Rainbow Electronics DS1990A - Manual (Page 4)

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.