DS2761

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SPECIAL FEATURE REGISTER

The format of the special feature register is shown in Figure 12. The function of each bit is described in

detail in the following paragraphs.

Figure 12. SPECIAL FEATURE REGISTER FORMAT

Address 08

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

PS

PIO

MSTR

X

X

X

X

X

PS — PS Pin Latch. This bit latches a low state on the PS pin, and is cleared only by writing a 1 to this

location. Writing this bit to a 1 immediately upon reading of a 0 value is recommended.

PIO—PIO Pin Sense and Control. See the Programmable I/O section for details on this read/write bit.

MSTR—SWAP Master Status Bit. This bit indicates whether a device has been selected through the

SWAP command. Selection of this device through the SWAP command and the appropriate net address

results in setting this bit, indicating that this device is the master. A 0 signifies that this device is not the

master.

X—Reserved Bits.

1-WIRE BUS SYSTEM

The 1-Wire bus is a system that has a single bus master and one or more slaves. A multidrop bus is a 1-

Wire bus with multiple slaves. A single-drop bus has only one slave device. In all instances, the DS2761

is a slave device. The bus master is typically a microprocessor in the host system. The discussion of this

bus system consists of four topics: 64-bit net address, hardware configuration, transaction sequence, and

1-Wire signaling.

64-BIT NET ADDRESS

Each DS2761 has a unique, factory-programmed 1-Wire net address that is 64 bits in length. The first

eight bits are the 1-Wire family code (30h for DS2761). The next 48 bits are a unique serial number. The

last eight bits are a cyclic redundancy check (CRC) of the first 56 bits (see Figure 13). The 64-bit net

address and the 1-Wire I/O circuitry built into the device enable the DS2761 to communicate through the

1-Wire protocol detailed in the 1-Wire Bus System section of this data sheet.

Figure 13. 1-WIRE NET ADDRESS FORMAT

8-BIT CRC

48-BIT SERIAL NUMBER

8-BIT FAMILY

CODE (30H)

MSb

LSb

CRC GENERATION

The DS2761 has an 8-bit CRC stored in the most significant byte of its 1-Wire net address. To ensure

error-free transmission of the address, the host system can compute a CRC value from the first 56 bits of

the address and compare it to the CRC from the DS2761. The host system is responsible for verifying the

CRC value and taking action as a result. The DS2761 does not compare CRC values and does not prevent