Datasheet

whether there is one slave or more than one. If there is only one slave device, the Master skips
the selection process. If more than one are present, the Master selects a slave by sending out
the ID for the selected slave, indicating to the remaining slaves that any further commands should
be ignored until after the next reset.
■ The Master sends a command followed by a sending or receiving of data. While most commands
are implemented using bytes read or written, some commands use bytes where some bits are
written and some read. Since a read looks like a write 1, if the line is held and driven low for less
than 15 μs and the slave does not know it is supposed to reply, the line remains untouched and
is read as a 1 (and written as a 1).
■ Usually at the end of one or more commands to that slave, a reset is issued again.
The 1-Wire module supports reset and answer-to-reset on request (although answer-to-reset can
be suppressed). It also supports byte operations. The software chooses which read, write, or mixed
operation to use. Partial byte (bit or multi-bit) is also supported.
22.3.3 Overdrive
The 1-Wire Module also allows for an "overdrive" speed to be selected for shorter line lengths.
Overdrive is about ten times faster because it uses smaller time periods. It is agreed by both master
and slave before enabling. Overdrive is selected using the ODRV bit of the 1-Wire Control and
Status (ONEWIRECS) register.
In overdrive mode, the time periods shrink. Table 22-2 on page 1510 shows the active time for 1-Wire
operations in overdrive mode. For example, the Master typically drives and holds the line low for
15 µs for a write 1, but in overdrive mode this is 1 µs.
Table 22-2. Active Time Periods in Overdrive Mode
NotesTimeOperation
Less than 2 µs low1 µs lowWrite 1
Up to 10 µs low7.5 µs lowWrite 0
Master samples at 2 µs1 µs lowRead 1
Slave holds at least 7 µs9 µs lowRead 0
Note 2 µs delay before reset for safety70 µs lowReset
Master samples at 8.5 µs4 µs to 48 µsAnswer to Reset
22.3.4 Timing Override
The 1-Wire module can manually override timing for any main action. Each non-zero value in the
1-Wire Timing Override (ONEWIRETIM) register replaces the default normal or overdrive time
rule.
The time factors that can be controlled:
Table 22-3. Bit Field Definitions for 1-Wire Timing and Override (ONEWIRETIM) Register
Default
(overdrive)
Default
(normal)
ScaleRange (if used)MeaningName
1 µs6 µs1 µs units1 to 15 µsAmount of time to drive and hold line low for a
write 1
W1TIM
7.5 µs60 µs4 µs units4 to 124 µsAmount of time to drive and hold line low for
write 0
a
W0TIM
December 13, 20131510
Texas Instruments-Advance Information
1-Wire Master Module