DS1990A

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TRANSACTION SEQUENCE

The sequence for accessing the DS1990A via the 1-Wire port is as follows:

§ Initialization

§ ROM Function Command

§ Read Data

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 master followed by a presence pulse(s) transmitted by the

slave(s).

The presence pulse lets the bus master know that the DS1990A 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 four ROM function commands. All

ROM function commands are eight bits long. A list of these commands follows (refer to flowchart in

Figure 4):

Read ROM [33h] or [0Fh]

This command allows the bus master to read the DS1990A’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 DS1990A 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). The DS1990A Read ROM function will occur with a

command byte of either 33h or 0Fh in order to ensure compatibility with the DS1990, which will only

respond to a 0Fh command word with its 64-bit ROM data.

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

The complete 1-Wire protocol for all Dallas Semiconductor iButtons contains a Match ROM and a Skip

ROM command. (See the Book of DS19xx iButton Standards.) Since the DS1990A contains only the 64-

bit ROM with no additional data fields, the Match ROM and Skip ROM are not applicable and will cause

no further activity on the 1-Wire bus if executed. The DS1990A does not interfere with other 1-Wire parts

on a multidrop bus that do respond to a Match ROM or Skip ROM (example DS1990A and DS1994 on

the same 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 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. The ROM search 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 master performs this simple 3-step routine on each bit of the ROM.

After one complete pass, the bus master knows the contents of the ROM in one device. The remaining

number of devices and their ROM codes may be identified by additional passes. See Chapter 5 of the

Book of DS19xx iButton Standards for a comprehensive discussion of a ROM search, including an actual

example.