Rainbow Electronics DS2422 - Manual (Page 40)

DS2422
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1-Wire SIGNALING
The DS2422 requires strict protocols to ensure data integrity. The protocol consists of four types of signaling on
one line: Reset Sequence with Reset Pulse and Presence Pulse, Write-Zero, Write-One and Read-Data. Except for
the presence pulse the bus master initiates all these signals. The DS2422 can communicate at two different
speeds, standard speed, and Overdrive Speed. If not explicitly set into the Overdrive mode, the DS2422 will
communicate at standard speed. While in Overdrive Mode the fast timing applies to all waveforms.
To get from idle to active, the voltage on the 1-Wire line needs to fall from V
PUP
below the threshold V
TL
. To get
from active to idle, the voltage needs to rise from V
ILMAX
past the threshold V
TH
. The time it takes for the voltage to
make this rise is seen in Figure 15 as ' e' and its duration depends on the pull-up resistor (R
PUP
) used and the
capacitance of the 1-Wire network attached. The voltage V
ILMAX
is relevant for the DS2422 when determining a
logical level, not triggering any events.
The initialization sequence required to begin any communication with the DS2422 is shown in Figure 15. A Reset
Pulse followed by a Presence Pulse indicates the DS2422 is ready to receive data, given the correct ROM and
memory function command. If the bus master uses slew-rate control on the falling edge, it must pull down the line
for t
RSTL
+ t
F
to compensate for the edge. A t
RSTL
duration of 690µs or longer will exit the Overdrive Mode returning
the device to standard speed. If the DS2422 is in Overdrive Mode and t
RSTL
is no longer than 80µs the device will
remain in Overdrive Mode.
Figure 15. Initialization Procedure “Reset and Presence Pulses”
RESISTOR MASTER DS2422
t
RSTL
t
PDL
t
RSTH
t
PDH
MASTER TX “RESET PULSE” MASTER RX “PRESENCE PULSE”
V
PUP
V
IHMASTER
V
TH
V
TL
V
ILMAX
0V
e
t
F
t
REC
t
MSP
After the bus master has released the line it goes into receive mode (RX). Now the 1-Wire bus is pulled to V
PUP
through the pullup resistor or, in case of a DS2480B driver, by active circuitry. When the threshold V
TH
is crossed,
the DS2422 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
DS2422 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 DS2422 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 16.
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 DS2422 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.