DS28E05 データシートの表示（PDF） - Maxim Integrated

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DS28E05

1-Wire EEPROM

Maxim Integrated 

DS28E05

1-Wire EEPROM

A multidrop bus consists of a 1-Wire bus with multiple

slaves attached. The DS28E05 supports overdrive speed

of 76.9kbps (max) only and cannot be used together with

standard speed or dual-speed 1-Wire slaves on the bus.

The value of the pullup resistor primarily depends on the

1-Wire pullup voltage, network size and load conditions.

The DS28E05 requires a pullup resistor of maximum

1.5kΩ.

The idle state for the 1-Wire bus is high. If for any reason

a transaction must be suspended, the bus must be left in

the idle state if the transaction is to resume. If this does

not occur and the bus is left low for more than 16µs, one

or more devices on the bus could be reset.

Transaction Sequence

The protocol for accessing the DS28E05 through the

1-Wire port is as follows:

• Initialization

• ROM Function Command

• Memory 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 DS28E05 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 five ROM function commands that the

DS28E05 supports. All ROM function commands are 8

bits long. A list of these commands follows (see the flow-

chart in Figure 8).

Read ROM [33h]

The Read ROM command allows the bus master to read

the DS28E05’s ROM ID (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 family code and

48-bit serial number as read by the master are unlikely to

match the CRC.

Match ROM [55h]

The Match ROM command, followed by a 64-bit ROM ID,

allows the bus master to address a specific DS28E05 on

a multidrop bus. Only the DS28E05 that exactly matches

the 64-bit ROM ID responds to the following 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 ROM ID numbers. By taking advantage of the

wired-AND property of the bus, the master can use a pro-

cess of elimination to identify the ID of all slave devices.

For each bit of the ID number, starting with the least sig-

nificant 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 ID number bit. On the

second slot, each slave device participating in the search

outputs the complemented value of its ID 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 choos-

ing which state to write, the bus master branches in the

search tree. After one complete pass, the bus master

knows the ROM ID number of a single device. Additional

passes identify the ID 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 sys-

tem by allowing the bus master to access the memory

functions without providing the 64-bit ROM ID. If more

than one slave is present on the bus and, for example,

a read command is issued following the Skip ROM com-

mand, data collision occurs on the bus as multiple slaves

transmit simultaneously (open-drain pulldowns produce a

wired-AND result).

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