DS1996

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Match ROM [55H]

The match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a

specific DS1996 on a multidrop bus. Only the DS1996 that exactly matches the 64-bit ROM sequence

will respond to the subsequent memory function command. All slaves that do not match the 64-bit ROM

sequence will wait for a reset pulse. This command can be used with a single or multiple devices on the

bus.

Skip ROM [CCH]

This command can save time in a single drop bus system by allowing the bus master to access the

memory 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).

Search ROM [F0H]

When a system is initially brought up, the bus master might not know the number of devices on the 1-

Wire bus or their 64-bit ROM codes. The search ROM command allows the bus master to use a process

of elimination to identify the 64-bit ROM codes of all slave devices on the bus. The search ROM process

is the repetition of a simple 3-step routine: read a bit, read the complement of the bit, then write the

desired value of that bit. The bus master performs this simple, 3-step routine on each bit of the ROM.

After one complete pass, the bus master knows the contents of the ROM in one device. The remaining

number of devices and their ROM codes may be identified by additional passes. See Chapter 5 of the

Book of DS19xx iButton Standards for a comprehensive discussion of a search ROM, including an actual

example.

Overdrive Skip ROM [3CH]

On a single-drop bus this command can save time by allowing the bus master to access the memory

functions without providing the 64-bit ROM code. Unlike the normal Skip ROM command the Overdrive

Skip ROM sets the DS1996 in the Overdrive Mode (OD=1). All communication following this command

has to occur at Overdrive Speed until a reset pulse of minimum 480 µs duration resets all devices on the

bus to regular speed (OD=0).

When issued on a multidrop bus this command will set all Overdrive-capable devices into Overdrive

mode. To subsequently address a specific Overdrive-capable device, a reset pulse at Overdrive speed has

to be issued followed by a Match ROM or Search ROM command sequence. This will shorten 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 DS1996 on a multidrop bus and to simultaneously set it

in Overdrive Mode. Only the DS1996 that exactly matches the 64-bit ROM sequence will respond to the

subsequent memory function command. Slaves already in Overdrive mode from a previous Overdrive

Skip or Match command will remain in Overdrive mode. All other slaves that do not match the 64-bit

ROM sequence or do not support Overdrive will return to or remain at regular speed and wait for a reset

pulse of minimum 480 µs duration. The Overdrive Match ROM command can be used with a single or

multiple devices on the bus.