48–bit serial num-

ber, and 8–bit CRC. This command can be used only if

there is a single DS2407 on the bus. If more than one

slave is present on the bus, a data collision will occur

when all slaves try to transmit at the same time (open

drain will produce a wired–AND result).

Match ROM [55h]

The match ROM command, followed by a 64–bit ROM

sequence, allows the bus master to address a specific

DS2407 on a multidrop bus. Only the DS2407 that

exactly matches the 64–bit ROM sequence will respond

to the subsequent memory function command. All

slaves that do not match the 64–bit ROM sequence will

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

single or multiple devices on the bus.

Skip ROM [CCh]

This command can save time in a single drop bus sys-

tem 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 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).

Search ROM [F0h]

When a system is initially interrogated, the bus master

may not know the number of devices on the 1–Wire bus

or their 64–bit ROM codes. The Search ROM command

allows the bus master to use a process of elimination to

identify the 64–bit ROM codes of all slave devices on the

bus. This process of elimination involves repeated

application of a simple three–step procedure where the

bus master starts by reading a bit position in the 64–bit

ROM, followed by reading the complement of that bit

position, and finally writing to all the devices still

involved in the search the desired logic value for that bit

position. A detailed example and a flowchart for the

search algorithm can be found in the “Book of DS19xx

iButton Standards.”

After one complete pass, the bus master knows the con-

tents of the 64–bit ROM in one device. Subsequent

passes will reveal the total number of devices and their

individual ROM codes. In addition, after each complete

pass of the search that successfully determines the

64–bit ROM 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 are

waiting for a reset pulse.

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