DS1985 Maxim, DS1985 Datasheet - Page 3

• Manufacturer Part Number: DS1985
• Description: The DS1985 16Kb Add-Only iButton is a rugged read/write data carrier that identifies and stores relevant information about the product or person to which it is attached
• Manufacturer: Maxim

DS1985
DS1985 BLOCK DIAGRAM Figure 1
64-BIT LASERED ROM
Each DS1985 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 3.)
The 64-bit ROM and ROM Function Control section allow the DS1985 to operate as a 1-Wire device and
follow the 1-Wire protocol detailed in the section “1-Wire Bus System.” The memory functions required
to read and program the EPROM sections of the DS1985 are not accessible until the ROM function
protocol has been satisfied. This protocol is described in the ROM functions flow chart (Figure 8). The 1-
Wire bus master must first provide one of four ROM function commands: 1) Read ROM, 2) Match ROM,
3) Search ROM, or 4) Skip ROM. After a ROM function sequence has been successfully executed, the
bus master may then provide any one of the memory function commands specific to the DS1985 (Figure
5).
8
5
4
The 1-Wire CRC of the lasered ROM is generated using the polynomial X
+ X
+ X
+ 1. Additional
information about the Dallas Semiconductor 1-Wire Cyclic Redundancy Check is available in the Book
of DS19xx iButton Standards. The shift register acting as the CRC accumulator is initialized to 0. Then
starting with the least significant bit of the family code, 1 bit at a time is shifted in. After the 8th bit of the
family code has been entered, then 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 should return the
shift register to all 0s.
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