CRC5 creation method and example.

[ldong.1]

Hi. I'm working with L9369 H-bridge driver and i want to know how to calculate CRC code in C programming.

it's polynomial is X^5+X^2+1 and initial value is 0x1F(1 1111)

1. Do you have any documentation regarding CRC implementation in C code?

i got CRC documentable attched document, but i think it's 26 bit and responding CRC value is not matched.

2.i want to know X^5+X^2+1 polynomial's CRC Lookup Table and how to use lookup table in C code. i calculated it by manual, and have some result. i want to know i calculate it correctly.

0->0x0

1-> 0x5

2->0xa

3->0xf

4->0x14

5->0x11

.....

255-> 0x14 

3. i want to know how to use lookup table below 2 cases. and sample code.

when i try to calucate CRC value with lookup-table, it's result is different to the value calculated by manual.

(please let me know how to calculate in case of CRC initial(0x1F) value apply, not apply case)

0x1234 ( 0001 0010 0011 0100)

0x2D40CFF(0010 1101 0100 0000 1100 1111 1111)

4. I attached excel file that i calculate CRC value by manual.

atttched document says 26 bit stream is  "0 0000:0011 0 0000:0000:0000:0000" and CRC value is 0x0B. My CRC value is 0x15(with initial CRC), 0x1A(with out initial CRC)

i don't know why the result is different to me. please help to solve it.

thank you.

thank you.

[ldong.1]

i think LSB is 5bit. not 6bit. isn't it? (CRC-5 's polynomial is 100101(6bit), so CRC value must be 5bit. [4..0].

you are right if (26 bit data) + (6 bit CRC) = 32bit, but i think CRC bit stream is 5 bit.

And in case of 24bit or 25bit, i think above code is not working properly.

because of initial seed value calculation, i think alignment between data & shifted initial value will not match. corret?

[Tesla DeLorean · Accepted solution]

I'm computing the CRC over 26 bits, when completed the high order 5-bits are the remainder, then I pull them back into the low order[4..0]

crc = (crc >> 27) & 0x1F; // Recover, align, mask computed value

Now if I appended the 5-bit CRC and ran those 31 bits, the remainder for the correct CRC would drop to zero.