DS2436 Просмотр технического описания (PDF) - Maxim Integrated

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DS2436

Battery ID/Monitor Chip

Maxim Integrated 

DS2436

CRC GENERATION

The DS2436 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 DS2436 to determine if the ROM data has been received error-free by the bus master. Additionally,

each page read appends one CRC byte. The equivalent polynomial function of this CRC is:

CRC = X8 + X5 + X4 + 1

Xn = bit at the n-th stage

+ = "exclusive-or" function

The DS2436 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 DS2436 (for ROM reads) or the 8-bit CRC value computed within the DS2436

scratchpad (which is read as a 33rd byte when the 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 DS2436 that prevents a command sequence from proceeding if the CRC stored in or calculated

by the DS2436 does not match the value generated by the bus master. Proper use of the CRC 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 6. Additional information about the Dallas 1-Wire CRC is available in an

application note entitled “Understanding and Using Cyclic Redundancy Checks with Dallas

Semiconductor Touch Memory Products” (App Note #27).

In the circuit in Figure 6, the shift register bits are initialized to 0. 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,

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 0s.

64-BIT LASERED ROM Figure 4

8-BIT CRC CODE

48-BIT SERIAL NUMBER

8-BIT FAMILY CODE (1B)

MSB

LSB MSB

LSB MSB

LSB

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