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DS1920
ROM FUNCTION COMMANDS
Once the bus master has detected a presence pulse, it can issue one of the five ROM function commands.
All ROM function commands are eight bits long. A list of these commands follows (see the flowchart in
Figure 5).
Read ROM [33h]
This command allows the bus master to read the DS1920’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 DS1920 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).
Match ROM [55h]
The match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a
specific DS1920 on a multidrop bus. Only the DS1920 that exactly matches the 64-bit ROM sequence
will respond to the subsequent 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.
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). The
Skip ROM command is useful to address all DS1920s on the bus to do a temperature conversion. Since
the DS1920 uses a special command set, other device types will not respond to these commands.
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, 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. Refer to Chapter 5 of the
Book of iButton Standards for a comprehensive discussion of a ROM Search, including an actual
example.
Alarm Search [ECh]
The flowchart of this command is identical to the Search ROM command; however, the DS1920 will
respond to this command only if an alarm condition has been encountered at the last temperature
measurement. An alarm condition is defined as a temperature higher than TH or lower than TL. The
alarm condition remains set as long as the DS1920 is powered up or until another temperature
measurement reveals a nonalarming value. For alarming, the trigger values stored in EEPROM are taken
into account. If an alarm condition exists and the TH or TL settings are changed, another temperature
conversion should be done to validate any alarm conditions .
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