Datasheet

DS2408
21 of 39
PLS of the Control/Status Register (address 008Dh), and the state of the PIO channels. See Figure 7 for a
description of the Conditional Search logic. The device also responds to the Conditional Search if the
PORL bit is set. The Conditional Search ROM provides an efficient means for the bus master to deter-
mine devices on a multidrop system that have to signal an important event, such as a state change at a
PIO pin caused by an external signal. After each pass of the conditional search that successfully deter-
mined the 64-bit ROM for a specific device on the multidrop bus, that particular device can be individu-
ally accessed as if a Match ROM had been issued, since all other devices will have dropped out of the
search process and will be waiting for a reset pulse.
Skip ROM [CCh]
This command can save time in a single-drop bus system by allowing the bus master to access the control
functions without providing the 64-bit ROM code. If more than one slave is present on the bus and a
Read command is issued following the Skip ROM command, data collision will occur on the bus as
multiple slaves transmit simultaneously (open-drain pulldowns will produce a wired-AND result).
Resume Command [A5h]
In a typical application the DS2408 can be accessed several times to complete a control or adjustment
function. In a multidrop environment this means that the 64-bit ROM sequence of a Match ROM com-
mand has to be repeated for every access. To maximize the data throughput in a multidrop environment,
the Resume Command function is implemented. This function checks the status of the RC flag and, if it is
set, directly transfers control to the control functions, similar to a Skip ROM command. The only way to
set the RC flag is through successfully executing the Match ROM, Search ROM, Conditional Search
ROM, or Overdrive-Match ROM command. Once the RC flag is set, the device can be repeatedly
accessed through the Resume Command function. Accessing another device on the bus will clear the RC
flag, preventing two or more devices from simultaneously responding to the Resume Command function.
Skip ROM [3Ch]
On a single-drop bus this command can save time by allowing the bus master to access the control
functions without providing the 64-bit ROM code. Unlike the normal Skip ROM command, the
Overdrive Skip ROM sets the DS2408 in the overdrive mode (OD = 1). All communication following
this command has to occur at overdrive speed until a reset pulse of minimum 480µs duration resets all
devices on the bus to standard speed (OD = 0). When issued on a multidrop bus this command will set all
overdrive-supporting devices into overdrive mode. To subsequently address a specific overdrive-
supporting device, a reset pulse at overdrive speed has to be issued followed by a Match ROM or Search
ROM command sequence. This will speed up the time for the search process. If more than one slave
supporting overdrive is present on the bus and the Overdrive Skip ROM command is followed by a Read
command, data collision will occur on the bus as multiple slaves transmit simultaneously (open-drain
pulldowns will produce a wired-AND result).
Overdrive Match ROM [69h]
The Overdrive Match ROM command followed by a 64-bit ROM sequence transmitted at overdrive
speed allows the bus master to address a specific DS2408 on a multidrop bus and to simultaneously set it
in overdrive mode. Only the DS2408 that exactly matches the 64-bit ROM sequence will respond to the
subsequent control function command. Slaves already in overdrive mode from a previous Overdrive Skip
or Match command will remain in overdrive mode. All overdrive-capable slaves will return to standard
speed at the next Reset Pulse of minimum 480µs duration. The Overdrive Match ROM command can be
used with either single or multiple devices on the bus.