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1024-Bit EEPROM iButton
V PUP
V IHMASTER
V TH
V TL
V ILMAX
MASTER Tx "RESET PULSE"
ε
t MSP
MASTER Rx "PRESENCE PULSE"
0V
t RSTL
t PDH
t PDL
t REC
t F
t RSTH
RESISTOR
Figure 10. Initialization Procedure: Reset and Presence Pulse
MASTER
DS1972
Read/Write Time Slots
Data communication with the DS1972 takes place in
time slots that carry a single bit each. Write time slots
transport data from bus master to slave. Read time
slots transfer data from slave to master. Figure 11 illus-
trates the definitions of the write and read time slots.
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 DS1972 starts its internal
timing generator that determines when the data line is
sampled during a write time slot and how long data is
valid during a read time slot.
Master-to-Slave
For a write-one time slot, the voltage on the data line
must have crossed the V TH threshold before the write-
one low time t W1LMAX is expired. For a write-zero time
slot, the voltage on the data line must stay below the
V TH threshold until the write-zero low time t W0LMIN is
expired. For the most reliable communication, the volt-
age on the data line should not exceed V ILMAX during
the entire t W0L or t W1L window. After the V TH threshold
has been crossed, the DS1972 needs a recovery time
t REC before it is ready for the next time slot.
Slave-to-Master
A read-data time slot begins like a write-one time slot.
The voltage on the data line must remain below V TL
until the read low time t RL is expired. During the t RL
window, when responding with a 0, the DS1972 starts
pulling the data line low; its internal timing generator
determines when this pulldown ends and the voltage
starts rising again. When responding with a 1, the
DS1972 does not hold the data line low at all, and the
voltage starts rising as soon as t RL is over.
The sum of t RL + δ (rise time) on one side and the inter-
nal timing generator of the DS1972 on the other side
define the master sampling window (t MSRMIN to
t MSRMAX ), in which the master must perform a read
from the data line. For the most reliable communication,
t RL should be as short as permissible, and the master
should read close to but no later than t MSRMAX . After
reading from the data line, the master must wait until
t SLOT is expired. This guarantees sufficient recovery
time t REC for the DS1972 to get ready for the next time
slot. Note that t REC specified herein applies only to a
single DS1972 attached to a 1-Wire line. For multide-
vice configurations, t REC must be extended to accom-
modate the additional 1-Wire device input capacitance.
Alternatively, an interface that performs active pullup
during the 1-Wire recovery time such as the DS2482-
x00 or DS2480B 1-Wire line drivers can be used.
______________________________________________________________________________________
17
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