Rainbow Electronics DS1993 - Manual (Page 4)

DS1992/DS1993/DS1994
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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 will provide 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 may still be
read normally.
64-BIT LASERED ROM
Each DS199X contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family
code. The next 48 bits are a unique serial number. The last eight 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 as shown in Figure 3. The polynomial is X
8
+ X
5
+ X
4
+ 1. Additional information about the
Dallas 1-Wire 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, one 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 eight bits of CRC should return the shift register to all zeros.
64-BIT LASERED ROM Figure 2
FAMILY
CODE
04h = DS1994
06h = DS1993
08h = DS1992 LSB
SERIAL
NUMBER
48-BIT
UNIQUE
NUMBER
CRC 8 BITS
MSB
1-WIRE CRC CODE Figure 3