64-Bit Lasered 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 gen-

erator consisting of a shift register and XOR gates as

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 contains 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 soft-

ware 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 micro-

processor 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.

Serial Number iButton

4

_______________________________________________________________________________________

MSB

8-BIT

CRC CODE

48-BIT SERIAL NUMBER

MSB

MSB

LSB

LSB

LSB

8-BIT FAMILY CODE

(01h)

MSB

LSB

Figure 2. 64-Bit Lasered ROM

1ST

STAGE

2ND

STAGE

3RD

STAGE

4TH

STAGE

7TH

STAGE

8TH

STAGE

6TH

STAGE

5TH

STAGE

INPUT DATA

Figure 3. 1-Wire CRC Generator