DS28E17 datasheet(8/27 Pages) MAXIM

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64-Bit ROM ID

Each DS28E17 contains a unique ROM ID that is 64 bits

long. The first 8 bits are a 1-Wire family code. The next

48 bits are a unique serial number. The last 8 bits are a

cyclic redundancy check (CRC) of the first 56 bits. See

Figure 2 for details. The 1-Wire CRC is generated using

a polynomial generator consisting of a shift register and

XOR gates as shown in Figure 3. The polynomial is X8 +

X5 + X4 + 1. Additional information about the 1-Wire CRC

is available in Application Note 27: Understanding and

Using Cyclic Redundancy Checks with Maxim iButton®

Products.

The shift register bits are initialized to 0. Then, starting

with the least significant bit of the family code, one bit at

a time is shifted in. After the 8th bit of the family code has

been entered, the serial number is entered. After the last

bit of the serial number has been entered, the shift reg-

ister contains the CRC value. Shifting in the 8 bits of the

CRC returns the shift register to all 0s.

1-Wire Bus System

The 1-Wire bus is a system that has a single bus master

and one or more slaves. In all instances the DS28E17 is

a slave device. The bus master is typically a microcon-

troller. The discussion of this bus system is broken down

into three topics: hardware configuration, transaction

sequence, and 1-Wire signaling (signal types and timing).

The 1-Wire protocol defines bus transactions in terms of

the bus state during specific time slots, which are initiated

on the falling edge of sync pulses from the bus master.

Hardware Configuration

The 1-Wire bus has only a single line by definition; it is

important that each device on the bus be able to drive

it at the appropriate time. To facilitate this, each device

attached to the 1-Wire bus must have open-drain or

three-state outputs. The 1-Wire port (i.e., IO pin) of the

DS28E17 is open drain or an input to read data.

iButton is a registered trademark of Maxim Integrated Products, Inc.

Figure 2. 64-Bit ROM ID

Figure 3. 1-Wire CRC Generator

DS28E17

1-Wire-to-I2C Master Bridge

www.maximintegrated.com

Maxim Integrated │ 8

MSb

8-BIT

CRC CODE

48-BIT SERIAL NUMBER

MSb

LSb MSb

LSb

8-BIT FAMILY CODE

(19h)

MSb

LSb

1ST

STAGE

2ND

STAGE

3RD

STAGE

4TH

STAGE

7TH

STAGE

8TH

STAGE

6TH

STAGE

5TH

STAGE

POLYNOMIAL = X8 + X5 + X4 + 1

INPUT DATA

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