DS2408
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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 VPUP below the VTL threshold.
To get from active to idle, the voltage needs to rise from VILMAX past the VTH threshold. The VILMAX
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 tRSTL + tF to compensate for the edge. A tRSTL duration of 480s or longer will exit
the overdrive mode returning the device to standard speed. If the DS2408 is in overdrive mode and tRSTL
is no longer than 80s 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 VPUP via the pullup resistor or, in case of a DS2480B driver, by active circuitry. When the VTH
threshold is crossed, the DS2408 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 DS2408 is ready for data communication. In a mixed population network, tRSTH should be
extended to a minimum of 480s at standard speed and 48s at overdrive speed to accommodate other 1-
Wire devices.