DS1977
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Figure 8. HARDWARE CONFIGURATION
SPU = Strong Pullup
RX = Receive
TX = Transmit
READ ROM [33H]
This command allows the bus master to read the DS1977’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 DS1977 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 resultant family code and 48-bit serial number will result in a mismatch of the CRC.
MATCH ROM [55H]
The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a specific
DS1977 on a multidrop bus. Only the DS1977 that exactly matches the 64-bit ROM sequence will respond to the
following memory function command. All slaves 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 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 search ROM process is the repetition of a simple
three-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, three-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 Application Note 187 for a comprehensive discussion of the 1-Wire search
algorithm.
SKIP ROM [CCH]
This command can save time in a single-drop bus system by allowing the bus master to access the memory
functions without providing the 64-bit ROM code. If more than one slave is present on the bus and a Read
command is issued following the Skip ROM command, data collision will occur on the bus as multiple slaves
transmit simultaneously (open drain pulldowns will produce a wired-AND result).
