LTC3300-2_15 Datasheet, PDF(34/42 Page) Linear Technology – Addressable High Efficiency Bidirectional Multicell Battery Balancer

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LTC3300-2_15 Datasheet, PDF (34/42 Pages) Linear Technology – Addressable High Efficiency Bidirectional Multicell Battery Balancer

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LTC3300-2

APPLICATIONS INFORMATION

How to Calculate the CRC

One simple method of computing an n-bit CRC is to perform

arithmetic modulo-2 division of the n+1 bit characteristic

polynomial into the m bit message appended with n ze-

ros (m+n bits). Arithmetic modulo-2 division resembles

normal long division absent borrows and carries. At each

intermediate step of the long division, if the leading bit

of the dividend is a 1, a 1 is entered in the quotient and

the dividend is exclusive-ORed bitwise with the divisor.

If the leading bit of the dividend is a 0, a 0 is entered in

the quotient and the dividend is exclusive-ORed bitwise

with n zeros. This process is repeated m times. At the end

of the long division, the quotient is disregarded and the

n-bit remainder is the CRC. This will be more clear in the

example to follow.

For the CRC implementation in the LTC3300-2, n = 4 and

m = 12. The characteristic polynomial employed is x4 + x

+ 1, which is shorthand for 1x4 + 0x3 + 0x2 + 1x1 + 1x0,

resulting in 10011 for the divisor. The message is the first

12 bits of the balance command. Suppose for example the

desired balance command calls for simultaneous charging

of Cell 1 and synchronous discharging of Cell 4. The 12-bit

message (MSB first) will be 110000010000. Appending

4 zeros results in 1100000100000000 for the dividend.

The long division is shown in Figure 13a with a resultant

CRC of 1101. Note that the CRC bits in the write balance

command are inverted. Thus the correct 16-bit balance

command is 1100000100000010. Figure 13b shows the

same long division procedure being used to check the

CRC of data (command or status) read back from the

LTC3300-2. In this scenario, the remainder after the long

division must be zero (0000) for the data to be valid. Note

that the readback CRC bits must be inverted in the dividend

before performing the division.

An alternate method to calculate the CRC is shown in

Figure 14 in which the balance command bits are input to

a combinational logic circuit comprised solely of 2-input

exclusive-OR gates. This âbrute forceâ implementation is

easily replicated in a few lines of C code.

READBACK = 1100000100000010

DIVIDEND = 1100000100001101

110101101011

(a) 1 0 0 1 1 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0

10011

110101101011

(b) 1 0 0 1 1 1 1 0 0 0 0 0 1 0 0 0 0 1 1 0 1

10011

10110

10011

01010

00000

10101

10011

01100

00000

11000

10011

10110

10011

01010

00000

10100

10101

10011

01110

01101

00000

11100

11010

10011

11110

10011

REMAINDER = 1 1 0 1 = 4-BIT CRC

REMAINDER = 0

0 0 1 0 = 4-BIT CRC INVERTED

33002 F13

Figure 13. (a) Long Division Example to Calculate CRC for Writes.

(b) Long Division Example to Check CRC for Reads

For more information www.linear.com/LTC3300-2

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