DS1922E
iButton High-Temperature Logger 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 DS1922E 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 maximize the
data throughput in a multidrop environment, the Resume
function was implemented. This function checks the sta-
tus of the RC bit and, if it is set, directly transfers control
to the memory/control functions, 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 simultaneously responding to the Resume
command function.
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-
Maxim Integrated
Skip ROM command sets the DS1922E in the Overdrive
Mode (OD = 1). All communication following this com-
mand 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 DS1922E
on a multidrop bus and to simultaneously set it in
Overdrive Mode. Only the DS1922E that exactly match-
es the 64-bit ROM sequence responds to the subse-
quent memory/control function command. Slaves
already in Overdrive Mode from a previous Overdrive-
Skip ROM or successful Overdrive-Match ROM com-
mand remain in Overdrive Mode. All overdrive-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.
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