DS1992/DS1993

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iButton DESCRIPTION

The DS1992/DS1993 memory iButtons (hereafter referred to as DS199_) are rugged read/write data

carriers that act as a localized database, easily accessible with minimal hardware. The nonvolatile

memory and optional timekeeping capability offer a simple solution to storing and retrieving vital

information pertaining to the object to which the iButton is attached. Data is transferred serially through

the 1-Wire protocol that requires only a single data lead and a ground return.

The scratchpad is an additional page that acts as a buffer when writing to memory. Data is first written to

the scratchpad where it can be read back. After the data has been verified, a copy scratchpad command

transfers the data to memory. This process ensures data integrity when modifying the memory. A 48-bit

serial number is factory lasered into each DS199_ to provide a guaranteed unique identity that allows for

absolute traceability. The durable MicroCan package is highly resistant to environmental hazards such as

dirt, moisture, and shock. Its compact coin-shaped profile is self-aligning with mating receptacles,

allowing the DS199_ to be easily used by human operators. Accessories permit the DS199_ to be

mounted on almost any surface including plastic key fobs, photo–ID badges, and PC boards.

Applications include access control, work-in-progress tracking, electronic travelers, storage of calibration

constants, and debit tokens.

OPERATION

The DS199_ have three main data components: 1) 64-bit lasered ROM, 2) 256-bit scratchpad, and 3)

1024-bit (DS1992) or 4096-bit (DS1993) SRAM. All data is read and written least significant bit first.

The memory functions are not available until the ROM function protocol has been established. This

protocol is described in the ROM functions flow chart (Figure 9). The 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 memory functions are accessible and the

master can then provide any one of the four memory function commands (Figure 6).

PARASITE POWER

The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry steals power whenever

the data input is high. The data line provides sufficient power as long as the specified timing and voltage

requirements are met. The advantages of parasite power are two-fold: 1) by parasiting off this input,

lithium is conserved, and 2) if the lithium is exhausted for any reason, the ROM can still be read

normally.

64-bit LASERED ROM

Each DS199_ contain 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.)

The 1-Wire CRC is generated using a polynomial generator consisting of a shift register and XOR gates

Cyclic Redundancy Check is available in the Book of DS19xx iButton Standards. The shift register bits

are initialized to zero. 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 zeros.