DS1985

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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).

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.