DS2438 Dallas Semiconducotr, DS2438 Datasheet - Page 11

• Manufacturer Part Number: DS2438
• Description: Smart Battery Monitor
• Manufacturer: Dallas Semiconducotr

DS2438
CRC Generation
The DS2438 has an 8-bit CRC stored in the most significant byte of the 64-bit ROM. The bus master can
compute a CRC value from the first 56 bits of the 64-bit ROM and compare it to the value stored within
the DS2438 to determine if the ROM data has been received error-free by the bus master. The equivalent
polynomial function of this CRC is:
8
5
4
CRC = X
+ X
+ X
+1
The DS2438 also generates an 8-bit CRC value using the same polynomial function shown above and
provides this value to the bus master to validate the transfer of data bytes. In each case where a CRC is
used for data transfer validation, the bus master must calculate a CRC value using the polynomial
function given above and compare the calculated value to either the 8-bit CRC value stored in the 64-bit
ROM portion of the DS2438 (for ROM reads) or the 8-bit CRC value computed within the DS2438
(which is read as a 9th byte when a scratchpad is read). The comparison of CRC values and decision to
continue with an operation are determined entirely by the bus master. There is no circuitry inside the
DS2438 that prevents a command sequence from proceeding if the CRC stored in or calculated by the
DS2438 does not match the value generated by the bus master. Proper use of the CRC as outlined in the
flowchart of Figure 6 can result in a communication channel with a very high level of integrity.
The 1-Wire CRC can be generated using a polynomial generator consisting of a shift register and XOR
gates as shown in Figure 4. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is
available in Application Note 27 entitled “Understanding and Using Cyclic Redundancy Checks with
Dallas Semiconductor Touch Memory Products.”
The shift register bits are initialized to 0. Then starting with the least significant bit of the family code,
th
1 bit at a time is shifted in. After the 8
bit of the family code has been entered, the serial number is
th
entered. After the 48
bit of the serial number has been entered, the shift register contains the CRC
value.
1-WIRE CRC CODE Figure 4
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