DS1921 datasheet(30/44 Pages) DALLAS

30 / 44 page

DS1921H/Z

30 of 44

INITIALIZATION PROCEDURE (RESET AND PRESENCE PULSES) Figure 14

RESISTOR

MASTER

DS1921H/Z

tRSTL

tPDL

tRSTH

tPDH

MASTER TX RESET PULSE

MASTER RX PRESENCE PULSE

VPUP

VIHMASTER

VTH

VTL

VILMAX

tREC

tMSP

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.

DS1921 Datasheet(PDF) 30 Page - Dallas Semiconductor