DS2755: High-Accuracy Battery Fuel Gauge with Snapshot

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TRANSACTION SEQUENCE

The protocol for accessing the DS2755 through the 1-Wire port is as follows:

Initialization

Net Address Command

Function Command

Transaction/Data

The sections that follow describe each of these steps in detail.

All transactions of the 1-Wire bus begin with an initialization sequence consisting of a reset pulse transmitted by the

bus master followed by a presence pulse simultaneously transmitted by the DS2755 and any other slaves on the

bus. The presence pulse tells the bus master that one or more devices are on the bus and ready to operate. For

more details, see the I/O Signaling section.

NET ADDRESS COMMANDS

Once the bus master has detected the presence of one or more slaves, it can issue one of the net address

commands described in the following paragraphs. The name of each command is followed by the 8-bit opcode for

that command in square brackets. Figure 17 presents a transaction flowchart of the net address commands.

Read Net Address [33h or 39h].

This command allows the bus master to read the DS2755’s 1-Wire net address.

This command can only be used if there is a single slave on the bus. If more than one slave is present, a data

collision occurs when all slaves try to transmit at the same time (open drain produces a wired-AND result). The

RNAOP bit in the Status Register selects the opcode for this command, with RNAOP = 0 indicating 33h and

RNAOP = 1 indicating 39h.

Match Net Address [55h].

This command allows the bus master to specifically address one DS2755 on the 1-Wire

bus. Only the addressed DS2755 responds to any subsequent function command. All other slave devices ignore

the function command and wait for a reset pulse. This command can be used with one or more slave devices on

the bus.

Skip Net Address [CCh].

This command saves time when there is only one DS2755 on the bus by allowing the

bus master to issue a function command without specifying the address of the slave. If more than one slave device

is present on the bus, a subsequent function command can cause a data collision when all slaves transmit data at

the same time.

Search Net Address [F0h].

This command allows the bus master to use a process of elimination to identify the 1-

Wire net addresses of all slave devices on the bus. The search process involves the repetition of a simple three-

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 three-step routine on each bit location of the net address. After one complete pass through all

64 bits, the bus master knows the address of one device. The remaining devices can then be identified on

additional iterations of the process. See Chapter 5 of the Book of DS19xx iButton

discussion of a net address search, including an actual example. This publication can be found on the

Maxim/Dallas website at www.maxim-ic.com.

FUNCTION COMMANDS

After successfully completing one of the net address commands, the bus master can access the features of the

DS2755 with any of the function commands described in the following paragraphs. The name of each function is

followed by the 8-bit opcode for that command in square brackets.

Read Data [69h, XX].

This command reads data from the DS2755 starting at memory address XX. The LSb of the

data in address XX is available to be read immediately after the MSb of the address has been entered. Because

the address is automatically incremented after the MSb of each byte is received, the LSb of the data at address XX

+ 1 is available to be read immediately after the MSb of the data at address XX. If the bus master continues to read

beyond address FFh, the DS2755 outputs logic 1 until a reset pulse occurs. Addresses labeled “reserved” in the

memory map contain undefined data. The Read Data command can be terminated by the bus master with a reset

pulse at any bit boundary.

iButton is a registered trademark of Dallas Semiconductor.