DS1921H/Z
INITIALIZATION PROCEDURE (RESET AND PRESENCE PULSES) Figure 14
MASTER TX RESET PULSE
MASTER RX PRESENCE PULSE
tMSP
ε
VPUP
VIHMASTER
VTH
VTL
VILMAX
0V
tF
tRSTL
tPDL
tRSTH
tPDH
tREC
RESISTOR
MASTER
DS1921H/Z
After the bus master has released the line it goes into receive mode (RX). Now, the 1-Wire bus is pulled
to VPUP via the pull-up resistor or, in case of a DS2480B driver, by active circuitry. When the threshold
VTH is crossed, the DS1921H/Z waits for tPDH and then transmits a presence pulse by pulling the line low
for tPDL. To detect a presence pulse, the master must test the logical state of the 1-Wire line at tMSP
.
The tRSTH window must be at least the sum of tPDHMAX, tPDLMAX, and tRECMIN. Immediately after tRSTH is
expired, the DS1921H/Z is ready for data communication. In a mixed population network, tRSTH should
be extended to minimum 480µs at standard speed and 48µs at Overdrive speed to accommodate other 1-
Wire devices.
Read/Write Time Slots
Data communication with the DS1921H/Z 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. The
definitions of the write and read time slots are illustrated in Figure 15.
All communication begins with the master pulling the data line low. As the voltage on the 1-Wire line
falls below the threshold VTL, the DS1921H/Z starts its internal timing generator that determines when
the data line will be sampled during a write time slot and how long data will be 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 VTH threshold after the
write-one low time tW1LMAX is expired. For a write-zero time slot, the voltage on the data line must stay
below the VTH threshold until the write-zero low time tW0LMIN is expired. The voltage on the data line
should not exceed VILMAX during the entire tW0L or tW1L window. After the VTH threshold has been
crossed, the DS1921H/Z needs a recovery time tREC before it is ready for the next time slot.
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DALLAS [ DALLAS SEMICONDUCTOR ]
