DS1977
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RESUME COMMAND [A5h]
The Resume Command function maximizes the data throughput in a multidrop environment. 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 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 will clear 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 functions
without providing the 64-bit ROM code. Unlike the normal Skip ROM command, the Overdrive Skip ROM sets the
DS1977 in the Overdrive mode (OD = 1). All communication following this command has to occur at Overdrive
speed until a reset pulse of minimum 480µs duration resets all devices on the bus to standard speed (OD = 0).
When issued on a multidrop bus this command will set all Overdrive-supporting devices into Overdrive mode. To
subsequently address a specific Overdrive-supporting device, a reset pulse at Overdrive speed has to be issued
followed by a Match ROM or Search ROM command sequence. This will speed 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 will occur on the bus as multiple slaves transmit simultaneously (open
drain pulldowns will 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 DS1977 on a multidrop bus and to simultaneously set it in Overdrive mode.
Only the DS1977 that exactly matches the 64-bit ROM sequence will respond to the subsequent memory function
command. Slaves already in Overdrive mode from a previous Overdrive Skip or Match command will remain in
Overdrive mode. All overdrive-capable slaves will return to standard speed at the next Reset Pulse of minimum
480µs duration. The Overdrive Match ROM command can be used with a single or multiple devices on the bus.
1-Wire SIGNALING
The DS1977 requires strict protocols to ensure data integrity. The protocol consists of five types of signaling on one
line: Reset Sequence with Reset Pulse and Presence Pulse, Write-Zero, Write-One Read-Data, and strong pullup
to supply power over the 1-Wire line. Except for the presence pulse the bus master initiates all these signals. The
DS1977 can communicate at two different speeds, standard speed and Overdrive Speed. If not explicitly set into
the Overdrive mode, the DS1977 will communicate at standard speed. While in Overdrive mode the fast timing
applies to all waveforms.
To get from idle to active, the voltage on the 1-Wire line needs to fall from V
PUP
below the threshold V
TL
. To get
from active to idle, the voltage needs to rise from V
ILMAX
past the threshold V
TH
. The time it takes for the voltage to
make this rise, in Figure 10 as ε , and its duration depends on the pullup resistor (RPUP) used and capacitance of
the 1-Wire network attached. The voltage V
ILMAX
is relevant for the DS1977 when determining a logical level, not
triggering any events.
The initialization sequence required to begin any communication with the DS1977 is shown in Figure 10. A Reset
Pulse followed by a Presence Pulse indicates the DS1977 is ready to receive data, given the correct ROM and
memory function command. If the bus master uses slew-rate control on the falling edge, it must pull down the line
for t
RSTL
+ t
F
to compensate for the edge. A t
RSTL
duration of 480µs or longer will exit the Overdrive mode returning
the device to standard speed. If the DS1977 is in Overdrive Mode and t
RSTL
is no longer than 80µs the device will
remain in Overdrive mode.
