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DS2890V-000

1-Wire® Digital Potentiometer

Dallas Semiconductor -> Maxim Integrated 

DS2890

SEARCH ROM [F0H]

When a multi-drop 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 3-step routine on each bit of the ROM. After

one complete pass, the bus master knows the 64-bit ROM code of one device. Additional passes will

identify the ROM codes of the remaining devices. See Chapter 5 of the Book of DS19xx iButton

Standards for a comprehensive discussion of a search ROM, including an actual example.

CONDITIONAL SEARCH ROM [ECH]

The Conditional Search ROM command operates similarly to the Search ROM command except that only

devices fulfilling the specified search condition will participate in the search. The device condition that

will cause individual DS2890s to participate in a Conditional Search is a wiper position located at the

power-on default setting (00h). This feature enables the bus master to easily determine whether a

potentiometer has gone through a power-on reset and needs to be re-configured with a required wiper

position setting.

SKIP ROM [CCH]

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

potentiometer functions without providing the 64-bit ROM code. If more than one slave is present on the

bus and, for example, a read command is issued following the Skip ROM command, data collision will

occur on the bus as multiple slaves transmit simultaneously (open drain pull-downs will produce a wired-

AND result).

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 DS2890 in the Overdrive Mode. All communication following this

command code has to occur at Overdrive Speed until a reset pulse of minimum 480 µs duration resets all

devices on the bus to regular speed.

When issued on a multi-drop 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

search process. If more than one Overdrive-supporting slave 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 pull-downs will produce a wire-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 DS2890 on a multi-drop bus and to simultaneously set

it in Overdrive Mode. Only the DS2890 that exactly matches the 64-bit ROM sequence will respond to

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

Overdrive Skip or a successful Overdrive Match command will remain in Overdrive mode. All Over-

drive-capable slaves will return to regular speed at the next Reset Pulse of minimum 480 µs duration. The

Overdrive Match ROM command can be used with a single or multiple devices on the bus.

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