Datasheet

DS1977
20 of 29
Figure 10. INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES”
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 pullup resistor or, in case of a DS2480B driver, by active circuitry. When the threshold V
TH
is crossed, the
DS1977 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
DS1977 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 DS1977 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 11.
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 DS1977 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
THMAX
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
THMIN
threshold until the write-zero low time t
W0LMIN
is 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
DS1977 needs a recovery time t
REC
before it is ready for the next time slot.
Figure 11. READ/WRITE TIMING DIAGRAM
Write-One Time Slot
RESISTOR MASTER
V
PUP
V
IHMASTER
V
TH
V
TL
V
ILMAX
0V
t
F
t
SLOT
t
W1L
ε