DS1991

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TRANSACTION SEQUENCE

The protocol for accessing the DS1991 via the 1-Wire port is as follows:

§ Initialization

§ ROM Function Command

§ Memory Function Command

§ Transaction/Data

INITIALIZATION

All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence

consists of a reset pulse transmitted by the bus master followed by presence pulse(s) transmitted by the

slave(s). The presence pulse lets the bus master know that the DS1991 is on the bus and is ready to

operate. For more details, see the “1-Wire Signaling” section

ROM FUNCTION COMMANDS

Once the bus master has detected a presence pulse, it can issue one of the four ROM function commands.

All ROM function commands are eight bits long. A list of these commands follows (refer to flowchart in

Figure 9).

Read ROM [33H]

This command allows the bus master to read the DS1991’s 8-bit family code, unique 48-bit serial number

and 8-bit CRC. This command can be used only if there is a single DS1991 on the bus. If more than one

slave is present on the bus, a data collision will occur when all slaves try to transmit at the same time

(open drain will produce a wired-AND result).

Match ROM [55H]

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

specific DS1991 on a multidrop bus. Only the DS1991 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 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 ROM search 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.