Datasheet

DS2408
25 of 39
Read/Write Time Slots
Data communication with the DS2408 takes place in time slots, which carry a single bit each. Write time
slots transport data from bus master to slave. Read time slots transfer data from slave to master. The
definitions of the write and read time slots are illustrated in Figure 14.
All communication begins with the master pulling the data line low. As the voltage on the 1-Wire line
falls below the threshold V
TL
, the DS2408 starts its internal time base. The tolerance of the slave time
base creates a slave-sampling window, which stretches from t
SLSMIN
to t
SLSMAX
. The voltage on the data
line at the sampling point determines whether the DS2408 decodes the time slot as 1 or 0.
Master-to-Slave
For a write-one time slot, the voltage on the data line must have crossed the V
THMAX
threshold after the
write-one low time t
W1LMAX
has expired. For a write-zero time slot, the voltage on the data line must stay
below the V
THMIN
threshold until the write-zero low time t
W0LMIN
has expired. For most reliable
communication, the voltage on the data line should not exceed V
ILMAX
during the entire t
W0L
window.
After the V
THMAX
threshold has been crossed, the DS2408 needs a recovery time t
REC
before it is ready for
the next time slot.
Figure 14. READ/WRITE TIMING DIAGRAM
Write-One Time Slot
V
PUP
V
IHMASTER
V
TH
V
TL
V
ILMAX
0V
DS2408
Sampling
Window
t
SLSMIN
t
F
t
SLOT
t
W1L
t
SLSMAX
ε
Write-Zero Time Slot
t
REC
V
PUP
V
IHMASTER
V
TH
V
TL
V
ILMAX
0V
DS2408
Sampling
Window
t
SLSMIN
t
F
t
SLOT
t
SLSMAX
t
W0L
ε