DS1977 Maxim, DS1977 Datasheet - Page 16

• Manufacturer Part Number: DS1977
• Description: The DS1977 is a 32KB EEPROM in a rugged iButton® enclosure
• Manufacturer: Maxim

Figure 8. HARDWARE CONFIGURATION
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).
HOST CPU
SIMPLE BUS MASTER
SPU
RX
TX
DS2480B BUS MASTER
Serial
Port
Open Drain
serial out
serial in
Port Pin
IRLMS6702
or equivalent
VDD
POL
RXD
TXD
DS2480B
SPU = Strong Pullup
RX = Receive
TX = Transmit
GND
VPP
1-W
NC
16 of 29
V
R
PUP
PUP
+5V
SEE
TEXT
To 1-Wire data
DATA
DS1977 1-Wire PORT
100 Ω
MOSFET
RX
TX
DS1977