Datasheet

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 X
8
+ X
5
+ X
4
+ 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 MSBLSB
LSB
LSB
8-BIT FAMILY CODE
(01h)
MSBLSB
1ST
STAGE
2ND
STAGE
3RD
STAGE
4TH
STAGE
7TH
STAGE
8TH
STAGE
6TH
STAGE
5TH
STAGE
X
0
X
1
X
2
X
3
X
4
POLYNOMIAL = X
8
+ X
5
+ X
4
+ 1
INPUT DATA
X
5
X
6
X
7
X
8