64-Bit Unique ROM

Each DS1990A contains a unique ROM code 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 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 Cyclic Redundancy Check

(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 48th bit of

the serial number has been entered, the shift register con-

tains the CRC value. Shifting in the 8 bits of 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 DS1990A

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

controller or PC. For small configurations, the 1-Wire

communication signals can be generated under software

control using a single port pin. Alternatively, the DS2480B

1-Wire line driver chip or serial-port adapters based on

this chip (DS9097U series) can be used. This simplifies

the hardware design and frees the microprocessor from

responding in real time. 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 that are

initiated on the falling edge of sync pulses from the bus

master. For a more detailed protocol description, refer to

Chapter 4 of the Book of iButton Standards.

Figure 2. 64-Bit Unique ROM

Figure 3. 1-Wire CRC Generator

www.analog.com

Analog Devices │ 4

DS1990A

Serial Number iButton

MSB

8-BIT

CRC CODE

48-BIT SERIAL NUMBER

MSB

MSB

LSB

LSB

LSB

8-BIT FAMILY CODE

(01h)

MSB

LSB

1ST

STAGE

2ND

STAGE

3RD

STAGE

4TH

STAGE

7TH

STAGE

8TH

STAGE

6TH

STAGE

5TH

STAGE

X0

X1

X2

X3

X4

POLYNOMIAL = X8 + X5 + X4 + 1

INPUT DATA

X5

X6

X7

X8