DS2890

10 of 27

Figure 9. DS2890 TO-92 CONFIGURATION BLOCK DIAGRAM

64-BIT LASTERED ROM

Each DS2890 contains a unique ROM registration number that is 64 bits long; the format of this value is

shown in Figure 10. The first 8 bits are a 1-Wire family code; the family code for the DS2890 and future

1-Wire Potentiometers is 2Ch.. The next 48 bits are a unique serial number that is administered by Dallas

Semiconductor. The last 8 bits are a CRC of the first 56 bits. The 1-Wire CRC is generated using a

polynomial generator consisting of a shift register and XOR gates as shown in Figure 11. Operationally,

the CRC generator works as follows: The shift register bits are first initialized to zero. Then starting with

the least significant bit, the 8-bit family code is shifted in. After the 8th bit of the family code has been

entered, the 48-bit serial number is shifted in. After shifting in the 48th bit of the serial number the shift

register contains the CRC value. Shifting in the 8 bits of CRC should return the shift register to an all

zeros value. Detailed information about the Dallas 1-Wire Cyclic Redundancy Check is available in the

Book of DS19xx iButton Standards. The 64-bit ROM and the 1-Wire Function Controller portions of the

DS2890 allow the device to operate as a 1-Wire device and follow the protocol detailed in the section

“TRANSACTION SEQUENCE”.

Figure 10. 64-BIT LASERED ROM

MSB

LSB

8-Bit CRC Code

48-Bit Serial Number

8-Bit Family Code (2Ch)

MSB

LSB

MSB

LSB

MSB

LSB