7

Analog Devices

Initialization

All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence

consists of a reset pulse transmitted by the bus controller followed by presence pulse(s) transmitted by

the peripheral(s). The presence pulse lets the bus controller know that the DS1904 is on the bus and is

ready to operate. For more details, see the 1-Wire Signaling section.

ROM Function Commands

Once the bus controller has detected a presence, it can issue one of the four ROM function commands

that the DS1904 supports. All ROM function commands are eight bits long. A list of these commands

follows (refer to flowchart in Figure 7):

Read ROM [33h]

This command allows the bus controller to read the DS1904’s 8-bit family code, unique 48-bit serial

number, and 8-bit CRC. This command should only be used if there is a single peripheral on the bus. If

more than one peripheral is present on the bus, a data collision will occur when all peripherals try to

transmit at the same time (open drain will produce a wired-AND result). The resultant family code and

48-bit serial number read by the controller will be invalid.

Match ROM [55h]

The match ROM command, followed by a 64-bit ROM sequence, allows the bus controller to address a

specific DS1904 on a multidrop bus. Only the DS1904 that exactly matches the 64-bit ROM sequence

will respond to the following clock function command. All peripherals 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.

Search ROM [F0h]

When a system is initially brought up, the bus controller might not know the number of devices on the 1-

Wire bus or their 64-bit ROM codes. The search ROM command allows the bus controller to use a

process of elimination to identify the 64-bit ROM codes of all peripheral devices on the bus. The search

ROM process is the repetition of a simple 3-step routine: read a bit, read the complement of the bit, then

write the desired value of that bit. The bus controller performs this 3-step routine on each bit of the ROM.

After one complete pass, the bus controller knows the 64-bit ROM code of one device. Additional passes

will identify the ROM codes of the remaining devices. See Chapter 5 of the Book of iButton® Standards

for a comprehensive discussion of a search ROM, including an actual example.