DS1922L/DS1922T
iButton Temperature Loggers
with 8KB Data-Log Memory
Skip ROM [CCh]
This command can save time in a single-drop bus sys-
tem by allowing the bus master to access the memory
functions without providing the 64-bit ROM code. For
example, if more than one slave is present on the bus
and a read command is issued following the Skip ROM
command, data collision occurs on the bus as multiple
slaves transmit simultaneously (open-drain pulldowns
produce a wired-AND result).
Resume [A5h]
The DS1922L/DS1922T must be accessed several times
before a mission starts. In a multidrop environment this
means that the 64-bit ROM code after a Match ROM
command must be repeated for every access. To maxi-
mize the data throughput in a multidrop environment,
the Resume command was implemented. This com-
mand checks the status of the RC bit and, if it is set,
directly transfers control to the memory/control func-
tions, similar to a Skip ROM command. The only way to
set the RC bit is through successfully executing the
Match ROM, Search ROM, or Overdrive-Match ROM
command. Once the RC bit is set, the device can
repeatedly be accessed through the Resume command
function. Accessing another device on the bus clears
the RC bit, preventing two or more devices from simulta-
neously responding to the Resume command.
Overdrive-Skip ROM [3Ch]
On a single-drop bus this command can save time by
allowing the bus master to access the memory/control
functions without providing the 64-bit ROM code. Unlike
the normal Skip ROM command, the Overdrive-Skip
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ROM command sets the DS1922L/DS1922T in the over-
drive mode (OD = 1). All communication following this
command must occur at overdrive speed until a reset
pulse of minimum 690μs duration resets all devices on
the bus to standard speed (OD = 0).
When issued on a multidrop bus, this command sets all
overdrive-supporting devices into overdrive mode. To
subsequently address a specific overdrive-supporting
device, a reset pulse at overdrive speed must be
issued followed by a Match ROM or Search ROM com-
mand sequence. This speeds up the time for the
search process. If more than one slave supporting
overdrive is present on the bus and the Overdrive-Skip
ROM command is followed by a read command, data
collision occurs on the bus as multiple slaves transmit
simultaneously (open-drain pulldowns produce a wired-
AND result).
Overdrive-Match ROM [69h]
The Overdrive-Match ROM command followed by a
64-bit ROM sequence transmitted at overdrive speed
allows the bus master to address a specific DS1922L/
DS1922T on a multidrop bus and to simultaneously set
it in overdrive mode. Only the DS1922L/DS1922T that
exactly matches the 64-bit ROM sequence respond to
the subsequent memory/control function command.
Slaves already in overdrive mode from a previous
Overdrive-Skip ROM or successful Overdrive-Match
ROM command remain in overdrive mode. All over-
drive-capable slaves return to standard speed at the
next reset pulse of minimum 690μs duration. The
Overdrive-Match ROM command can be used with a
single or multiple devices on the bus.
Maxim Integrated