DS1921H Datasheet, PDF (30/45 Pages) Maxim Integrated Products – High-Resolution Thermochron iButton

DS1921H/Z
INITIALIZATION PROCEDURE (RESET AND PRESENCE PULSES) Figure 14
After the bus master has released the line it goes into receive mode (RX). Now, the 1-Wire bus is pulled
to V_PUP via the pull-up resistor or, in case of a DS2480B driver, by active circuitry. When the threshold
V_TH is crossed, the DS1921H/Z waits for t_PDH and then transmits a presence pulse by pulling the line low
for t_PDL. To detect a presence pulse, the master must test the logical state of the 1-Wire line at t_MSP.
The t_RSTH window must be at least the sum of t_PDHMAX, t_PDLMAX, and t_RECMIN. Immediately after t_RSTH is
expired, the DS1921H/Z is ready for data communication. In a mixed population network, t_RSTH 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 V_TL, 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 V_TH threshold after 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. The voltage 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 DS1921H/Z needs a recovery time t_REC before it is ready for the next time slot.
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