Datasheet

DS2408
24 of 39
1-WIRE SIGNALING
The DS2408 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 DS2408 can
communicate at two different speeds, standard speed, and overdrive speed. If not explicitly set into the
overdrive mode, the DS2408 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 V
TL
threshold.
To get from active to idle, the voltage needs to rise from V
ILMAX
past the V
TH
threshold. The V
ILMAX
voltage is relevant for the DS2408 when determining a logical level, not triggering any events.
Figure 13 shows the initialization sequence required to begin any communication with the DS2408. A
Reset Pulse followed by a Presence Pulse indicates the DS2408 is ready to receive data, given the correct
ROM and control 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 480µs or longer will exit
the overdrive mode returning the device to standard speed. If the DS2408 is in overdrive mode and t
RSTL
is no longer than 80µs the device will remain in overdrive mode.
Figure 13. INITIALIZATION PROCEDURE “RESET AND PRESENCE PULSES”
After the bus master has released the line it goes into receive mode (RX). The 1-Wire bus is then pulled
to V
PUP
via the pullup resistor or, in case of a DS2480B driver, by active circuitry. When the V
TH
threshold is crossed, the DS2408 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 DS2408 is ready for data communication. In a mixed population network, t
RSTH
should be
extended to a minimum of 480µs at standard speed and 48µs at overdrive speed to accommodate other 1-
Wire devices.