DS2422 Maxim, DS2422 Datasheet - Page 36

• Manufacturer Part Number: DS2422
• Description: The DS2422 temperature/data logger combines the core functions of a fully featured data logger in a single chip
• Manufacturer: Maxim

DS2422
1-Wire ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the eight ROM function commands that the
DS2422 supports. All ROM function commands are 8 bits long. A list of these commands follows (refer to flowchart
in Figure 14).
READ ROM [33h]
This command allows the bus master to read the DS2422’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 slave 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
DS2422 on a multidrop bus. Only the DS2422 that exactly matches the 64-bit ROM sequence will respond to the
following memory function command. All other slaves 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 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 registration
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 second slot,
each slave device participating in the search outputs the complemented value of its registration number 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 participating 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 romcode
tree. After one complete pass, the bus master knows the registration number 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.
CONDITIONAL SEARCH [ECh]
The Conditional Search ROM command operates similarly to the Search ROM command except that only those
devices, which fulfill certain conditions, will participate in the search. This function provides an efficient means for
the bus master to identify devices on a multidrop system that have to signal an important event. After each pass of
the conditional search that successfully determined the 64-bit ROM code for a specific device on the multidrop bus,
that particular device can be individually accessed as if a Match ROM had been issued, since all other devices will
have dropped out of the search process and will be waiting for a reset pulse.
The DS2422 will respond to the conditional search if one of the five alarm flags of the Alarm Status Register
(address 0214h) reads 1. The data and temperature alarm will only occur if enabled (see Temperature Sensor
Alarm and Serial Input Alarm). The BOR alarm is always enabled. The first alarm that occurs will make the device
respond to the Conditional Search command.
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, for example, 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).
RESUME COMMAND [A5h]
The DS2422 needs to be accessed several times before a mission will start. In a multidrop environment this means
that the 64-bit ROM code after a Match ROM command has to be repeated for every access. To maximize the data
throughput in a multidrop environment, the Resume function was implemented. This function checks the status of
the RC bit and, if it is set, directly transfers control to the Memory/Control functions, similar to a Skip ROM
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