One wire communication is executed in “time slots”.
The DS1481 generates either a read/write bit “time slot”
or a reset on the I/O pin. The operation performed is
determined by the states of the D/CLK and RES pins as
Read 0, Read 1, logic high logic high (see
logic low logic high (see
logic high logic low (see
After D/CLK and RES have been set, the time slot
begins when ENI is driven to its active state. A falling
edge on ENI causes the DS1481 to save the state of
D/CLK and RES. If the time slot is a 1–wire reset the
DS1481 will issue a busy signal by driving O1/BSY1 low
and O2/BSY2 high. After 2 µs O2/BSY2 is driven low.
Both outputs will remain low until the communication on
the I/O line is finished. A busy signal for a bit time slot
differs from the reset busy signal only in that both
O1/BSY1 and O2/BSY2 are driven low immediately.
While the busy signal is asserted, the host processor is
free to perform other tasks (including running the print
spooler). When the time slot is complete, the DS1481
restores both O1/BSY1 and O2/BSY2 to the states of I1
and I2 (see Figure 1).
When the host detects that one or both of the busy sig-
nals has returned high, it must query the result of the
time slot. This is accomplished by driving D/CLK low. If
the result of the time slot was low (Read 0, Write 0 or
presence detect) the DS1481 drives both O1/BSY1 and
O2/BSY2 low (this state is held until ENI returns high).
Otherwise it propagates the states of I1 and I2.
After the host reads the result of the time slot it must
drive ENI to its inactive state (high). The DS1481 will
then set O1/BSY1 and O2/BSY2 to the states of I1
1–WIRE TIMING GENERATION
For all time slots, the DS1481 samples the I/O pin at tSO
(see Figure 4). The DS1481 waits a minimun of 60 µs
from the start of the time slot and de–asserts O1/BSY1
When a reset is requested, the DS1481 drives the I/O
pin low for at least 480 µs and then releases it. During a
normal reset the I/O pin immediately begins to return
If a 1–wire device is present on the I/O line it pulls I/O low
after time T (15 µs ≤ T ≤ 60 µs) from the previous rising
edge. The 1–wire device(s) holds the I/O line low for 4T
and then releases it, allowing the I/O line to return high.
This is the presence detect pulse. The I/O line must
remain high (in its idle state) for at least 3T before the
1–wire device(s) is ready for further communication. To
insure this idle high time is satisfied, the DS1481 does
not release O1/BSY1 and O2/BSY2 for at least 960 µs
(measured from the 1st falling edge on the I/O pin).
If after 480 µs of low time the I/O line did not return high,
either the I/O line has been shorted to ground or there is
at least one 1–wire device connected to the I/O line
which is issuing an alarm interrupt (see Figure 6). In this
case the DS1481 waits for I/O to return high for an addi-
tional 3840 µs (64 * 60). If time expires the I/O line is
assumed to be shorted and the DS1481 releases
O1/BSY1 and O2/BSY2. If the I/O line returns high, the
DS1481 continues to monitor the presence detect por-
tion of the reset (as described above) as for the non–in-
terrupt case. Note that the 3T idle high time is still
required after the presence detect ends.
The DS1481 also supports overdrive communication
with overdrive capable 1–wire devices. When the
DS1481 powers up it is in normal mode (i.e., OD = 0,
Figure 1). To toggle to overdrive mode the host sets
D/CLK and RES low and drives ENI low. The DS1481
toggles the OD bit to a logic high and returns the states
of I1 and I2 on O1/BSY1 and O2/BSY2. Overdrive
mode is cleared in the same way. When overdrive is
turned off (OD = 0). O1/BSY1 and O2/BSY2 are driven
low to report the state of the OD bit.