Rainbow Electronics DS2890 - Manual (Page 11)

DS2890
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FIGURE 13. Typical Wiper Resistance vs Wiper Voltage at 25 o C,
Charge Pump OFF
0.1
1
10
100
0 2 4 6 8 10
Voltage Across Wiper (Volts)
W
iper
Resistance (Kohm
s)
V
PU
=2.8V
V
PU
=5V V
PU
=6V
64-BIT LASTERED ROM
Each DS2890 contains a unique and unalterable lasered ROM registration number that is 64 bits long; the
format of this value is shown in Figure 15. 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
16. 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. 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 15. 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