1-Wire ROM Function Commands

Once the bus master has detected a presence, it can

issue one of the ROM function commands the DS1990A

supports. All ROM function commands are 8 bits long.

A list of these commands follows. (See Figure 5 for a

flowchart.)

Read ROM [33h]

This command allows the bus master to read the

DS1990A’s 8-bit family code, unique 48-bit serial num-

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

there is a single slave device 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 pro-

duces a wired-AND result). The resultant family code and

48-bit serial number results in a mismatch of the CRC.

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 regis-

tration 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 sec-

ond slot, each slave device participating in the search

outputs the complemented value of its registration num-

ber 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 participat-

ing 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 ROM code tree. After one com-

plete pass, the bus master knows the registration num-

ber 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 discussion, including an example.

Match ROM [55h]/Skip ROM [CCh]

The minimum set of 1-Wire ROM function commands

includes a Match ROM and a Skip ROM command.

Because the DS1990A contains only the 64-bit ROM

without any additional data fields, Match ROM and Skip

ROM are not applicable. The DS1990A remains silent

(inactive) upon receiving a ROM function command

that it does not support. This allows the DS1990A to

coexist on a multidrop bus with other 1-Wire devices

that do respond to Match ROM or Skip ROM.

Serial Number iButton

6

_______________________________________________________________________________________

DS1990A Tx

PRESENCE PULSE

BUS MASTER Tx

RESET PULSE

BUS MASTER Tx ROM

FUNCTION COMMAND

DS1990A Tx

CRC BYTE

DS1990A Tx

FAMILY CODE

(1 BYTE)

DS1990A Tx

SERIAL NUMBER

(6 BYTES)

Y

33h

READ ROM

COMMAND?

N

BIT 0 MATCH?

N

N

N

F0h

SEARCH ROM

COMMAND?

N

Y

Y

DS1990A Tx BIT 0

DS1990A Tx BIT 0

MASTER Tx BIT 0

Y

BIT 1 MATCH?

BIT 63 MATCH?

DS1990A Tx BIT 1

DS1990A Tx BIT 1

MASTER Tx BIT 1

DS1990A Tx BIT 63

DS1990A Tx BIT 63

MASTER Tx BIT 63

Y

Figure 5. ROM Functions Flowchart